X-ray bursts as a probe of the corona: the case of XRB 4U 1636-536

To investigate the possible cooling of the corona by soft X-rays bursts, we have studied 114 bursts embedded in the known X-ray evolution of 4U 1636-536. We have grouped these bursts according to the ratio of the flux in the 1.5-12 keV band with respect to that in the 15-50 keV band, as monitored by Rossi X-ray Timing Explorer/All Sky Monitor and Swift/Burst Alert Telescope, respectively. We have detected a shortage at hard X-rays while bursting. This provides hints for a corona cooling process driven by soft X-rays fed by the bursts that occurred on the surface of neutron star. The flux shortage at 30-50 keV has a time lag of 2.4 +/- 1.5 s with respect to that at 2-10 keV, which is comparable to that of 0.7 +/- 0.5 s reported in bursts of IGR 17473-2721. We comment on the possible origin of these phenomena and on the implications for the models on the location of the corona.