  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 A4) on 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 A2 instrument and with that measured
to 400 keV by the HEAO A4 Medium Energy Detectors. The HEAO data set also joins
the recent results from COMPTEL on the Compton Gamma-Ray Observatory in the
1-10 MeV 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
fit 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 AGN components,
particularly Seyfert galaxies extending to cosmological distances, and that the
low energy gamma-rays may be due to emission from type 1a Supernovae, 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.
