Rapid X-ray variability of the superluminal source GRS1915+105

The superluminal X-ray transient source GRS 1915+105wasobservedduringJuly20-29,1996withtheIndian X-ray Astronomy Experiment (IXAE) on the Indian satellite IRS-P3 launched on March 21, 1996 from Shriharikota Range in India. During our observations covering the energy band 2- 18 keV, we have seen strong erratic intensity variations on time scale of 0.1s - 10s. Quasi Periodic Oscillations (QPOs) in a frequency range of 0.62 to 0.82 Hz were detected with a rms fraction of about 9%. The rapid X-ray intensity variations in GRS1915+105aresimilartothoseobservedinsomeotherblack hole binaries and thus provide further support for the hypothe- sis that this source is likely to be a black hole. We discuss the possible emission region and mechanism of the observed quasi- periodic oscillations. Comparing the observed QPOs with the ones observed in other neutron star and black-hole systems, we argue that GRS1915+105 is possibly a black-hole.