OPTICAL STUDY OF LMXBS WITH HIGH TEMPORAL RESOLUTION - EVIDENCE FOR NONTHERMAL FLARES FROM MXB-1735-44

We present a search for ultrafast optical variability among some low m ass X - ray binaries (LMXBs) using the fast photometer MANIA complex a ttached to the 2.15 m telescope of CASLEO, Argentina. Two flares of al most-equal-to 0.25 sec in duration were recorded from the MXB 1735-44 X - ray burster. Brightness of the object increased almost-equal-to 15 - 30 times during these flares with very steep forward fronts (charac teristic rising time being almost-equal-to 50 - 60 msec), displaying a fine structure on timescales of almost-equal-to 5 - 6 msec (confidenc e level greater-than-or-equal-to 95%. Brightness temperatures for diff erent phases of the flares ranged from 5 10(7)-degrees-K to 10(10)-deg rees-K; this can only be explained by non-thermal processes, or by a h ighly unprobable coherent thermal mechanism. These results are also an evidence that, at least, in the case of MXB 1735-44, there should exi st some departures from the standard model of accretion onto compact o bjects.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, 4 U1636-536 and 2S0921-630.