Gamma Ray Observations of Cygnus X-1 With OSSE

Abstract

Abstract. We report on 120 days of observations of Cygnus X-1 with OSSE onboard the Compton Observatory. Emission is detected in the range 50 keV to 1 MeV, and we nd evidence for a continuum of hard X-ray spectra over a broad range of uxes rather than the existence of distinct ux states. Comparisons of the source spectra with various theoretical models show that an exponentially truncated power law best describes the average spectrum in the OSSE energy band. Although we h a v e measured a new minimum in the hard X-ray ux from the source, no evidence was found for either a broad 1 MeV feature or a narrow 511 keV line previously reported in association with a low ux state. Upper limits on such emission features are an order of magnitude lower than earlier reported detections. The 5.6-day periodicity of the source measured at optical wavelengths was not detected with a sensitivity to the rms modulation fraction of 5 in the 60-140 keV energy band.

SMILE-II: Balloon-Borne Telescope for Background-Suppressed Soft Gamma-Ray Imaging

“…To test the feasibility of the signal detection, we chose three celestial point sources that are brightest in the sub-MeV band, Crab nebula, and black hole candidate Cyg X-1 and GRO J0422+32. We adopted as the flux model of the source the log parabolic power-law model of the Jourdan et al for Crab nebula [5], and the cutoff power-law models of Philips et al and Vieyro et al for Cyg X-1 and GRO J0422+32, respectively [14,20]. As well, the number of the photon N src within ψ degrees of the source yields,…”