OSSE Observations of the Soft Gamma-Ray Continuum from the Galactic Plane at Longitude 95°

We present the results of OSSE observations of the soft gamma-ray continuum emission from the Galactic plane at longitude 95°. Emission is detected between 50 and 600 keV where the spectrum is fitted well by a power law with photon index -2.6 ± 0.3 and flux (4.0 ± 0.5) × 10-2 photons s-1 cm-2 rad-1 MeV-1 at 100 keV. This spectral shape in this range is similar to that found for the continuum emission from the inner Galaxy, but the amplitude is lower by a factor of 4. This emission is due to either unresolved and previously unknown point sources, or diffuse electron bremsstrahlung, or a combination of the two. Simultaneous observations with OSSE and smaller field-of-view instruments operating in the soft gamma-ray energy band, such as X-ray Timing Explorer or Beppo-SAX, would help resolve this issue. If it is primarily diffuse emission due to nonthermal electron bremsstrahlung, as is the >1 MeV Galactic ridge continuum, then the power in low-energy cosmic-ray electrons exceeds that of the nuclear component of the cosmic rays by an order of magnitude. This would have profound implications for the origin of cosmic rays and the energetics of the interstellar medium. Alternatively, if the emission is diffuse and thermal, then there must be a component of the interstellar medium at temperatures ~109 K.