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Astron. Astrophys. 358, L29-L32 (2000)

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1. Introduction

GRS 1915+105 is a transient X-ray source discovered in 1992 with WATCH (Castro-Tirado, Brandt & Lund 1992). Since then it has probably never switched off completely and it has remained as a highly variable bright X-ray source (see Sazonov et al. 1994; Paciesas et al. 1996; Bradt et al. 2000). It is the first Galactic object that was found to show superluminal expansion in the radio (Mirabel & Rodríguez 1994). The interpretation of this phenomenon in terms of relativistic jets (Rees 1966) implies bulk velocities of the ejecta of [FORMULA] at an angle of 60-70 degrees to the line of sight (Mirabel & Rodriguez 1994, Fender et al. 1999, Rodríguez & Mirabel 1999). Because of the high value of the extinction on the line of sight, no optical counterpart is available, but an infrared counterpart has been found (Mirabel et al. 1994). The source is suspected to host a black hole because of its high X-ray luminosity and its similarity with another Galactic superluminal source GRO J1655-40 (Zhang et al. 1994), for which a dynamical estimate of the mass is available (Orosz & Bailyn 1997).

Four years of monitoring with the All-Sky Monitor (ASM) on board RXTE showed that the 2-10 keV flux of GRS 1915+105 is extremely variable, considerably more than any other known X-ray source (see Bradt et al. 2000). See Belloni et al. (2000) for a complete reference list of RXTE observations of the source.

Belloni et al. (1997a,b), from the analysis of selected X-ray spectra, showed that the X-ray variability of the source can be interpreted as the repeated appearance/disappearance of the inner portion of the accretion disk, caused by a thermal-viscous instability. During the low-flux intervals, when the source spectrum hardens considerably, the inner disk up to a certain radius becomes unobservable and is slowly re-filled again. A more complete picture of these variations, where the observations were classified into twelve different classes and another type of (soft) low-flux intervals was presented, was shown by Belloni et al. (2000). Additional spectral analysis has been presented by Markwardt et al. (1999) and Muno et al. (1999), who analyzed in detail the connection between QPOs and energy spectra in GRS 1915+105. One of the problems caused by the exceptional variability of the source is that it is difficult to estimate the accretion rate through the disk or even to rate observations according to accretion rate.

Quasi-periodic variability in the radio, infrared and millimetre bands has been discovered (Pooley 1995, Pooley & Fender 1997; Fender et al. 1997; Fender & Pooley 2000). Fender et al. (1997) suggested that these oscillations could correspond to small ejections of material from the system. Indeed, these oscillations have been found to correlate with the disk-instability as observed in the X-ray band (Pooley & Fender 1997; Eikenberry et al. 1998,2000; Mirabel et al. 1998). This suggests that (some of) the gas is ejected from the inner disk during each low-flux interval. On longer time scales an analogous pattern is observed in the form of major relativistic ejections occurring at the end of a 20-day X-ray dip or `plateau' (Fender et al. 1999).

In this Letter we present the results of detailed time-resolved spectral analysis of RXTE/PCA data of observations when (quasi-)simultaneous infrared data are available. We estimate the value of the accretion rate through the disk for each observation and show that it is anticorrelated with the estimated jet power.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000