Different Types of X‐Ray Bursts from GRS 1915+105 and Their Origin

Abstract

We report X-ray observations of the Galactic X-ray transient source GRS 1915]105 with the pointed proportional counters of the Indian X-ray Astronomy Experiment (IXAE) onboard the Indian satellite IRS-P3, which show remarkable richness in temporal variability. The observations were carried out on 1997 June 12È29 and August 7È10, in the energy range of 2È18 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 bursts in the other. The regular bursts are found to have two distinct timescales and to persist over extended durations. There is a strong correlation between the preceding quiescent time and the burst duration for the quasi-regular and irregular bursts. No such correlation is found for the regular bursts. The ratio of average Ñux during the burst time to the average Ñux during the quiescent phase is high and variable for the quasi-regular and irregular bursts, while it is low and constant for the regular bursts. We present a comprehensive picture of the various types of bursts observed in GRS 1915]105 in the light of the recent theories of advective accretion disks. We 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 between the low-hard state and the high-soft state near critical accretion rates in a very short timescale, giving rise to the irregular and quasi-regular bursts. The fast timescale for the transition of the state is explained by invoking the appearance and disappearance of the advective disk in its viscous timescale. The periodicity of the regular bursts is explained by matching the viscous timescale with the cooling timescale of the postshock region. A test of the model is presented using the publicly available 13È60 keV RXT E/PCA data for irregular and regular bursts concurrent with our observations. It is found that the 13È60 keV Ñux relative to the 2È13 keV Ñux shows clear evidence for state change between the quiescent phase and the burst phase. The value of this ratio during burst is consistent with the values observed during the high-soft state seen on 1997 August 19, while its value during quiescent phase is consistent with the values observed during the low-hard state seen on 1997 May 8.

The complex behaviour of the microquasar GRS 1915+105 in theρclass observed withBeppoSAX

“…According to the most accreditated hypothesis, the disk is a thin standard one extending up to a few tens of kilometers from the central source (Fender & Belloni 2004). However, the presence of a slim disk (Abramowicz et al 1988) has also been advocated to describe some of the spectra observed from this source (Ueda et al 2009;Vierdayanti et al 2010;Yadav et al 1999). The phenomenological power-law model is used to describe a Comptonization component from an optically thick corona above the disk with an electron population containing either thermal or non-thermal particles (Zdziarski et al 2001) and where energy can be also gained from the gas bulk motion (Titarchuk et al 1997).…”

Observational evidence for mass ejection during soft X-ray dips in GRS 1915+105

“…It has been exhibiting different types of X-ray variability characteristics (Morgan, Remillard, & Greiner 1997;Muno et al 1999;Yadav et al 1999;Belloni et al 2000a). The radio emission from this source also demonstrates its chaotic nature by means of time to time huge radio flares (Mirabel & Rodriguez 1994;Fender et al 1999), long episodes of Send offprint requests to: S.V.…”

“…Fender et al (1999) suggested that the repeated ejections of the inner accretion disk (Belloni et al 1997) might be responsible for such flares. It was pointed out that such oscillations, having hard dips are not always accompanied by high radio emission Yadav et al 1999). This suggests that some other mechanism is responsible for such huge radio flares.…”

Role of Shocked Accretion Flows in Regulating the Quasi-periodic Oscillation of Galactic Black Hole Candidates

“…The characteristic features of these QPOs suggest that such oscillations are a diagnostic of the accretion processes in the inner regions of the accretion disks around the central compact objects, like shock oscillations in sub-Keplerian advective accretion disks (Chakrabarti & Titarchuk 1995, hereafter CT;Molteni, Sponholz, & Chakrabarti 1996, hereafter MSC; Kazanas, Hua, & Titarchuk 1997;Titarchuk, Lapidus, & Muslimov 1998;Chakrabarti & Manickam 2000, hereafter CM). The Galactic microquasar GRS 1915ϩ 105 shows several properties indicating that QPOs may be generated as the result of the oscillation of the hot, dense inner regions of BH accretion disks (Paul et al 1998;Muno, Morgan, & Remillard 1999;Yadav et al 1999;Rutledge et al 1999;Smith, Heindl, & Swank 2002). A numerical simulation by MSC revealed that for a shocked nonspherical advective BH accretion flow, if the inflow and cooling parameters are properly tuned, the postshock cooling time becomes comparable to the advection timescale from the shock.…”