Quasar Variability as seen by Large Optical Sky Surveys

Abstract

Quasars are powered by accretion of hot gas onto a growing, supermassive black hole, at cosmological distances. Such an active galactic nucleus (AGN) in the heart of distant galaxies is a source of intense radiation spanning the electromagnetic spectrum. Light passing through intervening intergalactic medium allows studies of He II reionization history and Damped Lyman $\alpha$ Absorbers. Counting quasars across cosmic time and luminosity (Quasar Luminosity Function) help relate their evolution to galactic build-up of stellar mass. AGN are also laboratories for high-energy astrophysics studying accretion phenomena, and general relativity - only this year we witnessed the first direct observation of the relativistic shadow cast by the supermassive black hole of M87. Characteristics of quasar stochastic variability, such as observed timescales, and variability amplitude, are relevant to the physical properties of quasars: the black hole mass, bolometric luminosity, or Eddington ratio. In the era of large optical sky surveys (such as SDSS, CRTS, PTF, ZTF, PS1, LSST), an increasing availability of AGN time series makes it possible to combine these diverse sources of measurement. SDSS survey data broadly supported variability being consistent with the Damped Random Walk model. Newer CRTS data seemed to indicate an enhanced variability on monthly timescales, but in this dissertation I show that this was due to underestimated photometric uncertainties of CRTS. Having shown that DRW is an appropriate description of quasar variability for timescales ranging from months to years, with the currently available data I proceed to combine the SDSS and PS1 data. I provide better constraints on the underlying DRW parameters and update the coefficients correlating the variability parameters to quasar physical properties. I also make prediction about the improvement that will be afforded with the arrival of the LSST. Compared to its predecessors, LSST will have a larger etendue (300 m$^{2}$ deg$^{2}$) and will survey the sky to a new depth. For example, the SDSS etendue was only 5.9 m$^{2}$ deg$^{2}$. At the single-visit depth of 24.5 mag (r-band), crowdedness will become an issue even at moderate galactic latitudes. In preparation for LSST, I address the source of systematics in photometry resulting from the use of the LSST science pipelines, by reprocessing the DECAPS data. I show that with moderate software improvement it will be possible to fulfill the design requirements, and process the 18 000 deg$^{2}$ of the night sky.