8; a; ., if . R53' L“. .2 «a mm, A, 3 “5%er Kr... t. ‘ .5. . .5 a: a. . 2 flaws. : 052$ wt!- 3.. t... . U a A: , t 3113? . ~A. n arthfififiqml.5tu.ia I. 5‘12. 5»... “fur. . ’l 5.0.x! 3!. 2. :11. .is i « 113(36131. 314’s! ‘7‘": a" . t. :7 5.1;! 31‘7“.» :. ‘9 ‘3 34.... in 39‘! a 43am. 2902 LIBRARY Michigan State University This is to certify that the dissertation entitled He II Reverberation In N06 5548 presented by Aaron Patrick LaCluyzé has been accepted towards fulfillment of the requirements for the Ph.D. degree in Astrophysics and Astronomy '(LLK Q “KXMV Major Professor’s Signature 8/27/65 Date Doctoral Dissertation MSU is an affirmative-action, equal-opportunity employer PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 5108 KIProglAcc8PreleIRC/DateDue Indd HE II REVERBERATION IN NGC 5548 By Aaron Patrick LaCluyzé A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Astrophysics and Astronomy 2008 ABSTRACT HE II REVERBERATION IN NGC 5548 By Aaron Patrick LaCluyzé Despite decades of study, the exact nature of the central engines of Active Galactic Nuclei (AGN) is still a mystery to be solved. Although great strides have been made, there are some outstanding questions that still need to be resolved, and new tools are needed to answer them. The goal of this project was to craft a new method to accurately measure highly blended weak emission lines in AGN. In particular, the reverberation behavior of He II A4686 and He II A1640 were investigated to reconcile a discrepancy between theoretical models, which predicted that the two lines should have the same lagtime and previous observational studies, which found that the optical line had a reverberation lag several times longer than the UV line. The optical and UV He II emission lines were measured to have a reverberation lag time of 5 - 6 days, with good overlap between the two lines. Furthermore, a previously measured strong lines, C IV, which were also involved in the heavy blending, was measured to have a lagtime consistent with previous measurements. This helps to validate the technique that was used. This technique, which required a large software development effort, included using an automated fitting routine to simultaneously fit multiple emission line template profiles that were, themselves, created from observed data. One rather surprising result was the clear indication that He II and C IV are not created by the same gas within the BELR. This is contrary to what was predicted by the typical model of the BELR but consistent with previous measurements. Copyright by AARON PATRICK LACLUYZE 2008 For Lise, Dodger, Anna and Dclcnn iv ACKNOWLEDGMENTS First and formost, I must thank my wife, Jennifer, and my parents Paul and Mary. Without their help and support, I would not have made it this far. I also want to thank my advisor, Dr. Jack Baldwin. His patience has been saint—like. There have been a great many graduate students over the years who have helped me though all of this. Thanks to Pete, Bob, Mike, Fred, Ryan, and Chuck, all of whom helped me understand what graduate school is all about. Brian, Chris, Ken, Nathan, and Katie helped me make it through the long years that followed. Thanks also to the current pool of grad students who have kept me sane while I finished. Special thanks goes to Charles, who was willing to answer my silly questions about software. A very special thanks goes to Dan Reichart, who took a chance on me and has been very patient while I tied up loose ends. I am grateful to the National Science Foundation for their support of this work under grant AST-0305833, and to NASA for their support under ADP grant NAG5- 13075. I also thank Jack Baldwin, Mark Bottorff, Gary Ferland and Kirk Korista, the co—investigators on the latter grant, for their help. Thanks to Christopher Waters for the LATEX class used to format this thesis. TABLE OF CONTENTS List of Tables ........................................................ viii List of Figures ....................................................... ix 1 Introduction ...................................................... 1 1.1 A Brief history of AGN ...................... 2 1.2 AGN Standard Model ....................... 3 1.3 Reverberation Mapping ...................... 6 1.4 The Importance of He II Emission ................ 9 2 Data Set Description ............................................. 16 2.1 Background on AGN Watch and data set for NGC 5548 . . . . 16 2.2 Ultra-Violet Data From IUE and HST .............. 17 2.2.1 Non-linearity of IUE at Low Flux Levels ........... 18 2.3 Optical Data ............................ 23 2.3.1 Corrections to the Optical Data ................ 25 2.3.2 Observed vs. Rest Frame ................... 28 3 Data Fitting and Measurement .................................. 29 3.1 Automated Fitting Routine .................... 29 3.2 Building the Blend ......................... 32 3.2.1 Fe II Template ......................... 33 3.2.2 Other Templates ........................ 35 3.2.3 Fitting Parameters ....................... 38 3.2.4 Fitting Windows ........................ 40 3.3 Minuit and the X2 Minimization ................. 42 3.4 Error Estimation .......................... 46 3.5 Cross-Correlation Lags ...................... 47 4 Results ........................................................... 55 4.1 Cross Correlation Centroid Distribution ............. 55 4.2 Light Curve Error Estimation ................... 59 4.3 Sensitivity to Profile Templates .................. 61 5 Discussion ........................................................ 67 6 Summary ........................................................ 72 A Code for contin.f ................................................. 75 B Code for cfunk.f .................................................. 87 vi C Code for uvcontin.f ............................................... 91 D Code for uvfunk.f ................................................ 104 E Code for functions.f .............................................. 108 F Template Profiles ................................................. 115 References ....................................................... 137 vii 1.1 1.2 2.1 2.2 2.3 3.1 3.2 3.3 4.1 4.2 F.1 F2 F3 F4 F5 F6 F7 F8 LIST OF TABLES Reverberation Lags From Previous Studies ................. 13 Other C IV A1549 / He II A1640 Lag Ratios ................ 14 IUE UV Data ................................. 22 Optical Data Site List ............................ 24 Optical Data ................................. 27 Fitting Windows ............................... 42 Optical Profile Summary ........................... 54 UV Profile Summary ............................. 54 CCCD Results ................................ 57 Profile Permutations ............................. 63 C IV BELR Template ............................ 115 He II BELR Template ............................ 116 He I BELR Template ............................. 121 H6 BELR Template ............................. 125 C IV NELR Template ............................ 134 O III ...................................... 134 He II NELR Template ............................ 135 0 III 1663 BELR Template ......................... 135 viii LIST OF FIGURES 1.1 Simplified Cross-Sectional Representation of an AGN ........... 1.2 Simplified Illustration of Reverberation Mapping. ............. 1.3 Simplified Grotrian Diagram of He II .................... 1.4 NGC 5548 Optical Detail .......................... 1.5 NGC 5548 HST UV Detail .......................... 2.1 Typical UV spectrum. ............................ 2.2 Ratio of IUE to HST flux levels as a function of IUE detector counts 2.3 Typical optical spectrum. .......................... 3.1 Typical Optical Spectrum .......................... 3.2 Typical UV Spectrum ............................ 3.3 Fe II Optical Template ............................ 3.4 Fe II UV Template .............................. 3.5 Example Optical RMS Spectrum ...................... 3.6 UV RMS Spectrum .............................. 3.7 Ha BLR Profile ................................ 3.8 He II BLR Profile ............................... 3.9 He I BLR Profile ............................... 3.10 C IV BLR Profile ............................... 3.11 Optical Fit Overlay .............................. 3.12 UV Fit Overlay ................................ 3.13 UV Continuum Light Curve ......................... ix 10 11 12 19 30 31 34 35 36 37 38 39 40 41 44 45 49 3.14 3.15 3.16 3.17 4.1 4.2 4.3 4.4 4.5 4.6 4.7 5.1 Optical He II Light Curve .......................... 50 UV He II Light Curve ............................ 51 C IV Light Curve ............................... 52 H,3 Light Curve ................................ 53 Composite Centroids for Optical and UV He II .............. 56 Cross Correlation Centroid Distribution for C IV ............. 58 Composite Centroids for Optical and UV He II and C IV ......... 59 Cross Correlation Centroid Distribution for H6 .............. 60 He II Error Estimation Test ......................... 61 He II Error Estimation Test ......................... 62 He II A4686 Profile W'idth .......................... 64 Standard LOC Model ............................. 