Hard X-ray spectrum of 4U 1907 + 09

Abstract

We have studied the binary X-ray source 4U 1907+09 in the energy range 20-100 keV, using a xenon-filled multilayer proportional counter telescope flown with balloons from Hyderabad in 1985 and 1991. Results of these observations are reported. The source was detected above 20 keV at a significance level of 13 sigma in 1985 and marginally detected in 1991 when the flux was four times lower than that on the first occasion. A search for pulsation period near 437.5 s, observed in low-energy X-rays, gave null result. An energy spectrum between 20 and 100 keV obtained in 1985 is fitted with a power law of photon index 1.4. Results from an analysis of the High Resolution Imager and Monitor Proportional Counter data obtained from the Einstein Observatory data bank are also presented. The long term X-ray variability of the source is discussed from all the available data.