The X-Ray Variability of a Large, Serendipitous Sample of Spectroscopic Quasars

TL;DR

This study analyzes the X-ray variability of 264 quasars over long timescales, revealing that variability is common but not sufficient to explain flux ratio scatter, with no clear redshift dependence and spectral changes linked to brightness.

Contribution

It provides the largest sample analysis of quasar X-ray variability over extended timescales, including spectral and flux variation insights, and constrains flare rates and amplitude distributions.

Findings

30% of quasars show significant variability

Variability fraction increases with source counts, reaching 70% for >1000 counts

Extreme (>100%) changes are rare, occurring in less than 25% of observations

Abstract

We analyze the X-ray variability of 264 Sloan Digital Sky Survey spectroscopic quasars using the Chandra public archive. This data set consists of quasars with spectroscopic redshifts out to z~5 and covers rest-frame time scales up to Delta t_sys 2000 d, with 3 or more X-ray observations available for 82 quasars. It therefore samples longer time scales and higher luminosities than previous large-scale analyses of AGN variablity. We find significant (>3 sigma) variation in ~30% of the quasars overall; the fraction of sources with detected variability increases strongly with the number of available source counts up to ~70% for sources with >1000 counts per epoch. Assuming the distribution of fractional variation is Gaussian, its standard deviation is ~16% on >1 week time scales, which is not enough to explain the observed scatter in quasar X-ray-to-optical flux ratios as due to variability alone. We find no evidence in our sample that quasars are more variable at higher redshifts (z > 2), as has been suggested in previous studies. Quasar X-ray spectra vary similarly to some local Seyfert AGN in that they steepen as they brighten, with evidence for a constant, hard spectral component that is more prominent in fainter stages. We identify one highly-variable Narrow Line Seyfert 1-type spectroscopic quasar in the Chandra Deep Field-North. We constrain the rate of kilosecond-timescale flares in the quasar population using ~8 months of total exposure and also constrain the distribution of variation amplitudes between exposures; extreme changes (>100%) are quite rare, while variation at the 25% level occurs in <25% of observations. [OIII] 5007A emission may be stronger in sources with lower levels of X-ray variability; if confirmed, this would represent an additional link between small-scale (corona) and large-scale (narrow line region) AGN properties.