70 CHAPTER 1: INTRODUCTION Since their discovery by radio telescopes in the late 1950’s, quasars have captivated researchers. Despite decades Of intense study, only a general theoretical model exists with many details still unclear. Under the current standard model quasars are just one manifestation of a larger class of objects called active galactic nuclei (hereafter AGN). Because even the most nearby of AGN are at very large distances from us, direct imaging of their central cores is impossible. This makes verification of theoretical models difficult at best. We must therefore resort to indirect methods to attempt to gain insight into the actual physical workings within the core region. One technique that can be used to study the inner regions of AGN is reverberation mapping, where the lag time between a change in the driving continuum radiation and the response of an emission line is used to indirectly measure the distribution Of gas around the central engine. In this work, I present a new analysis of He II reverberation in N GC 5548, a relatively nearby and well studied low luminosity AGN, classified as a type 1 Seyfert galaxy. The remainder of this chapter will give a brief introduction to AGN, an overview of the reverberation mapping technique, and a discussion of why He II emission is both interesting and important to our understanding of AGN. 1.1 A BRIEF HISTORY OF AGN During the late 1950’s, radio astronomy was beginning to produce vast catalogs of radio sources. Using these catalogs, astronomers attempted to match radio sources with Optical counterparts. During this process, it became clear that several of the radio sources had fairly blue star-like point source optical companions. These strange objects were dubbed quasi-stellar radio sources (QSOS), or quasars for short. The most famous of these objects are 3C 48, the first to be observed, and 3C 273, the first to be identified with a bright (mu 2 12.8) stellar-appearing object; see Schmidt (1963), Oke & Schmidt (1963) and Greenstein & Schmidt (1964). The spectra of these objects caused both initial confusion and decades Of debate. Since the objects were star-like in appearance, the emission lines that were detected were not immediately identified. Schmidt and his collaborators made the first steps toward understanding by showing that in 3C 273 these unidentified emission lines were actually the well known Balmer lines of Hydrogen that had undergone a redshift of z 2 0.158. This sort of redshift had been seen before in galaxies, but a combination of being at a high redshift as well as being bright and stellar in appearance was both new and interesting. This led to debate within the community as to the nature of these Objects. They could be extremely bright, distant objects with a redshift due to the Hubble flow (the expansion of the Universe itself.) This would explain the point- like appearance, but would imply an extremely large luminosity. Or they could also be much less luminous star-like objects with the redshift arising from a large peculiar radial velocity relative to Earth. The latter hypothesis would imply a more ordinary luminosity, but would give no reason for the high velocity. While Schmidt and his colleagues assumed the former, there was no explanation at the time for how quasars could generate enough energy to be luminous enough to appear so bright at these vast extra—galactic distances. It is important to understand that like many Objects in astronomy, the definition of a quasar was originally based mainly on appearance. During the decades since, there have been a variety of Objects discovered that have many properties in common with quasars. This much larger group of seemingly re- lated objects are known collectively as active galactic nuclei. These AGN are galaxies in which the central core can outshine the rest of the stars in the galaxy combined. The defining characteristic is that the continuum radiation produced by these objects is not only luminous, but spans a large range in energies, extending from x-ray out to the far IR or radio regime. In most cases, there are also a variety of emission lines that, while common to most AGN, are of differing strengths depending on the object in question. The gas creating these emission lines is believed to be fed in from the host galaxy, making them a good tracer of chemical evolution in the central part of massive galaxies over a wide range Of lOOkback times. (Hamann & Ferland, 1999; Hamann et al., 2002) 1.2 AGN STANDARD MODEL \Nhile there has been a great. deal of debate on the exact structure of AGN, a “stan- dard model” has been embraced by a large portion Of the astronomical community. Under the standard model, all AGN, from radio galaxies to blazars, are the same type Of object, with distance, viewing angle, and the total output energy being the major factors that dictate which classification each object is given. The object of interest in this study, NGC 5548, is a type 1 Seyfert galaxy. Seyfert galaxies are a type of AGN that show clear emission lines, nuclear x-ray emission, and sometimes a small-scale radio jet but are otherwise radio quiet. Seyfert galaxies are the most common type of AGN. In the standard model (Gaskell, 2008, and references therein), the method of energy generation is accretion of matter onto a super-massive blackhole. This central black hole has a mass of about 106 —— 108M@ and is surrounded by an accretion disk that is on the order of a light day in diameter (see figure 1.1.) This size is inferred from variations in the output intensity that are on the order Of a day for lower energy objects like Seyfert galaxies, while more luminous QSOS show variability on time scales closer to a year, implying a correspondingly larger central engine. As gas falls- in toward the black hole, it collapses into an accretion disk due to angular momentum conservation. Internal friction within this disk causes the gas to heat up and begin to emit radiation like a blackbody. The closer the gas is to the center, the higher its angular speed, and the higher the temperature. This creates a superposition of blackbody radiation of varying temperatures generating a powerful and relatively flat continuum. Secondary processes then convert some of this thermal radiation into additional x—ray, IR, and radio light. While some of this radiation escapes the AGN, some Of it is reprocessed into emission lines by cooler gas that lies outside of the accretion disk. The first region Of gas that is encountered by the continuum radiation on its jour- ney out from the accretion disk is the so called broad emission line region (BELR, sometimes written as BLR). This gas is in close to the accretion disk, at a distance of several light days to tens Of light days from the central source in a typical Seyfert galaxy. Because the gas is in close to the central mass, it has a large velocity on the order of 10,000 km/s. This broadens any emission lines from the gas in the BELR, hence the name broad-line region. This Doppler-broadened emission, however, isn’t seen in all types of AGN . The standard model postulates that outside the BELR there is a large dusty torus, a doughnut-shaped collection of gas and dust that blocks the emission coming from the interior. The angle of the object to our line of sight will dictate our ability to observe the broad emission lines. This is the primary classifica- tion difference between type 1 and type 2 Seyfert galaxies, with type 1 showing broad and narrow emission lines and type 2 showing only narrow lines (see figure 1.1.) One consequence Of the location of the BELR is that it is quite sensitive to any variability in the continuum emission. The emission lines are generated by gas that Observer A NELR . C Black BELR . . H016 . . . O 0.0 . Q - " o O Accretion .0 Disk Dusty . -. Observer (Continuum Toms O O B . . Source) . . Figure 1.1 Simplified Cross-Sectional Representation of an AGN - This figure shows a typical AGN cross-section, seen edge-on. This figure is not to scale in any way and is meant as a general guide to the relative location of regions of interest in an AGN. When an observer is at position A, the emission from the interior BELR can be seen. When an observer is at position B, the dusty torus will block the BELR emission and only the lines from the N ELR will be seen. This leads to a distinction between Seyfert 1 galaxies, which show BELR emission, and Seyfert 2 galaxies, which do not show BELR emission. has been been excited by being continually bathed in radiation from the central en- gine. When the central engine’s emission changes in intensity, the broad line emission also changes on a time scale directly related to the light travel time from the central source to the emitting gas. This variability in the central continuum is Often seen in AGN, although the exact cause is not well known. It is also important to understand that the variability is not predictable or periodic as it would be in certain types of variable stars. As such, obtaining meaningful observations of this variability can be a long and difficult process spanning many months to years. Because this variability is not regularly periodic, extensive Observations need to be made to ensure that the Object is Observed in both a quiet baseline state as well as during a period of more intense activity. Outside of the dusty torus is another large region of gas that also reprocesses the continuum radiation into emission lines. This gas is much further from the central engine and as such does not exhibit the large Doppler broadening seen in the BELR. This region is therefore called the narrow emission line region (NELR, sometimes written as NLR). Unlike the BELR, the NELR is fairly insensitive to short term variations in the continuum intensity. As such, we can treat the narrow emission lines as being constant over the time scales during which the BELR is observed. 1.3 REVERBERATION MAPPING Although AGN are very luminous, it was quickly deduced that for the core to coher- ently vary in brightness on timescales of days or weeks as observed, it must be very small, only light-days to light-weeks across (Terrell, 1967). This means that even the most nearby AGN have cores, including the BELR, that are on the sub-arcsecond scale and cannot be resolved by either ground or space based instrumentation. Direct imaging of the cores of AGN is simply inconceivable for the foreseeable future. We can exploit the response of the BELR to changes in the continuum to indirectly measure the size of the BELR by making a few basic assumptions (Peterson & Horne, 2006). We assume that the continuum originates from a single central source. Because the size of the BELR is expected to be about a factor of 100 larger than the size of the accretion disk, this assumption is well justified. We also assume that the most important time scale to be considered is the light travel time required for photons to travel a distance r from the central source to the BELR clouds: 7.. There are two other time scales that should be considered because they may at first glance seem important. The first is the recombination time scale: —1 Tree = (71603) (1.2) Here a B is the Hydrogen case B recombination coefficient, and ne is the particle density of the line producing gas. Case B describes the situation where low-density ionized clouds form Hydrogen emission lines such that the higher Lyman lines scatter Often enough tO be degraded into a Balmer line and either a Lya or the two-photon continuum and the electron density must be low enough for collisions to be slow compared with spontaneous emission (Ferland, 1999). This is thought to describe line formation in most AGN (Osterbrock, 1989). A typical BELR density of 17.6 m 1010cm—3 gives a Tree 2: 0.lhr, much shorter than the light days of TLT. A second possibly important time scale is the dynamical time scale for the motion of the BELR gas itself. 