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A&A Supplement series, Vol. 128, February II 1998, 145-151

Received June 9, accepted June 20, 1997

X-ray variability of GRS 1915+105 during the low-hard state observed with the Indian X-ray astronomy experiment (IXAE)

B. Paultex2html_wrap938, P.C. Agrawaltex2html_wrap938, A.R. Raotex2html_wrap938, M.N. Vahiatex2html_wrap938, J.S. Yadav tex2html_wrap938, T.M.K. Marartex2html_wrap948, S. Seethatex2html_wrap948, and K. Kasturirangantex2html_wrap948

Send offprint request: B. Paul,

tex2html_wrap954  Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India
tex2html_wrap956  ISRO Satellite Centre, Airport Road, Vimanpura P.O. Bangalore 560 017, India


The galactic superluminal transient X-ray source GRS 1915+105 was observed with the pointed proportional counters (PPCs) onboard the Indian satellite IRS-P3 during 1996 July 23-27. We report here details of the behavior of this source during the relatively quiet and low luminosity state. Large intensity variations by a factor of 2 to 3, generally seen in black-hole candidates, are observed at a time scale of 100 ms to few seconds. No significant variation is detected over larger time scale of minute or more. The intensity variations are described as sum of shots in the light curve, and the number distribution of the shots are found to be exponential function of the fluence and duration of the shots. The cross correlation spectrum between 6-18  keV and 2-6  keV X-rays is found to have asymmetry signifying a delay of the hard X-rays by about 0.2 to 0.4 s. This supports the idea of hard X-rays being generated by Compton up-scattering from high energy clouds near the source of soft X-rays. Very strong and narrow quasi periodic oscillations in the frequency range 0.62 to 0.82 Hz are observed. We discuss about a model which explains a gradual change in the QPO frequencies with corresponding changes in the mass accretion rate of the disk.

keywords: accretion, accretion disks -- black hole physics -- X-rays: stars -- stars: individual: GRS 1915+105

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