An intrinsic link between long-term UV/optical variations and X-ray loudness in quasars

TL;DR

This study reveals a significant intrinsic link between X-ray loudness and UV/optical variability in quasars, especially over long timescales, suggesting a common underlying physical process related to magnetic turbulence in accretion disks.

Contribution

It demonstrates a long-term correlation between X-ray loudness and UV/optical variation in quasars, challenging the X-ray reprocessing paradigm and proposing a magnetic turbulence connection.

Findings

X-ray loudness correlates with UV/optical variability after controlling for other parameters.

The correlation is stronger on timescales up to 10 years, weaker below 100 days.

The results suggest a shared physical origin involving magnetic turbulence in accretion disks.

Abstract

Observations have shown that UV/optical variation amplitude of quasars depend on several physi- cal parameters including luminosity, Eddington ratio, and likely also black hole mass. Identifying new factors which correlate with the variation is essential to probe the underlying physical processes. Combining ~ten years long quasar light curves from SDSS stripe 82 and X-ray data from Stripe 82X, we build a sample of X-ray detected quasars to investigate the relation between UV/optical variation amplitude ($\sigma_{rms}$) and X-ray loudness. We find that quasars with more intense X-ray radiation (com- pared to bolometric luminosity) are more variable in UV/optical. Such correlation remains highly significant after excluding the effect of other parameters including luminosity, black hole mass, Ed- dington ratio, redshift, rest-frame wavelength (i.e., through partial correlation analyses). We further find the intrinsic link between X-ray loudness and UV/optical variation is gradually more prominent on longer timescales (up to 10 years in the observed frame), but tends to disappear at timescales < 100 days. This suggests a slow and long-term underlying physical process. The X-ray reprocessing paradigm, in which UV/optical variation is produced by a variable central X-ray emission illuminating the accretion disk, is thus disfavored. The discovery points to an interesting scheme that both the X-ray corona heating and UV/optical variation is quasars are closely associated with magnetic disc turbulence, and the innermost disc turbulence (where corona heating occurs) correlates with the slow turbulence at larger radii (where UV/optical emission is produced).