  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
onboard the Indian satellite IRS-P3. The observations were carried out from
1997 June 12 to June 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 time scale of about 7 s to 10 s and a sharp linear decay in 2 to 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 due to oscillations in a
shock front far away from the compact object.