7. In the case of a typical Seyfert galaxy, the type of AGN of interest in this work, Tdyn z 3 — 5years. This is much longer than the several months of the observing campaign used here, but can become an important factor with longer campaigns. A final assumption that needs to be made is that there is a simple relationship between the observed continuum and the ionizing continuum. Because the ionizing continuum is driving the line variations, we must assume that the ionizing continuum and the observed continuum vary in phase with one another. There is some evi- dence that there is a delay between the long wavelength continuum and the shorter wavelength continuum variations (Gaskell, 2008), but the time scale Of this is still significantly shorter than the timescale for the emission-line response. To better understand the principle behind this technique, imagine a situation where we have three test clouds at a common distance r from the central engine (the l 1’ iii Figure 1.2 Simplified Illustration Of Reverberation Mapping — The dark circle repre- sents the accretion disk central engine of the AGN, producing the broad spectrum continuum radiation. The three objects labeled “A”, “B”, and “C” are representa- tive BELR clouds all at the same distance r from the central engine. Because of the additional distance that lines emitted by clouds “B” and “C” must travel compared to “A”, there is a delay in the response of the observed broad emission to a change in continuum luminosity. black hole and its accretion disk, see figure 1.2.) If the central engine is variable and has a sudden increase in continuum luminosity, this pulse of increased intensity takes a finite amount of time to reach the gas which re—processes this continuum radiation, giving rise to the BELR lines. The pulse in the continuum luminosity continues onward to our Observer at some distance which is large compared to r. The response of cloud “A” to the increased continuum is seen by our observer almost immediately after the increase in the continuum as cloud “A” is along the line of sight from the observer to the central engine. The response of cloud “B” is seen after an additional time T = r/c corresponding to the additional distance that the emission line radiation must travel. The response of cloud “C” has an additional distance of 7‘ compared to cloud “B”, or 27‘ compared to cloud “A”. While it takes a finite amount of time for all Of the gas to respond, the calculation of a cross correlation between the light curves of the continuum and the responding emission line yields a mean response time for the emitting gas, which is reported as the characteristic lag time. This is the principle behind reverberation mapping and is the best tool currently available to study the inner structure of AGN. . This generalized discussion of the nature of the BELR is for the most part spec- ulative. Very little is actually known about the true structure of this region. While the cartoon picture of discrete clouds is useful for illustrative purposes, it may not reflect the true nature Of the region. 1.4 THE IMPORTANCE OF HE II EMISSION In order for an emission line to be produced efficiently, there must exist a specific combination of gas density and incident radiation strength. This is the crux Of the “Locally Optimally-emitting Cloud” (LOC) model put forth by Baldwin et a1. (1995). If the gas is too diffuse, there is not enough reprocessing to significantly affect the radiation output. If the gas is too far from the central engine, there isn’t enough incident flux to cause significant excitation or ionization/recombination. This is the central premise behind several models of the inner regions of AGN. Each emission line has an optimal location in space around the central engine where it can be produced efficiently. This corresponds to a characteristic distance from the center, assuming a roughly spherically symmetric distribution of gas. This distance is set by the ionization parameter, the ratio of incident ionizing flux to the gas density. In this work, the primary goal is to measure the reverberation lags of He II A4686 in the optical region and He II A1640 and C IV A1640 in the ultraviolet region in NGC 5548. The He II lines are produced by singly ionized helium which is hydrogenic in nature. This means that, having only one electron, He II behaves in many ways Similar to hydrogen and is therefore relatively easy to understand. Theoretical predictions (Baldwin et al., 1995) Show that the optimal conditions for producing both the A4686 and the A1640 lines are virtually identical as both emission lines should be produced by the same recombination and radiative cascading process (MacAlpine, 1981), as illustrated in figure 1.3. This means that they should both have about the same reverberation lag times. In addition, Bottorff et al. Showed that plasma simulations assuming a simple model for the BELR gas distribution (see discussion of LOC model in chapter 5) suggest that the He II lines (A1640 and A4686) Should have a Similar reverberation lag as C IV (A1549). 4 \O B 3 :1- CW Vv—d :25: 2 Hell <2 MOM omv CONN 1 Figure 1.3 Simplified Grotrian Diagram of He II, showing only the first four energy levels. The lines between energy levels are labelled by the approximate wavelength of the photons emitted when an electron drops from the higher to the lower energy level, or the wavelength of light absorbed to move an electron from the lower energy level to the higher one. Of particular note are the paths that would lead from level 4 to level 3 and from level 3 to level 2 that would produce A4686 and A 1640. From an Observational standpoint, there is only one Object, NGC 5548, where the A1640 and A4686 lines have been measured in the same campaign. Because the BELR is changing over time, it is important to measure both lines during the same time period in order to accurately measure the lag ratio. Wamsteker et al. (1990) studied the intensity ratio of the He II A1640 and A4686 in observations from both IUE and ground based data sources from 1978 - 1986. While reverberation mapping was not the goal of that study, the authors did find that the intensity ratio of the two lines, f(A1640)/f(A4686), varied from 4 to 10 during the study. A changing flux ratio would imply that the two lines are responding to the continuum changes on different time scales, implying different reverberation lag times for the two He II lines. While 10 Wamsteker et al. did some fit synthetic profiles to some Of the blended emission lines, they did so by using several generic line components rather than true template profiles generated from the Observations. 4 I l l l l l T HB 3.5 - Hy a 3 .— q ’6? Hel4471 U) 9 l 3,3 2 52 ............................................ ‘3’ 1.5 - ---------------------------------- q “- 1 He II 4686 0.5 - _ O l l 4 l l l 1 4200 4300 4400 4500 4600 4700 4800 4900 Wavelength (Angstroms) Figure 1.4 NCG 5548 Optical Detail - Example of NGC 5548 optical Spectrum, as used in Dietrich et al. (1993). The continuum was approximated as a straightline under the region of interest (dashed line.) The He II A4686 emission was defined to be in a simple bin above this pseudo-continuum from 4593 - 4770 A. This area includes contamination from the wing of H6 as well as He I and a weak iron multiplet that is contaminating the entire region. NGC 5548 was later a target for study by the International AGN watch (see chapter 2 for more information on this data set.) The UV He II A1640 and C IV A1549 (Clavel et al., 1991) and the optical He II A4686 (Dietrich et al., 1993) were among the emission lines studied to determine their reverberation lag times. These two studies found that the two He II lines may have different reverberation lags (between 4-10 days for the UV line and 7 days for the optical) and the He II A1640 did not agree well 11 with C IV (between 8-16 days). The authors did caution, however, that blending with other nearby emission lines is an issue (see figure 1.4.) The emission lines were defined to be in a simple wavelength bin, and all flux above a straight-line pseudo-continuum was assumed to be from the line in question. This does not correctly account for the wings of surrounding lines that could be contaminating the measurement. The optical He II A4686 line is contaminated by H6, He 1, and a Fe II multiplet that is under the entire region (see section 3.2 for more discussion on these contaminants.) 25 I I I I fi 20 - CIV - 70‘ g 15 - - (D 2’ m 3 i, 10 - A ‘x’ 3 E 5 '- -l 0 - Hell 1640+Olll]1663 ~ 1 450 1 500 1 550 1600 1 650 1 700 Wavelength (Angstroms) Figure 1.5 NCG 5548 HST UV Detail - Example of N GO 5548 UV spectrum, as used in Korista et al. (1995). The continuum was approximated as a straightline under the region of interest (dashed line.) The He II A1640 emission was defined to be in a simple bin above this pseudo-continuum. This area includes contamination from the wing of C IVA1549 as well as O III] A1663 and a weak iron multiplet that is contaminating the entire region. Korista et al. (1995) did a similar analysis for the UV data, but included measure— ments from HST as well as IUE. Once again, a straight line continuum and wavelength 12 bins were used in a manner similar to the Dietrich et al. study (see figure 1.5.) In the case of the UV Spectrum, the He II A1640 line is heavily blended with the O III] A1663 emission aS well as the wing of C IV and a weak Fe II multiplet that is contaminating the entire region. In this case, no attempt has been made to deblend the 0 III] A1663 emission, and the effects on the reverberation lag were simply estimated. Table 1.1: Reverberation Lags From Previous Studies - This table, taken from data in table 1 of Peterson & Wandel (1999), shows the best measured values for the reverberation lags of several emission lines from NGC 5548 using the 1989 AGN Watch data set. Line Lag (days) He 11 A1640 3.013;? He 11 A4686 asigj C 1v A1549 9.5iff, The AGN watch data was also further analyzed by Peterson & Wandel (1999) in order to estimate the mass of the central black hole. During this study, more careful attention was paid to the reverberation lag measurements. These values are shown in table 1.1. While it is true that the two He 11 lines do agree within formal errors, the UV He II and the C IV do not overlap, which is not predicted