In our 4 days of observations of the galactic superluminal source
GRS 1915+105 in the energy range 
keV, we have
observed intensity
variations over time scale of 100 ms to few seconds. The hard X-rays are
found to have time lag compared to the soft X-rays supporting the
Compton up-scattering models of hard X-ray production in black-hole
candidates. Strong QPOs in a narrow range of 0.62 Hz to 0.82 Hz are
from a narrow region in the disk which is stable during the quiescent
state of this source. We show that the observed QPO frequency
variations can be explained if the QPOs are generated by the
oscillations of the shock front in the accretion disk.
Acknowledgements
We gratefully acknowledge the support given by Shri K. Thyagrajan, the Project Director of IRS-P3 satellite, Shri R.N. Tyagi, Manager IRS programme, Shri R. Aravamudan, Director, ISAC and other technical and engineering staff of ISAC in making the IXAE project a success. We particularly thank M.R. Shah, J.P. Malkar, K. Mukerjee, D.K. Dedhia and P. Shah of our group and N. Upadhyaya, M.R. Sharma, N.S. Murthy, C.N. Umapathy and L. Abraham of Technical Physics Devision, ISAC, and other engineering, scientific and technical staff of other divisions of ISAC who were involved in the IRS-P3 project and ISRO tracking facility, for their efforts in making this experiment successful.