An Intrinsic Link between Long-term UV/Optical Variations and X-Ray Loudness in Quasars

Observations have shown that the UV/optical variation amplitude of quasars depends on several physical parameters including luminosity, Eddington ratio, and possibly black hole mass. Identifying new factors which correlate with the variation is essential to probing the underlying physical processes. Combining around 10 years of 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 (compared to bolometric luminosity) are more variable in the UV/optical. This correlation remains highly significant after excluding the effect of other parameters including luminosity, black hole mass, Eddington ratio, redshift, and rest frame wavelength (i.e., through partial correlation analyses). We further find that the intrinsic link between X-ray loudness and UV/optical variation is gradually more prominent on longer timescales (up to 10 yr 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 the UV/optical variation is produced by variable central X-ray emission illuminating the accretion disk, is thus disfavored. This discovery points to an interesting scenario in which both the X-ray coronal heating and UV/optical variation in quasars are closely associated with magnetic disc turbulence, and the innermost disc turbulence (where coronal heating occurs) correlates with slow turbulence at larger radii (where UV/optical emission is produced).