by simulations (Bottorff et al., 2002). In fact, Bottorff et al. argue that if this discrepancy is real, it is a significant failure in a general picture that otherwise fits together all of the reverberation results. Under any situation where the A1640 and A4686 lines are formed by cascades following recombination, as is thought to be the case, the two lines must be formed in the same volume of gas and therefore should vary together with the same reverberation time scales. Bottorff et al. further Show that the C IV A1549 should be produced under conditions similar enough to the region that favors He II 13 production and there should therefore be only a small difference in the reverberation lag times of C IV and either He II. While the He II A1640 and A4686 lines do formally overlap in the one object where both line are measured in the same campaign, there are several other AGN where only the He II A1640 and the C IV A1549 were measured. These measurements are summarized in table 1.2. In each case, the lag ratio of C IV A1549 to He II A1640 differs from unity by a convincing margin. Table 1.2: Other C IV A1549 / He II A1640 Lag Ratios - This table, taken from Bottorff et al. (2002) summarizes the (C IV A1549) / (He II A1640) lag ratio for several Objects, which Bottorff et al. considered to have reliable lag measurements of both lines. It is clearly Shown that this ratio is greater than 1 in all of the Objects. Object (1549) / (1640) Lag Ratio References NGC 5548 3.3 Bottorff et al. (2002) NGC 3783 5.0 Reichert et al. (1994) Fairall 9 3.4 Rodriguez-Pascual et a1. (1997) NGC 7469 2.4 Wanders et al. (1997) 3C 390.3 3.8 O’Brien et al. (1998) Average 3.6 The conclusions of Bottorff et al. (2002) concerning the existing data were that, “the best estimate of the lag times is that the A1640 feature varies 3 times more rapidly than A4686” and that “the He II A1640 lag times are measured to be several times Shorter than those of C IV A1549”. The first of these points would be extremely surprising, while the difference between the He II and C IV lag times is not predicted by the simplest models of the BELR. 14 One possibility, tested in this work, is that the He II lag measurements are simply in error. The He II emission lines are quite weak compared to most of the well- studied AGN emission lines. There is also considerable contamination by surrounding emission lines. In the case of He II A4686, it is contaminated by the blue wing of H6 (and possibly to a some extent the red wing of H7), He I A4471 and a Fe II multiplet. The UV He II A1640 line is Similarly contaminated by C IV A1549 and O III A1663 as well as a Fe II multiplet. Previous measurements of these lines, such as those described above, have not fully accounted for these blending problems. Emission lines were defined either by a simple bin or approximated using a blend of Gaussian profiles. Neither of these methods fully account for contamination by other emission lines that may be varying at slightly different rates. In this work, we present a new re—analysis of archival data of N GC 5548, a nearby, well Observed, type 1 Seyfert galaxy. Careful attention has been paid to separating contamination from the He II emission so as to present the most careful measurement of the reverberation lags to date. This has also been done in a more automated fashion than in previous studies in order to remove some of the the human subjectivity that is always present in data analysis. The software tools that were developed were designed to be compatible with previously existing software that is used by several collaborators. 15 CHAPTER 2: DATA SET DESCRIPTION This chapter will outline the basic details of the data used for this study. It is important to understand that gathering the amount of data needed for a study like this is a huge undertaking. For example, several years of observations are needed to get a really good baseline covering multiple reverberation events, and these should have a time sampling of at least one observation every few nights. In this case, archival data from a previous effort were used. This archival data set, despite some of its shortcomings, is the most complete set Of observations in terms of both spectral and temporal coverage Of any AGN that has been taken. 2.1 BACKGROUND ON AGN WATCH AND DATA SET FOR NGC 5548 The archival data set used here was taken as part of a project instigated by the International AGN Watch, a consortium of scientists who were interested in studying several AGN in detail, with careful consideration given to sufficient time resolution of Observations. To this end, one of the objects chosen for study was NGC 5548, a nearby Seyfert 1 galaxy. NGC 5548 is of interest because it is close (2 = 0.017175 2 71 Mpc) and although it is a fairly typical type 1 Seyfert galaxy, it is also highly variable. This allows for extensive study using reverberation mapping techniques. This work spanned more than a decade and produced a series of papers entitled “Steps Toward 16 Determination Of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei I-XVI”. Clavel et al. (1991), Peterson et al. (1991), Peterson et a1. (1992), and Dietrich et al. (1993) are of particular interest for the study described here. Because of the need to compare lines in the optical as well as the ultraviolet, data from both ground and space based Observations are necessary. The Earth’s own atmosphere blocks far too much UV radiation to allow for Observations in this wavelength regime to be taken from the ground. The UV data used here were taken using the International Ultraviolet Explorer (IUE), with some additional Observations taken with the Hubble Space Telescope (HST). 2.2 ULTRA-VIOLET DATA FROM IUE AND HST The International Ultraviolet Explorer (IUE) satellite was launched in January of 1978. It carried an 18-inch telescope with a UV Spectrograph. Although it initially had an expected service life of three years, it remained in operation until it was decommissioned on September 30, 1996. During 1988-1989, the AGN Watch used it for regular observations of NGC 5548 for roughly eight months (Clavel et al., 1991, and references therein.) During this time, a coordinated effort was made to obtain ground-based optical observations of NGC 5548 to complement the UV data from IUE. This IUE data set is the longest regularly sampled set of observations of NGC 5548 in the UV. Although the resolution is somewhat low at roughly 2 A per pixel, it is difficult to ignore this large data set. The observations span Julian Dates 2447510 - 2447745 (14 December 1988 - 7 August 1989) and were taken quite regularly about four days apart, with a total of 60 different days with Observations. On any given day, one LWP (1900-3300 A) and at least one SWP (1170-1970 A) spectrum were taken. Observations were taken in low resolution mode (1000 km 8’1 F WHM) with the large aperture (10” x 20”.) In the analysis described here, the SWP and LVV P 17 Spectra from any given day were combined together to create a single Spectrum for each date, with multiple SW P Spectra averaged together when more than one was taken on the same day. One drawback, in both the IUE and optical data sets, is the sampling rate. Ideally, observations would be taken on a time scale shorter than the shortest reverberation lag that is being studied in order to ensure sufficient temporal coverage of the emission line’s reaction to the changes in the continuum. In the case of IUE, spectra were taken quite regularly at about four day intervals. The primary lines of interest for the original AGN Watch study have lag times on the order of tens Of days and would be adequately sampled by measurements every four days. Due to the techniques used for the reverberation analysis, this data can still be used for He II, and aliasing due to the regular sampling also should not be an issue (see section 3.5 for further discussion Of this point.) 2.2.1 NON—LINEARITY OF IUE AT Low FLUX LEVELS The IUE data have a variety of known problems. Because of the age and nature Of the instrument, several different corrections need to be made to the raw data before they are useful for scientific measurements. These corrections can be as complex as taking into account the angle of the instrument relative to the magnetic field of the Earth. These corrections are well known and have been applied to the data Obtained from the AGN Watch. Koratkar et al. (1997) made a detailed comparison Of near simultaneous spectra of several AGN taken with both IUE and the HST F OS. They found that there were three objects that had observations taken with both instruments no more than 24 hours apart. These objects were Mrk 509, NGC 3783, and NGC 5548. While detailed comparisons of the two instruments had previously been done, such as Bohlin et al. (1990), they were typically done with bright spectrophotometric standard stars. It 18 60 I I I I I I I I I I 50~ - A4OP — U) 2 e 330~ - a) 3 fig 20- - ‘x’ 2 u. 10- _ o- _ 40 I I I I I I I I I I 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 Wavelength (Angstroms) Figure 2.1 Full Typical UV Spectrum - While the emission lines of interest all lie on the left half of this Spectrum, the extended coverage allows a more accurate fit of the ionizing continuum. was thought that since the Spectral energy distribution of AGN is both significantly different in shape and much fainter than that of standard stars, a direct comparison of actual AGN might be more illustrative Of the differences between IUE and HST. Koratkar et al. found that while the IUE and HST measurements of strong emission lines agreed to within 15%, the results for weaker lines disagreed by a much greater amount, reaching a factor of six for the faintest lines. Koratkar et al. concluded that IUE did have a nonlinear response at low flux levels, but they were unable to characterize it well enough to accurately evaluate its effect on their data set. Koratkar et al. analyzed the reduced, flux-calibrated Spectra, in which the total counts in each pixel had been divided by the exposure time and multiplied 19 by a detector response function. However the actual non-linearities in IUE’S SIT- Vidicon detectors are expected to depend only on the charge on the photocathode, which is proportional to the counts recorded after conversion in the ADC. We therefore repeated their analysis in terms Of raw detector counts to see if a better non—linearity correction function could be derived. The data for this comparison