Correlated X-ray/optical variability in the quasar MR 2251−178

• Published in: Monthly notices of the Royal Astronomical Society Vol. 389; no. 3; pp. 1479 - 1488
• Main Authors: Arévalo, P., Uttley, P., Kaspi, S., Breedt, E., Lira, P., McHardy, I. M.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 21-09-2008; Blackwell Science; Oxford University Press
• Subjects: Accretion disks; Astronomy; Astrophysics; Earth, ocean, space; Exact sciences and technology; galaxies: active; Quasars; Stars & galaxies; X-ray astronomy; galaxies: active; Active galaxies; Quasars; Fluctuations; Light emission; Correlations; Active galaxy nuclei; Accretion rate; Variability; Galaxy nuclei; Models; Accretion disks
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2008.13719.x

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 interband correlations is still not well established. Here we present the results of our X-ray and optical monitoring campaign of the quasar MR 2251−178, 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 d and not larger than 4 d 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.