Hard X-ray spectra of some X-ray binaries from balloon-borne observations

Abstract

We have studied X-ray binaries 4U1907 + 09, GX1 + 4 and Cygnus X-1 in the energy range 20-100 keV, using balloon-borne xenon-filled multilayer proportional counters flown from Hyderabad, India. Results of these observations are reported in this paper. X-ray binary 4U1907 + 09 was observed in 1985 and 1991. During 1985 the source was detected above 20 keV at a significance level of 13 sigma and was only marginally detected in 1991, indicating that the flux was four times lower than that on the first occasion. Energy spectrum between 20-100 keV obtained in 1985 was fitted with power-law with photon index -1.4. GX1 + 4 was observed in December 1991 and April 1992. The source showed a factor of two decrease in flux during the two observations. Hard X-rays spectra obtained on two occasions are found to be consistent with spectrum obtained from High Energy X-Ray Experiment (HEXE) experiment on board Kvant module. Cygnus X-1 was observed on several occasions from 1984 to 1992. There is an indication of Quasi-Periodic Oscillation (QPO) in April 1992 Cygnus X-1 observations as seen by SIGMA on board GRANAT and BATSE on board GRO.