came from near simultaneous observations of NGC 5548 using both IUE and HST on 5-6 July 1992, Observations that were also used by Koratkar et al. in their 1997 paper. The IUE observation (LWP23450) was retrieved from the archive in both the raw format and as a NEWSIPS (de La Pena et al., 1994) extracted final spectrum. IRAF was used to extract a spectrum from the raw file and parameters were adjusted until the extracted spectrum matched the NEWSIPS spectrum as well as possible as a way Of finding the number Of raw counts that went into each pixel in the fully-reduced N EWSIPS extracted spectrum. This was, unfortunately, a very subjective process, but Should be sufficient to decide if further investigation would be fruitful. In the end, we simply wish to determine if the ratio of the flux from IUE to the flux from HST is a simple function of raw detector counts. The HST data have 0.4 A per pixel resolution, much higher than those of IUE. To better facilitate a comparison, a boxcar smoothing was applied to the HST data until a resolution similar to that of the IUE data was achieved. A scale factor was applied so that the HST and IUE data had similar flux levels for the Mg II emission around 2850A before a ratio of the IUE flux to the HST/FOS flux was computed for each separate pixel in the IUE spectrum. This ratio was then plotted as a function Of the raw count numbers from the raw IUE spectra (see figure 2.2). This did show a good deal of discrepancy at low counts (less than 100 raw counts). At very low count levels, this ratio varied drastically. There was no apparent pattern to the fluctuations, and as such there was no straightforward way to remove this effect. While the IUE detector does have some problems at low count levels, no systematic non—linearities 20 4 I I I I I I I + 3.5 r - + + 3 - ++ + ‘ + o 2.5 '- i ++ + + ‘ T3 ++++ + I 2 - ¥fi¢ + + -1 '— ++fi§> ++ + + 9:) + + + 1;}: + ++ \ 1 5 " 4* 4’+ +;++ + ‘ “DJ + ++ + ++:+++++# + + + + _ 1 - + +++ i+fiif+¢i + + d + +4?- £+ 4" + + + + 05 - *3 3+ + + q . *4? 4* 0 r H r + _05 I I l I l I I 0 5O 1 00 1 50 200 250 300 350 400 IUE Detector Counts Figure 2.2 Ratio of IUE to HST flux levels as a function of IUE detector counts for near simultaneous observations of NGC5548 - It is clear that there is an issue with nonlinearity at low count levels, but it is unclear how to accurately characterize this effect in order to correct for it. were measurable above the noise level. Fortunately, Since even the relatively faint emission lines such as He II 1640A are floating atop the continuum, these problems with extremely low raw counts should not be significant in this present study. The IUE data are too important to ignore, but they must be used with care due to both the low resolution and the known linearity issues. The HST data have much higher resolution and are of better quality. Unfortunately, while sampled daily, there is only about one month of data. This is far too short a time period to do significant reverberation mapping. 21 5.35533 55552355? oméwcfitvm E5.>:vw©5? mmhmo5vvm 55$:mmw5? mmhcm5vvm 5:55:33? me. 3553& 5545355? dewwtém 5:55:83? mwémchvvm 5:55:33? 3435va 5555335? m5.5m553& 5555555? 5m.55©53& 5545553? mw.5$5vvm 5:55:535? mm.5mm5vvm H$545535? V5.mm553& 55.25355? mm.M5w5vvm 555353? 3555va 5:55:23? m.mmm5$m 5:55:33? m5.mm55vvm H55>:mm55? 5m.m©©53m 55.25255? 3,2653% 5:55:83? mwdvmtwvm 55_.>:mvm5? uEm55$~m 5545355? 3585va 5:55:33? 5o.©oo5vvm 5555255? 5o.©vm53& 5:55:33? am. 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Telescopes used in the AGN Watch study ranged in size from 1.8m to 6m and a variety of spectrographs were used. A complete listing of all observatories used in the AGN Watch study can be found in the notes for table 1 in Peterson et al. (1991). Because of the range of resolution and quality, only a subset of these observations were chosen for analysis in this work. This subset is listed in table 2.2, and the actual observations used are listed in table 2.3. Both the signal to noise and the spectral coverage varied substantially from site to site. Even within one site’s data, spectral coverage could be different from one set of observations to the next. Because He II was not the line of primary interest in the original project, the optical observations are not entirely ideal for its measurement. Even so, there are some observations with both sufficient signal strength and spectral coverage to make an attempt at a He II measurement. After carefully comparing the merits of the different data sets, we chose to use only the subset of observations that were used in Dietrich et al. (1993), paper IV of the larger AGN Watch study. The file names listed in table 2.3 include a letter designating the observatory site code given in table 2.2. In some cases, more than one observation was taken on a given night at a given site. In this case, not only will there be a letter suffix to denote site, but an alphabetical letter is added to uniquely identify it. If more than one observation was taken from a given site and they were all usable for this study, the multiple observations were averaged together before fitting and a suffix of “x” was added. In the cases where more than one observation was taken on a given night from multiple sites, the spectra were fit individually and the resultant flux measurements were averaged together before the cross correlation function was calculated. 23 7 T l I l I 6 P _ a (D 9 3 4 - q 13 X E 3t ‘ 2 - U . 1 l L l l l 4000 4500 5000 5500 6000 6500 Wavelength (Angstroms) Figure 2.3 Full Typical Optical Spectrum - This optical spectrum was taken from site H, which has fairly good signal to noise and spectral coverage. Not all of the optical observations are of this quality. Table 2.2: Optical Data Site list - This lists the site code as well as the correction scale factor for the optical observations found in table 2.3 as derived in Dietrich et a1. (1993) Data Source Site Code P.A. Aperture Geometry Scale Factor Ohio State CCD A 90° 5.0” x 7.6”, 1.0” x 7.6” 0.953 :l: 0.020 INT 2.5m C 0, 72.1, 75.4 1.5 x 6.0 0.930 i 0.000 Hale 5m D 61, 66.9 1.0 X 40,10 x 7.0 0.918 :l: 0.000 4.0 x 8.0 Ohio State IDS G . . . 0, 7.0 0.953 i 0.020 3m Shane H 59,... ,130 2.1 x 7.9 0.903 t 0.064 Steward I 60, 90, 130 4.5 x 27.2 0.924 :1: 0.053 24 Table 2.2: Optical Data Site List (continued) Data Source Site Code P.A. Aperture Geometry Scale Factor MDM K 0 1.7 x 3.0, 2.4 x 3.0, 0.949 :t 0.014 6.7 x 3.0 Calar Alto M 0,90 1 x 10.0 . . . 4 x 10.0 1.000 i 0.000 1m Nickel N 0 4.6 x 19.2 0.963 :1: 0.000 McDonald CCD J 0,90 8.0 x 9.0, 7.0 x 7.2 1.116 :1: 0.000 McDonald IDS Q 90 4.4 x 4.4 0.920 :t 0.000 2.3.1 CORRECTIONS To THE OPTICAL DATA Unlike the IUE data which were taken from a very stable environment above the Earth’s atmosphere, the optical spectra are affected by transmission changes and smearing due to the atmosphere, and also by instrumental flexure as the telescope tracks the object. Some differences in the instruments used for the optical observa- tions, such as spectrograph slit width, also must be taken into account. The UV data set does not need this sort of correction because the UV observations were all taken with a single instrument. In order to accurately compare the optical data, they first needed to be rescaled and wavelength corrected. Since observing conditions may have changed from one observation to the next, the strong [O III] line at A5007 was used both to scale the flux and to fix any minor wavelength calibration issues that might exist. Because [O III] is formed in the narrow line region, it should remain insensitive to the short time scale variations that are powering the BELR variations and should therefore have the same flux in all observations. This rescaling was accomplished by manually measuring the flux and the central peak using IRAF. All observations were then sealed 25 such that their slit width corrected flux (see discussion below) for [O III] A5007 was the canonical value of 5.53 X 10‘13 erg/s/cm2. This value is taken from Dietrich et a1. (1993) 26 gungm £5fi8©§>£ ammwfivm :Emkgwmwnmm mmmuvvm 55. “magma mvgvrvm :Emdmvtmw omcugm 55.636an ommnvnvm zanmécmwmhmm wmhnvvm sawfiwmmpnmm 83.3% :Emkdmmwhmm wwmhfltm Jahanwmhfi omtvvm :Emkdmgumm ovchvvm :Eaxfivcnfi vwmbgm nfimdmwmnmu mfitgm :Emhcgnmm #3:.va sawsmvwna Rmnvvm :Ewfimnmnfl wmtvvm 5.5.5.2an Stwvm €5.me :53 mmwnvvm 55.53an mhmnvvm £34553th owtgm 5365:an motvvm :Eivotmm mmcbvvm sfimkgwmwhfi mnmuvvm :Emfiafimnmm $.5va 55%mean wounvvm :Eafimotmm amazes—um :Efivmmwnfl hmmbgm £5233an mtgvm 55.53.5an Nowhvvm €538th Hmmnwvm nEwfimoumm ommnvvm :Em.8avmn& wtnvvm 5.5.3:th 3:.va :Emdfiothm omwnwvm 55583.3 uvmngm 53.8333 wwtvvm favxfiotmw Stvvm 55.22:.an Scngm :Emsfiwfiwnfi ovmnvrvm sawammmnfl nwhnwvm £22.23th cowhwvm €5.38an 2.3.va :Emhgcumm cmmfimvm €5.33th nonnvvm 22:253.an vownvvm 556.835an 33.3% :Ewfimfiwnfi mmmngm :Em.x€mmn& wmtvvm 55.8%th mcwhfltm :Eagmwopmw caesium :Emkxommhmta ofimnvvm :Efiosmomhfi 3:.va €5.53.an ~8qu £5253an mammvvm :Eaammmhmm ofimnvvm :Efixnmombmm OH. 23 Q5 BE DH. 23 OH. 2E .mzoficiomno m5 m0 85¢ :33”. 23 fig mac? 35% £5 E com: 895 fix? msosciomno 10.2qu :5 mo pm: 5 mm “335:8 53H. - San— EQBQO ”Wm 24$. 27 There are a variety of challenges associated with using the ground based optical data. Due to variations in observing conditions, the absolute flux level from one observation to the next must be corrected. A strong narrow line such as [O III] A5007 can be used as a reference standard to scale the data. Since the narrow emission line region is both large in size and at a large distance from the central source, it is relatively insensitive to continuum variations on short time scales (Peterson, 1993). We can therefore assume that the flux in [O III] A5007 remains constant over the time it was observed. Furthermore, we can use this line to correct for any errors in the wavelength calibration since its peak wavelength should also not change. Unfortunately, the NELR that generates [O III] narrow lines is of a large enough angular size that the amount of flux recorded is influenced by the slit width of the spectrograph used. This means that in order to make any meaningful comparison between the various ground based sites, a correction for slit width must be made to the narrow lines. Dietrich et al. (1993) went to great lengths to determine these correction factors for slit width, and we use their corrections here (as listed in the last column of table 2.2) before scaling the observed spectra to have the same flux in [O III]. Another important limitation of the optical data is the same as that of the IUE data, poor spectral resolution. Even the best of the ground based optical data only have a resolution of about 2 A per pixel. Some of the lower resolution data from the AGN watch archive were not suitable for accurately measuring the He II emission. 