Quasi-regular X-ray bursts from GRS 1915+105 observed with the IXAE: Possible evidence for matter disappearing into the event horizon of the black hole

Three different types of very intense, quasi-regular X-ray bursts have been observed from the Galactic superluminal X-ray transient source GRS 1915+105 with the pointed proportional counters of the Indian X-Ray Astronomy Experiment on board the Indian satellite IRS-P3. The observations were carried out from 1997 June 12 to 29 in the energy range of 2-18 keV and revealed the presence of persistent quasi-regular bursts with different structures. Only one of the three types of bursts is regular in occurrence, revealing a stable profile over extended durations. The regular bursts have an exponential rise with a timescale of about 7-10 s and a sharp linear decay in 2-3 s. The X-ray spectrum becomes progressively harder as the burst evolves, and it is the hardest near the end of the burst decay. The profile and energetics of the bursts in this black hole candidate source are distinct from both the type I and type II X-ray bursts observed in neutron star sources. We propose that the sharp decay in the observed burst pattern is a signature of the disappearance of matter through the black hole horizon. The regular pattern of the bursts can be produced by material influx into the inner disk that is due to oscillations in a shock front far away from the compact object.