The ensemble photometric variability of over $10^5$ quasars in the Dark Energy Camera Legacy Survey and the Sloan Digital Sky Survey

TL;DR

This study analyzes the long-term photometric variability of over 100,000 quasars using data spanning 15 years from DECaLS and SDSS, revealing correlations with physical properties and supporting accretion disk instability models.

Contribution

It provides a comprehensive ensemble variability analysis of quasars over a 15-year baseline, exploring how variability relates to multiple quasar properties with a large dataset.

Findings

Variability amplitude positively correlates with redshift.

Variability amplitude negatively correlates with bolometric luminosity.

A significant negative correlation exists between variability and Eddington ratio.

Abstract

We present the ensemble variability analysis results of quasars using the Dark Energy Camera Legacy Survey (DECaLS) and the Sloan Digital Sky Survey (SDSS) quasar catalogs. Our dataset includes 119,305 quasars with redshifts up to 4.89. Combining the two datasets provides a 15-year baseline and permits analysis of the long timescale variability. Adopting a power-law form for the variability structure function, $V=A(t/1yr)^{\gamma}$, we use the multi-dimensional parametric fitting to explore the relationships between the quasar variability amplitude and a wide variety of quasar properties, including redshift (positive), bolometric luminosity (negative), rest-frame wavelength (negative), and black hole mass (uncertain). We also find that $\gamma$ can be also expressed as a function of redshift (negative), bolometric luminosity (positive), rest-frame wavelength (positive), and black hole mass (positive). Tests of the fitting significance with the bootstrap method show that, even with such a large quasar sample, some correlations are marginally significant. The typical value of $\gamma$ for the entire dataset is $\gtrsim 0.25$, consistent with the results in previous studies on both the quasar ensemble variability and the structure function. A significantly negative correlation between the variability amplitude and the Eddington ratio is found, which may be explained as an effect of accretion disk instability.