2.3.2 OBSERVED vs. REST FRAME While N GC 5548 is relatively nearby, it is still far enough away that cosmological redshift needs to be taken into account. It lies at a redshift of z = 0.017175 (de Vaucouleurs et al., 1991) which needs to be corrected for in order to put the emission into a rest frame. To accomplish this, the IRAF task “dopcor” was utilized. 28 CHAPTER 3: DATA FITTING AND MEASUREMENT Previous reverberation measurements (e.g. Clavel et al., 1991; Dietrich et al., 1993, and many others) directly Show that the emission lines from different ions respond to continuum variations on different timescales. This is interpreted to mean that the BELR covers a wide range of distances from the continuum source, and lines coming from different ionization states are formed at different radial distances. Since the He II lines are spectroscopically observed in blended features that also include emission lines from ions of very different ionization levels (e.g. Fe+, H+, C+3, and 0+2), it is crucial to be able to accurately measure the flux in each of the separate components in the blend in order to accurately measure the He II reverberation time scale. Some previous studies simply defined continuum-subtracted wavelength ranges, or bins, for each emission line. While this method is a decent approximation, it does not accurately measure contamination from adjacent emission lines. In order to more accurately fit these highly blended varying emission lines, new software tools as well as reasonable approximations of the individual emission line profiles were needed. 3.1 AUTOMATED FITTING ROUTINE The central difficulty to be overcome is the de-blending of spectral lines in the data. The emission spectrum of an AGN is a very complicated structure since there can be multiple overlapping emission lines that vary with slightly different lagtimes. To accurately de-blend these components, one must fit each component individually at 29 every time slice. This can be done by hand, but it would be both time consuming and very subjective; a fit by hand does not give a true “best fit” to the observations but instead is simply what the observer feels looks best. An automated fit using some mathematical technique would be efficient, consistent, and give at least some measure of the quality of the fit. To create the automated fitting routine, it was important to build upon existing software tools used by my advisor and his collaborators. This would both make the automated fitting routine useful for purposes other than this study and allow easy use of previous measurements of N GC 5548 as a starting point. This should help to explain the sometimes cryptic nature of the input parameters as well as the choice to use FORTRAN. 6.5 I I I I [o m] 6 - 5007 l l 5.5 5 - _ 4.5 *- - 4 r [0 III] _ HB 4959 3.5 ~ Hy + [0 III] 4363 He II 4686 3 .- Flux (1 e-14 ergs/cm/s) He | 4471 l 1.5 - - Fell 1 l J l l 4200 4400 4600 4800 5000 5200 Wavelength (Angstroms) 2.5 2 Figure 3.1 Typical Optical Spectrum of NGC 5548 - This spectrum, from the file n57621h, is a detail region of a typical N GO 5548 optical spectrum. Several of the important emission lines are labeled. 30 Figure 3.1 shows a portion of a typical optical spectrum for N GC 5548. While the primary feature of interest for this study is the A4686 He II line, it is surrounded by contamination from other lines in the blend. The wing of the Balmer H5 line as well as a Fe II multiplet and the He 11 line from the NELR all contribute to the measured flux around A4686. There is also contamination from the He I A4471 as discussed by Vestergaard & Peterson (2005). To properly measure the flux from the He II A4686 line, all of the other contaminating lines must be fitted as well. In addition, the underlying continuum component, which is also variable, was included in the fit. 30 I I I I I I I C|V1549 25 — ~ ‘3” 20 - - Q U) 9 Q) 15 - ~ 3' NW] om 1663 g 1486 l :5: 1O _ He“ _, n- 1640 5 ’ a 0 l l l l l l l 1400 1 450 1 500 1550 1600 1 650 1 700 1750 1800 Wavelength (Angstroms) Figure 3.2 Typical UV Spectrum of N GO 5548 - This spectrum, from the file r57530uv, is a detail region of a typical NGC 5548 UV spectrum. Several of the important emission lines are labeled. At its core, the automated fitting routine (hereafter AFR) uses a X2 minimization technique to fine tune an initial guess at the fit. The goodness-of-fit statistic X2 is 31 basically defined as: x2 = Zn: (fig—liy (3.1) 2:1 where A and B are the data spectrum and the blend built from profiles, respectively. The a is some measure of the noise in the data spectrum. In the UV, since all of the spectra were taken with the same instrument, this value was simply estimated by looking at a relatively flat, uncontaminated area, and a value of 0.5 X 10‘14 w chosen. Because the optical data come from different instruments, this value was estimated in the program by applying a boxcar smoothing to the data in the fitting windows and seeing how far the observed values deviated from this smoothed version. This value would stay the same for any individual spectrum that was fit, so the actual value of a is not all that important. 3.2 BUILDING THE BLEND The continuum radiation was modeled as a simple power law: A ’7 t' = A —— .2 Con lnuum ( 5600) (3 ) with both A and ’y allowed to vary within reasonable ranges. While the exact form of the continuum emission from the central engine is not known, it is thought to be a superposition of blackbody spectra of nearly continuously varying temperatures coming from an accretion disk undergoing viscous heating. A power law does not precisely describe this situation mathematically, but it is a reasonable approximation for the relatively small wavelength ranges at which we are looking and is a good deal more accurate than a simple straight line approximation which is often used. To make the fit as realistic as possible, template profiles of the emission lines are used whenever practical. In some previous studies (such as Dietrich et al., 1993), emission lines, including both those arising from the NELR and BELR, were measured 32 in a simpler way. If the emission line was deemed to be weak, a wavelength range (or bin) was defined for that line, and all flux within that range that was above the continuum level (which was itself defined as a straight—line under the bin) was assumed to be from the emission line in question. Contamination from adjacent lines was either ignored or simply estimated. If the line was deemed to be strong, it was sometimes modeled using a superposition of several Gaussians. If the shape was complicated, more Gaussians were added until a shape that more closely approximated the true shape of the actual emission line was created. Neither of these techniques truly described the physical situation, and a new approach was needed. To this end, it was decided to use observed spectra from actual AGN to create template profiles that more accurately reflected the shape of the real observed emission lines. 3.2.1 FE II TEMPLATE First noted by Phillips (1977, 1978), the Fe II multiplet of Seyfert 1 galaxies are very similar from object to object. The primary difference between any two given Seyfert 1 AGN is the amount of Doppler broadening. With this fact in mind, the Seyfert 1 galaxy I Zw 1 has long been of interest in order to generate a template profile for the Fe II emission in AGN. I Zw 1 has quite narrow broad lines such as Fe II and H13, some of the narrowest permitted lines observed in any Seyfert 1 galaxy. Phillips demonstrated that if the Fe II profile was Doppler broadened to match the width of H6, the I ZW 1 template fit other Seyfert 1 galaxies well. Véron-Cetty et al. (2004) have created a revised and improved version of the I Zw 1 optical template (see figure 3.3.) Vestergaard & Wilkes (2001) created a similar template for the UV emission from data of I Zw 1 taken with HST (see figure 3.4) Vestergaard & Peterson (2005) used these templates successfully to study the Fe II emission in NGC 5548 using the same AGN watch data set used in this study. Although their focus was not He II, they did find that the use of these templates improved the measurement of other 33 3 I I I I I I I I 2.5 - '- «2‘ ‘i' 2 E o '1- a: a) 4 _ 5 1.5 to l o C x 1 I 2 LL 0.5 ., l l L l l l l 0 3500 4000 4500 5000 5500 5000 6500 7000 7500 8000 Wavelength (A) Figure 3.3 Fe II Optical Template - based on a broadened version of the Véron-Cetty et al. (2004) optical template. emission lines in the regions contaminated by Fe II. This present work is the obvious extension of that idea, using all of the template profiles at our disposal to disentangle these multiple overlapping lines. M. Vestergaard kindly provided us with a set of Fe II template profiles with a variety of different Doppler broadenings. We used the same templates that were used by Vestergaard & Peterson (2005) in their study of NGC 5548 using a Doppler broadening of 6250 km/ sec, similar to the broadening used in their paper and which seemed to give a good fit to the N GO 5548 data. The templates shown in figures 3.3 and 3.4 show this particular broadening. 34 Flux (10’15 erg s‘1 cm’2 A) 0) 2 - _ 1 _ -1 O " 1 _1 1 1 1 l 1000 1500 2000 2500 3000 3500 Wavelength (A) Figure 3.4 Fe II UV Template - based on a broadened version of the Vestergaard & Wilkes (2001) UV template. 