Grs 1915+105: Ten Years After

It is now ten years since the first microquasar GRS 1915+105 was discovered. More than six years of observations with RossiXTE have shown a level of variability never observed in any other X-ray source. Here I try to address some issues, based on X-ray observations only, that have relevance for theoretical modeling. First, I ignore these peculiarities and concentrate on the similarities with other X-ray transients. Then I focus on the peculiar variability and present a number of obervational facts that need to be addressed by theoretical models. As practically all articles about GRS 1915+105 start, the source was discovered ten years ago, in 1992, with the Watch instrument on board GRANAT [1]. Figure 1 shows the original discovery light curve from that work, where one can see that the source was very variable from the beginning. As a comparison, the full RossiXTE/ASM light curve up to 2002 July is shown in the bottom panel of Fig. 1. Already from these plots, it is evident that the variability of GRS 1915+105 is rather unique and that in this source we are observing something which is not observed in other sources. When examining the variability at shorter time scales, as observed with the RossiXTE/PCA, things become even more complex (see Fig. 2), showing a definite structure, which in this particular case repeats almost unchanged after about 25 minutes. The complexity of these light curves, first shown by [2], is analyzed and categorized by [3], who classify the light curves in a dozen variability classes and identify three basic spectral states called A, B and C. In these ten years, many papers have been devoted to the analysis and interpretation of the X-ray observations of GRS 1915+105, and it is not possible to review them all here. What I want to do is to put this unique source in perspective, comparing it with other transient BHCs. First, I will emphasize the similarities with other systems, then I will concentrate on the peculiarities. 2. GRS 1915+105 as a transient Looking at GRS 1915+105 as an X-ray transient, if we ignore the most obvious peculiarities, we can identify some common features which the system shares with all other BH systems. One is the shape of the Power Density Spectra (PDS) above, say, 1 Hertz (see e.g. [4, 5]). A second one is the shape of the broad-band energy spectra as observed with BeppoSAX and …