Correlated X-ray/Optical Variability in the Quasar MR2251-178

TL;DR

This study presents a 2.5-year monitoring of quasar MR2251-178, revealing close X-ray and optical variability correlations, with complex amplitude behaviors and challenges to simple reprocessing models, suggesting intrinsic accretion rate fluctuations influence emissions.

Contribution

It provides detailed observational evidence of correlated X-ray and optical variability in a quasar, challenging existing reprocessing models and supporting intrinsic accretion rate variations as a key driver.

Findings

X-ray and optical bands are nearly simultaneously correlated with delays up to 4 days.

Rapid X-ray fluctuations have small optical counterparts, while long-term trends are stronger in optical.

Simple reprocessing models cannot fully explain the observed variability patterns.

Abstract

Emission from Active Galactic Nuclei is known to vary strongly over time over a wide energy band, but the origin of the variability and especially of the inter-band correlations is still not well established. Here we present the results of our X-ray and optical monitoring campaign of the quasar MR2251-178, covering a period of 2.5 years. The X-ray 2-10 keV flux is remarkably well correlated with the optical B, V and R bands, their fluctuations are almost simultaneous with a delay consistent with 0 days and not larger than 4 days in either direction. The amplitude of variations shows an intriguing behaviour: rapid, large amplitude fluctuations over tens of days in the X-rays have only small counterparts in the optical bands, while the long-term trends over hundreds of days are stronger in the B band than in X-rays. We show that simple reprocessing models, where all the optical variability arises from the variable X-ray heating, cannot simultaneously explain the discrepant variability amplitudes on different time-scales and the short delays between X-ray and optical bands. We interpret the variability and correlations, in the optically-thick accretion disc plus corona scenario, as the result of intrinsic accretion rate variations modulating both X-ray and optical emission, together with reprocessing of X-rays by the accretion disc.