3.2.2 OTHER TEMPLATES The creation of the remainder of the template profiles was a rather subjective proce- dure. As a starting point, a RMS (root mean squared) spectrum was created from scaled and shifted data spectra (see figures 3.5 and 3.6.) In this way, we are left with primarily the highly variable BELR emission with the nearly constant NELR emission removed. From there, some smoothing was typically applied, and any sharp upturns remaining on the edges were clipped to try to leave a smoothly changing template profile. As this was a fairly individualized process, tabular copies of the templates used are included in appendix F of this work. The strong Ha line, seen on the right side of figure 3.5, was used to create a template for the Hydrogen Balmer lines (see figure 3.7.) The Balmer lines should all 35 5 - Hell . . . Hel Flux (1 e-15 ergs/cm/s) 1 l l l l l 4500 5000 5500 6000 6500 7000 Wavelength (Angstroms) Figure 3.5 Optical RMS Spectrum - Most of the non-variable N ELR lines have been eliminated, leaving only the highly variable BELR emission. This can be used to construct template profiles for the BELR emission that give a better approximation of the true shape of the emission lines. The regions used for the creation of the Ha, He I, and He II BELR templates are marked. have similar profile shapes because they are all emitted by the same gas. This profile can therefore be rescaled to fit H5 or H7 as needed. The He II A4686 emission from the RMS spectrum was used to generate a BELR He II template (see figure 3.8) for fitting both the A4686 and the A1640 broad line components. This is important to note, as we have used the exact same template profile to fit both the UV and the optical He II emission lines. Because we expect the emission to originate from the same region within the AGN, they should have the same shape. The relatively strong He I line at A5876 was used to create a He I template (see figure 3.9) for fitting the He I line at A4471, a line which contaminates the 36 30 I I I I I I I 25— « g 20- ~ Q U) 9 315— - :53 310- 4 LL 5&- -1 MM cw 0 1 1 00 1 200 1 300 1400 1 500 1 600 1 700 1 800 1 900 Wavelength (Angstroms) Figure 3.6 UV RMS Spectrum - This is a detail of the region of interest of the UV rms Spectrum. Most of the non-variable N ELR lines have been eliminated, leaving only the highly variable BELR emission. Like the optical rms spectrum, this can be used to construct template profiles for the BELR emission. The C IV BELR profile was created from the strong emission labeled in this spectrum. measurement of the He II A4686 emission line. Templates for O III] A1663 and the narrow line component of the He 11 A1640 blend were created in a similar manner, but an average rather than an RMS spectrum was used as the starting point. In an average spectrum, the NELR emission is not removed as in a RMS spectrum. These lines could also have been described with a simple Gaussian or blend of Gaussians of appropriate widths, if necessary. 37 5 l I ‘r l 4.5 - 4 4 — 4 .2 6'. 3.5 ~ ~ 6 .— 3 - - '(D a: _ a a.) 2.5 :2 'o 2 - 1 g 1.5 - - u. 1 r- — 0.5 — _ o l 1 l l l 1 l 6200 6300 6400 6500 6600 6700 6800 6900 7000 Wavelength (A) Figure 3.7 Ha BELR Profile 3.2.3 FITTING PARAMETERS Each profile will be Shifted in wavelength (actually in velocity) and scaled by a mul- tiplicative factor as part of the automated fitting procedure. So for each template profile that is to be used, we have to specify an original wavelength (Aold), a Shifted wavelength (Anew), and a multiplicative constant. These parameters are given in the input file, called “blendsin”. While three parameters are input, only the last two are allowed to vary. In the input files for the fitter, the multiplicative constant is actually expressed as a fraction of the total flux in the data spectrum. For example, if a particular emission line carries 10% of the total flux, the scaling factor would be 0.1. This method is used in order to mirror more accurately the input files used in the program Blends. (Blends, and several of the subroutines called by the automated fitter were developed by Dr. Jack Baldwin. They are used extensively by both him 38 4 I I I I *I I I I 3.5- _ A 3~ _ .< “1' 25A - E a 0 '0, 2— . 9 Q) 59 1.5- - '52 2: 1~ - 3 i: 0.5- . 0,. - _05 I 1 J I I I I I .4450 4500 4550 4600 4650 4700 4750 4800 4850 4900 Wavelength (A) Figure 3.8 He II BELR Profile and his collaborators, the author of this work included.) The only significant change of which to be aware is that Blends assumes that the continuum shape has been removed and that there is only an additive constant still included. This constant continuum level is one part of the input file for Blends but is ignored by the AFR which instead fits a power-law continuum described by equation 3.2. It still must be included in the input file to preserve formatting. The long term aim here was to produce a general-purpose automated fitting program for which the starting values could be determined by using the existing Blends program to fit by hand, and then the Blends output file would be used as the input file for the automated fitter. 39 4.5 I I I I I I I 4 _ ., 115 - 4 .2 <1 3 “ ‘ E v- 2.5 _ i '0) a - fl 5 2 :2 '9 1.5 — - E 1 ~ ‘ E 0.5 r _ O ._ .. _05 I I I I I I I ' 5700 5750 5800 5850 5900 5950 6000 6050 6100 Wavelength (A) Figure 3.9 He I BELR Profile 3.2.4 FITTING WINDOWS While the spectra we are using may cover several thousand angstroms, we are typically only interested in a small region around an interesting emission feature. In addition, we may need to fit in areas away from the line of interest in order to constrain the Shape of the continuum. There also may be regions of the spectrum where we do not have a profile for the emission that is present. To accommodate these facts better, the AFR does not calculate contributions to X2 everywhere in the spectrum but rather only in certain regions of interest specified by an input file to define fitting windows. These windows are defined by a pair of wavelengths, a starting and an ending wavelength, with one pair per line in an input file called “window.txt”. In choosing what regions to fit, it is important to fit only in areas where we have profiles or where there is little or no emission (just continuum.) During testing, it was 40 25 I I I I I 20- . 4? ‘1‘ E P 15— - IUD 9 a) to To 10~ - 3‘? 2 LL 5- - O 1 I I I l 1480 1500 1520 1540 1560 1580 1600 Wavelength (A) Figure 3.10 C IV BELR Profile found that the NELR lines can cause difficulties with the automated fitter in some circumstances. The width of the NELR lines is directly tied to the width of the Slit used on the spectrographic detector. Because a number of different instruments with different Slit widths were used for the optical observations, a single template narrow line cannot be used for all of the optical spectra. Since these lines are formed far away from the continuum source, they are essentially constant in flux over the timescales we are investigating. Therefore, fitting them in each individual spectrum is unnecessary, and when possible only the wings corresponding to the broad line emission are fit. In this way we are fitting only the broad edges of the emission line, which is, after all, the part in which we are interested. This is fairly easily accomplished with the optical spectra, where the NELR lines create well defined spikes, typically near the center of the BELR line. The UV spectra, taken with IUE, don’t suffer from the changes in 41 NELR line width that affect the optical data. In this case, the He II narrow line is included in the fit, but kept constant. This is possible because the UV observations were all taken with the same slit width on the same instrument. The actual values used for the fitting windows are listed in table 3.1. In addition to these values, the file “window.txt” requires a number at the end of each line specifying the amount of smoothing to apply when estimating the amount of noise in the fitting region for the purposes of calculating X2- This number must be odd, and a value of 7 was chosen for the optical data. This value is ignored in the actual fit of the UV data (the amount of noise is hardcoded in) but a value must still be present in the input file. Table 3.1: Fitting Windows - This table summarizes the windows in which X2 is actually calculated. It is important to fit only in regions where we have template profiles or where very little emission is contaminating the continuum. In the case of the optical data, we have left gaps where the N ELR emission occurs. Optical (A) UV (A) 4225 - 4275 1425 - 1730 4420 - 4676 1798 - 1836 4703 - 4838 2400 - 2650 5100 - 5170 5650 - 5700 3.3 MINUIT AND THE X2 MINIMIZATION Once all of the physical corrections have been made to the observations and template profiles as well as fitting windows have been chosen, the automated fitter takes over. 42 At its heart is a function minimization package called Minuit. Minuit was developed at CERN, the European Organization for Nuclear Research as a generalized multi- parameter function minimizer for use with FORTRAN programs. It is important to understand that Minuit is only a function minimizer; it does not know any of the actual details of the function it is minimizing. It is up to the end user to come up with a useful quantity to calculate as well as some way of bringing all of the pieces together. To that end, a fitter for the optical data called “contin.f” (see appendix A) and a fitter for the UV data called “uvcontin.f” (see appendix C) were developed. These each call the Minuit package as a subroutine. As mentioned in the previous section, we are able to fit the NELR emission in the UV Spectra. Because of this, there are some additional constraints that we can put on some of the components of the blend. It is for this reason that there are two different versions of the fitting code. A more elegant generalized version of this fitter should be able to handle both the optical and the UV, and will hopefully be the subject of a future project. When the fitter is run, it takes a list of input spectra from the file “sorted2.in”. The spectra listed in this file must be in IRAF “.imh” format because of some of the subroutines that are used to read in the data. The fitter also takes the list of input template spectra and the windows it should use for calculating X2 (“blends.in” and “window.txt”, discussed in the two previous sections.) If no fitting windows are Specified, the fitter assumes that it should attempt to fit the entire spectrum. The initial guesses at the parameter values for each of the template profiles, as well as any constraints on those parameters, are then put into arrays that are passed to Minuit. Minuit in turn passes these values to a user-supplied subroutine where the function to be minimized is calculated. In the case of the optical data this subroutine is called “cfunk” (see appendix B) and in the case of the UV data this subroutine is called “uvfunk” (see appendix D). The subroutine then takes the parameters it is given and creates the blended 43 2.2 I l 17 l I I I I 2 ~ 4 0'1" 1.8 - — E 0 'w 16 - - 9 m ,‘I é L4 - I u E d u E 12 .