High energy X-ray spectrum of Her X-1

Abstract

OBSERVATIONS of Her X–1 taken during the on-state of its 35-d cycle¹ between 31 August and 12 September 1977 with the high-energy X-ray detector on board OSO 8 are reported here. A detailed analysis of the time-variable background spectrum suggests that the line features observed in our uncorrected time-averaged spectrum and those previously reported at 25, 60 and 135 keV can be caused by a ∼1% change in detector gain. After correcting for this effect, a 3-σ upper limit of 4×10⁻³ photons cm⁻² s⁻¹ is obtained on the line flux at 60 keV in the time-averaged spectrum, a flux significantly lower than that observed by Coe et al.² 6 months earlier. A 3-σ upper limit of 2×10⁻³ photon cm⁻² s⁻¹ obtained for our pulsed spectrum at the same energy is higher than, and hence consistent with, the positive result reported by Trümper^3,4 from a balloon flight during the same time interval.