THE GALACTIC NOVA RATE : ESTIMATES FROM ALL-SKY TIME DOMAIN SURVEYS

There is a significant discrepancy between recent predictions of the Milky Way classical novarate of ∼ 60 per year and the annual discovery rate of ∼ 10. Why the recovery fraction of these events is ∼ 15% even with large advancements to observational time domain astronomy remains largely unexplained. Because of the location of Earth within the Galaxy, discovering Galactic transients requires a large field of view. For the past few decades, observations from a network of amateur astronomers were largely responsible for discovering classical novae, so the sky-coverage as a function of position and depth was difficult to model. Fortunately, in the past decade, many time domain surveys with fields of view that cover large areas of the sky have been commissioned, making the sky-coverage more well defined. To date, there are no Galactic nova rate predictions made using data from a time domain survey that is capable of observing the entire sky. In this thesis, the first estimate of the Galactic nova rate using observations from two all-sky surveys is made. The All-Sky Automated Survey for Supernovae (ASAS-SN) is the first survey to systematically observe the entire sky every night, providing unprecedented cadence of the sky for transients and variable stars. The space-based survey Gaia has a broad observing filter and a fine (∼ 0.1 arcsecond) pixel scale, so it is capable of detecting Galactic plane transients in crowded fields that are heavily affected by extinction. These are the only two all-sky surveys to report classical nova candidates, and they have contributed to marginally increasing the discovery rate of Galactic novae to 13 per year on average since 2017. In addition to the increase in discoveries, this thesis exploits the systematic observing patterns to estimate what fraction of the Galaxy’s novae these surveys detect. To make this estimate, I have constructed a statistical model of Galactic classical novae by utilizing the recently published models of stellar density and extinction of the Milky Way. Using ASAS-SN photometry, I measure the outburst amplitude of novae to be normally distributed with mean and standard deviation μ = 11.43 ± 0.25 mag and σ = 2.57 ± 0.20 mag, respectively. By using recently available all-sky 3D dust maps, I estimate that ∼ 50% of Galactic novae are hidden by extinction from being detected by ASAS-SN. Finally, I estimate that the recovery fraction of the global population of Galactic novae for ASAS-SN ≈ 33%, Gaia ≈ 42%, and a joint effort of the two surveys ≈ 54%, predicting that the Galactic nova rate is 26 ± 5 yr−1, significantly lower than recent estimates.