— ,1 W -" —l l“ H 3.... m . _ " WV“ 1 l | l | l Continuum He l Fe II He II HB 0'8 1 I l I I I I I I 4300 4400 4500 4600 4700 4800 4900 5000 5100 5200 Wavelength (A) Figure 3.11 Optical Fit Overlay - This figure shows the relative postions of the various components in the optical blend. spectrum. This is done by first shifting the emission line template spectrum in velocity space using a subroutine adapted from Blends. This “align” routine (as well as several other useful subroutines) can be found in the listing for “functionsf” (see appendix E) The routine from Blends was used because it has been well debugged and properly handles the complexities of flux correction when moving a spectrum in velocity space. The template is then scaled by some factor. This is done for each of the templates in the “blends.in” list. A power-law continuum as described by equation 3.2 is then added together with each of the shifted and scaled templates to create the final blended spectrum. (Examples can be seen in figures 3.11 and 3.12 for the optical and UV spectra respectively. Compiled lists of templates used can be found in tables 3.2 and 3.3 at the end of this chapter.) The subroutine then compares the blended 44 Spectrum’s flux value in each wavelength bin with that of the input spectrum that is being fit (from “sorted2.in”) to calculate X2. This X2 value is then handed back to Minuit. Minuit makes adjustments to the parameters and then calls the subroutine again. This process is repeated until the value being returned is no longer changing by more than some threshold value or until a set number of iterations has been reached (each of these parameters are set by the user in the main fitting program code.) The values used to create the blend are then used to calculate the flux in the template profiles as well as in the power-law continuum. These are then output into a file called, for historical reasons, “slope.txt”. This entire process is then repeated with the next input spectrum from “sorted2.in.” 7 I I I I 6.5 - - A 6 - _ ‘1' E ,° 5.5 - _ I”) ‘ ‘ 9 5 1 OJ ' _ II. l ' Sb llkl“ l J l 5 5. . , my - x l 41.. m - I ll:- Continuum ‘A "h! L. 4 - ClV NELR Fell ‘w-ll — om] Hell NELR 35 _ C IV BELR He H BELR NELR Olll]BELR 3 I I I I 1450 1500 1550 1600 1650 1700 Wavelength (A) Figure 3.12 UV Fit Overlay — This figure shows the relative positions of the various components in the UV blend. Unfortunately, due to the nuances of data output when using FORTRAN, the 45 output file “slope.txt” is in a less than optimal state. Some simple manipulation with, for instance, a Perl script can put this into a more user friendly format that can be used for further analysis. In the end, we need separate light curve files that contain the Julian date and flux in each emission line, as well as one for the continuum. The Julian dates for each observation was deduced using a combination of IRAF header information, file naming convention, and records from previous papers, in particular Dietrich et al. (1993). These dates are listed in tables 2.1 and 2.3. Some constraints were put on Minuit to keep results within physically plausible ranges. The wavelength Anew was only allowed to shift by :l:7A, and the initial guess for the flux in a line was only allowed to vary within a range of 0.1x and fix of the original value. This was done because it was found that weak lines in a noisy spectrum could easily be driven to zero or pushed well away from their intended area of fitting. In addition, the NELR lines in the UV fit were held to constant values. These values were chosen by first allowing them to change flux during a fit and seeing what the median value was. This value was tweaked as necessary to allow for a good fit and then hardcoded into the “uvcontin.f” and “uvfunkf” files. Because of the higher quality overall of the calibration of the UV data, certain lines were also tied together in Anew to prevent Minuit from driving them outside of physical ranges. The lack of resolution and quality of the calibration prevented this in the optical data, so only the wavelength range was constrained. 3.4 ERROR ESTIMATION Minuit determines statistical errors during its fitting such that a change in a parameter that results in an increase of 1 in the reduced X2 is treated as a 10 change in the parameter. We have made several design choices that make this value given by Minuit a bit suspect. In order both to make the final emission lines physically reasonable and to ensure that the fitter does not accidentally set a weak line to zero to generate 46 a fit that looks better, we had to constrain the range of values that most parameters could have. This is something that is not recommended by the author of the Minuit Reference Manual, as it can lead to strange results for the error bars. It was, however, necessary to ensure that weak lines such as He II 4686 would not be lost when fitting these low resolution, noisy spectra. The error in any given parameter is also influenced by how well the fitter has done at fitting the adjacent blended lines. These factors seem to help explain the perplexing error values that we found from Minuit. The error bars on the parameters, when used to recalculate the integrated flux in any given line can be orders of magnitude smaller or larger than the calculated flux. This renders these error bars essentially useless for the purposes of further calculation. Due to these issues, the error in the flux needs to be estimated. AS will be shown in the discussion of the cross correlation function, the final reverberation lag is insensitive to modest changes in the size of the error bars, so it is only important that we arrive at a value that is reasonable physically when we take into account the quality of the source data. Given these facts, we have chosen to take as an error 10% of the median flux value for the line in question. This error is Similar to the value estimated by Dietrich et al. (1993). 3.5 CROSS-CORRELATION LAGS As was well illustrated by Kaspi (2007), Blandford & McKee (1982) first gave a name to and laid down the mathematical foundation that governs the technique of “reverberation mapping”. The response of an emission line to continuum variations is described by L(v,t) = /w(’v,t — T)C(T)d7' (3.3) 47 Here, C(T) is the continuum light curve and L(v, t) is the responding emission—line light curve. The quantity 20(1), T) is defined as the transfer function; that is to say the function that governs the response of the emission-line, where v is the velocity field of the BELR that causes the shape and broadening of the BELR emission. This definition assumes that the object is able to be studied in great detail, with very well sampled light curves over a long period of time. In practice this is not typically the case. When using low resolution spectra and poorly sampled light curves, the two- dimensional transfer function, 211(0, 7') collapses to a one-dimensional transfer function, 1,0(7'). This is expressed as the Single parameter of the time lag between the continuum light curve and the emission-line light curve. FCCFU) = macaw-1: C(t)L(t + T) (3.4) t Here, N is the number of points in the sum, 7' is the lag, and ac and 0L are the rms of the light curves of the continuum and the emission-line respectively. The centroid of this CCF is the measure of the Size of the BELR, and is often denoted as RBLR 01’ RBELR- Using this formalism, we are able to calculate a characteristic radius at which an emission-line is produced by creating a light curve for both the continuum and the emission-line in question and calculating a cross-correlation function for them. Dr. Brad Peterson and his collaborators have developed software to compute this CCF in a standardized way. There are two basic techniques, discussed in detail in Peterson (1993). In the first method, the light curves for the actual observations are interpolated into a smoothly varying function. This function is then sampled regularly to calculate the cross-correlation function. In the second method, called the discrete correlation function (DCF) method, only real data points are used without any interpolation. The DCF method has some further subtleties such as exclusion of points that have a zero time difference, that is to say it will not attempt to correlate 48 ill 1 ll ill) i :25 l i ll 1 ill 3 5jl ( (l lllHlll - O L I I I I I I I I 7500 7530 7560 7590 7620 7650 7680 7710 7740 7770 7800 Julian Date (—2440000) Figure 3.13 UV Continuum Light Curve — as measured at A1337, as per Dietrich et al. (1993) points taken from the same spectrum. This is done to eliminate artificial “lags” at very short times due to correlated errors in the measurement or calibration of the spectrum. Unfortunately, as noted by Peterson (1993), the DCF method does not work well when used on poorly sampled data. Because the data used in this study does not have a sampling rate that is much smaller than the expected reverberation lag time, we used only the interpolated method. The implementation of the interpolated method was kindly provided to us by B. Peterson in the form of code called lager'rb'. This code allows for the comparison of two light curves for the purposes of calculating a cross correlation centroid distribution (CCCD) The interpolation method makes repeated attempts at the creation of a cross correlation function. On each attempt lagerrb’ first performs a flux randomization of 49 Flux (1 e-13 ergs/cm/s) (.0 __1 1 I 1 1 I I l I I 7500 7530 7560 7590 7620 7650 7680 7710 7740 7770 7800 Julian Date (-2440000) Figure 3.14 Optical He II Light Curve the input data curve. It allows each point to have a value within the error bars provided with a Gaussian weighting before creating a smooth interpolation between the points and taking a subsample for the purposes of calculating the cross correlation function. If the CCF is above some target threshold, in this case a CCF factor of 0.4, it is deemed a “good” CCF. For a CCF that is “good”, a centroid of the distribution above some cutoff value is determined. In the case of this study, the threshold is set at 0.8 of the maximum. An alternate method is to simply look at the peak value of the CCF rather than the centroid of the CCF. The relative merits of the two methods has long been debated, however, B. Peterson has a preference for the centroid method (see the discussion in the appendix of Peterson et al., 2004). Therefore we constrain our discussion here to the centroid method. The process above is then repeated some large number of times, in this case 50 14 I I I I I I I I I 55- l Flux (1 e—1 3 ergs/cm/s) 0') l—+—1 t—-+—