The X-ray transient source GRS 1915+105 was discovered in 1992
in hard X-rays using the WATCH all sky X-ray monitor onboard the GRANAT
satellite
(Castro-Tirado et al. 1994). Large intensity variations in the source over
time scales of a few hours to a few days were detected. During two
years of hard X-ray observations by WATCH, two powerful bursts
were discovered during which the source luminosity was as high
as 1039 erg s -1 . The hard X-ray spectrum was found to
fit well with a power law spectrum with a photon index 
of -2.5 (Sazonov et al. 1994).
A probable optical counterpart of the source has been observed only in the I band at 23.4 magnitude (Boeer et al. 1996). The optical faintness is probably owing to a low mass companion and very high extinction at a distance of 12.5 kpc. H and He emission lines in the infrared were found to be narrow and no Doppler shift was observed. Absence of high velocity signatures in the line profiles in the IR band suggests that the companion is a low mass star (Castro-Tirado et al. 1996). However, high resolution spectral observations in the near-infrared revealed similarity with spectra of very high mass stars of Oe or Be spectral type (Mirabel et al. 1997).
Superluminal motion of two symmetric radio emitting jets of GRS 1915+105 was discovered by Mirabel & Rodriguez (Mirabel & Rodriguez 1994) which prompted the object being termed as a micro-quasar. Correlated enhanced radio and hard X-ray emissions were discovered from the source in a long time monitoring during 1994 September to 1996 March (Foster et al. 1996). In a previous outburst during 1993 December to 1994 April decreases or dips in the hard X-ray flux were observed during the radio flares. This observation suggested an interaction between the hard X-ray emission and the jet production. Redirection of the accreted material onto jets can cause the observed X-ray and radio intensity fluctuations (Harmon et al. 1997). Radio flares are thought to be synchrotron emission of outgoing plasmoids from the central object. Infrared jets at the same position angle as that of the radio jets were discovered by Sams et al. (1996). The I-R emission from this source is described to be free-free emission of a wind flowing out of the disk. Correlation between the length of a jet, its brightness temperature and central source luminosity was observed which is similar to that seen in AGNs and quasars. This opened up the possibility of understanding the processes in the centers of the AGNs by studying GRS 1915+105 for which evolution time scale of the jets is a much shorter one.
The X-ray luminosity increased to a very high level in 1996 and the source was observed on several occasions by the pointed proportional counters (PPCs) onboard the Indian satellite IRS-P3 (Agrawal et al. 1996a) and also by PCA and ASM detectors on board the RXTE (Bradt 1996). From 1996 February to March 28 the intensity varied erratically around a mean intensity level of about 1 Crab and this state is termed as "chaotic state". During March 28 to May 17 the source was in a very high intensity state in a declining phase but the intensity variations were relatively less and this is named "bright state" of the source. During May 17 to July 16 very large intensity flares were observed. In this period the X-ray intensity is found to increase by a factor as large as 6 with time scale of a few days. GRS 1915+105 was found in the same state again during August 15 to December 30, and this state is called "flaring state". In between these two flaring states i.e., during July 16 to August 15 and after the end of the second flaring state until the present time ASM observations indicate a stable state of the source with luminosity about 500 mCrab in the 1.5-12 keV observation range of ASM. The relatively quiet state of the source is termed "low-hard" state because the spectrum was harder during this low state which is common in black hole candidates.
Strong, narrow, quasi periodic oscillations of varying frequency were discovered in GRS 1915+105 with the PPC observations (Agrawal et al. 1996b; Paul et al. 1997). Intensity dependent narrow QPOs were detected with the PCA. Strong harmonics were also seen at lower frequencies which vanished with increase in intensity and QPO frequency (Chen et al. 1997). The timing behavior, is on some occasions similar to that of the black-hole candidates GS 1124-68 and GX 339-4 (Belloni et al. 1997). The X-ray intensity was found to vary on a variety of time scales ranging from sub-seconds to days during the flaring state, and the spectrum also changed during the brightness variations (Greiner et al. 1996). In the color intensity diagram two separate branches are clearly observed. According to the intensity states, timing behavior and spectral informations, the source has four different states none of which is typical of a black hole state (Morgan et al. 1997).
During our observations in the 2-18 keV energy band spanning 5 days the source was in the low-hard state. The power density spectrum (PDS) of GRS 1915+105 during the low-hard state has a broken power law shape which is almost flat below 1 Hz and decreases sharply above the QPO frequency (Paul et al. 1997).