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Hard X-ray view of microlensing events in RX J1131-1231

RX J1131-1231 is a gravitationally lensed system which includes four images of a quasar lensed by an elliptical galaxy. The flux in the individual images is known to be affected by microlensing effect in the visible and X-ray bands. We study the multi-wavelength properties of RX J1131-1231 over a broad energy range, from optical to hard X-ray, during the periods of the microlensing caustic crossings. We aim to constrain the spatial extent of the X-ray emission region at different energies. We combine the data of the source monitoring in the visible band with the X-ray data of the Burst Alert Telescope (BAT) on board of SWIFT satellite and Chandra X-ray observatory. Inspecting the broad band spectrum and lightcurves of the source we identify several microlensing caustic crossing events, and study the details of variability of the source during these events. The caustic crossings of image A on MJD 55150 and 55500 produce strong variations of the overall X-ray flux from the source. In the soft X-ray band, the caustic crossing events are characterised by the exponential growth / decay of the flux ratio between the source images with the exponentiating time scale T_d~60 d, which corresponds to the caustic transit time of the source of the size ~10^{14} cm. Variability induced by the microlensing dominates the overall variability of the sum of the fluxes of four images of the source which should also be detectable in the hard X-ray lightcurve, if the hard X-ray source is comparably compact. Non-detection of the caustic crossing events in the SWIFT/BAT lightcurve of the source indicates that geometry of the region of emission of X-rays with energies above 15 keV is different from that of the lower energy X-rays, with the hard X-ray emission an order of magnitude larger in size.