Improving Damped Random Walk parameters for SDSS Stripe 82 Quasars with Pan-STARRS1

TL;DR

This study extends quasar light curves with PS1 data to improve damped random walk model parameter estimates, revealing stronger correlations with black hole mass and aiding in identifying changing-look quasars.

Contribution

It combines SDSS and PS1 data to extend light curves, enhancing DRW parameter constraints and introducing a new method for discovering changing-look quasars.

Findings

Longer baseline improves DRW parameter constraints.

Variability amplitude correlates more strongly with black hole mass.

Identifies 40 candidate changing-look quasars with significant brightness changes.

Abstract

We use the Panoramic Survey Telescope and Rapid Response System 1 Survey (Pan-STARRS1, PS1) data to extend the Sloan Digital Sky Survey (SDSS) Stripe 82 quasar light curves. Combining PS1 and SDSS light curves provides a 15 yr baseline for 9248 quasars - 5 yr longer than prior studies that used only SDSS. We fit the light curves with the damped random walk (DRW) model model - a statistical description of their variability. We correlate the resulting DRW model parameters: asymptotic variability amplitude SF$_{\infty}$, and characteristic timescale $\tau$, with quasar physical properties - black hole mass, bolometric luminosity, and redshift. Using simulated light curves, we find that a longer baseline allows us to better constrain the DRW parameters. After adding PS1 data, the variability amplitude is a stronger function of the black hole mass, and has a weaker dependence on quasar luminosity. In addition, the characteristic timescale $\tau$ dependence on quasar luminosity is marginally weaker. We also make predictions for the fidelity of DRW model parameter retrieval when light curves will be further extended with Zwicky Transient Facility (ZTF) and the Rubin Observatory Legacy Survey of Space and Time (LSST) data. Finally, we show how updated DRW parameters offer an independent method of discovering changing-look quasar candidates (CLQSOs). The candidates are outliers in terms of differences in magnitude and scatter between SDSS and PS1 segments. We identify 40 objects (35 newly reported) with tenfold increase in variability timescale between SDSS and SDSS--PS1 data, which is due to a large change in brightness (over 0.5 mag) - characteristic for CLQSOs.