Different types of X-ray bursts from GRS 1915+105 and their origin

We report X-ray observations of the Galactic X-ray transient source GRS 191 5+105 with the pointed proportional counters of the Indian X-ray Astronomy Experiment (IXAE) onboard the Indian satellite IRS-P3, which show remarkabl e richness in temporal variability. The observations were carried out on 19 97 June 12-29 and August 7-10, in the energy range of 2-18 keV and revealed the presence of very intense X-ray bursts. All the observed bursts have a slow exponential rise, a sharp linear decay, and broadly can be put in two classes: irregular and quasi-regular bursts in one class, and regular burst s in the other. The regular bursts are found to have two distinct timescale s and to persist over extended durations. There is a strong correlation bet ween the preceding quiescent time and the burst duration for the quasi-regu lar and irregular bursts. No such correlation is found for the regular burs ts. The ratio of average flux during the burst time to the average flux dur ing the quiescent phase is high and variable for the quasi-regular and irre gular bursts, while it is low and constant for the regular bursts. We prese nt a comprehensive picture of the various types of bursts observed in GRS 1 915+105 in the light of the recent theories of advective accretion disks. W e suggest that the peculiar bursts that we have seen are characteristic of the change of state of the source. The source can switch back and forth bet ween the low-hard state and the high-soft state near critical accretion rat es in a very short timescale, giving rise to the irregular and quasi-regula r bursts. The fast timescale for the transition of the state is explained b y invoking the appearance and disappearance of the advective disk in its vi scous timescale. The periodicity of the regular bursts is explained by matc hing the viscous timescale with the cooling timescale of the postshock regi on. A test of the model is presented using the publicly available 13-60 keV RXTE/PCA data for irregular and regular bursts concurrent with our observa tions. It is found that the 13-60 keV flux relative to the 2-13 keV flux sh ows clear evidence for state change between the quiescent phase and the bur st phase. The value of this ratio during burst is consistent with the value s observed during the high-soft state seen on 1997 August 19, while its val ue during quiescent phase is consistent with the values observed during the low-hard state seen on 1997 May 8.