The Extreme Ultraviolet Variability of Quasars

TL;DR

This study investigates the variability of extreme ultraviolet emission in high luminosity quasars, revealing a threshold time scale linked to the size of the EUV emitting region and consistent with slim disk accretion models.

Contribution

It identifies a characteristic variability time scale in quasar EUV emission and connects it to the physical size of the emission region within slim disk models of accretion.

Findings

Significant EUV variability occurs for time intervals > 2×10^7 sec.

Approximately 55% of quasars show EUV variability at longer time scales.

The variability time scale aligns with the radial infall time in slim disk models for rapidly spinning black holes.

Abstract

We study the extreme ultraviolet (EUV) variability (rest frame wavelengths 500 - 920 $\AA$) of high luminosity quasars using HST (low to intermediate redshift sample) and SDSS (high redshift sample) archives. The combined HST and SDSS data indicates a much more pronounced variability when the sampling time between observations in the quasar rest frame is $> 2\times 10^{7}$ sec compared to $< 1.5\times 10^{7}$ sec. Based on an excess variance analysis, for time intervals $< 2\times 10^{7}$ sec in the quasar rest frame, $10\%$ of the quasars (4/40) show evidence of EUV variability. Similarly, for time intervals $>2\times 10^{7}$ sec in the quasar rest frame, $55\%$ of the quasars (21/38) show evidence of EUV variability. The propensity for variability does not show any statistically significant change between $2.5\times 10^{7}$ sec and $3.16\times 10^{7}$ sec (1 yr). The temporal behavior is one of a threshold time interval for significant variability as opposed to a gradual increase on these time scales. A threshold time scale can indicate a characteristic spatial dimension of the EUV region. We explore this concept in the context of the slim disk models of accretion. We find that for rapidly spinning black holes, the radial infall time to the plunge region of the optically thin surface layer of the slim disk that is responsible for the preponderance of the EUV flux emission (primarily within 0 - 7 black hole radii from the inner edge of the disk) is consistent with the empirically determined variability time scale.