The Spectrum of Diffuse Cosmic Hard X‐Rays Measured withHEAO 1

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The spectrum of the diffuse isotropic component of cosmic X-rays over the 13-180 keV range was determined by the UCSD/MIT Hard X-Ray and Gamma-Ray instrument (HEAO 1 A-4) on board the High Energy Astronomical Observatory 1 (HEAO 1). The instrument consists of a complex of actively shielded and collimated scintillation counters, including the low-energy detector set from which the data reported here were obtained. These data join smoothly with the spectrum at lower energies reported by the GSFC HEAO 1 A-2 instrument and with that measured to 400 keV by the HEAO 1 A-4 medium-energy detectors. The HEAO 1 data set also joins the recent results from COMPTEL on the Compton Gamma Ray Observatory (CGRO) in the 1-10 range, which failed to confirm the existence of an MeV bump in this range. Although the spectrum over the entire range 3 keV ≤ E ≤ 100 GeV can be fitted by a simple empirical analytic expression, the origin is likely due to a number of distinct source components. The prevailing idea for the origin is that the hard X-ray spectrum is due to X-rays from various active galactic nucleus components, particularly Seyfert galaxies extending to cosmological distances, and that the low-energy gamma rays may be due to emission from Type Ia supernovae, which is also integrated to cosmological distances. The higher energy gamma-ray spectrum defined by EGRET, also on the CGRO, may be due to unresolved gamma-ray-emitting blazars. Models of production by these source components, extrapolated to the present epoch, must reproduce the observationally derived spectrum.