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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.

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