Correlated X-ray/optical variability in the quasar MR2251-178
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 interband 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 2.5years. The X-ray 2-10keV flux is remarkably well correlated with the optical B, V and R bands, their fluctuations are almost simultaneous with a delay consistent with 0d and not larger than 4d 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2008
- DOI:
- 10.1111/j.1365-2966.2008.13719.x
- arXiv:
- arXiv:0807.2451
- Bibcode:
- 2008MNRAS.389.1479A
- Keywords:
-
- galaxies: active;
- Astrophysics
- E-Print:
- accepted for publication in MNRAS