1994A&A...289..141B

We present a search for ultrafast optical variability among some low mass X-ray binaries (LMXBs) using the fast photometer MANIA complex attached to the 2.15 m telescope of CASLEO, Argentina. Two flares of ≃0.25 sec in duration were recorded from the MXB 1735-44 X-ray burster. Brightness of the object increased ≃15-30 times during these flares with very steep forward fronts (characteristic rising time being ≃50-60msec), displaying a fine structure on timescales of ≃5-6msec (confidence level ≥95%). Brigthness temperatures for different phases of the flares ranged from 5 10⁷degK to 10¹⁰degK; this can only be explained by non-thermal processes, or by a highly unprobable coherent thermal mechanism. These results are also an evidence that, at least, in the case of MXB 1735-44, there should exist some departures from the standard model of accretion onto compact objects. On the other hand we have detected no stochastic variations on timescale from 10^–7 to 10 sec for 4U1659-487, 4U1822-371, GX339-4, 4U1636-536 and 2S0921-630.