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

Abstract

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 fit 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 four. This emission is either due to unresolved and previously unknown point sources or it is of diffuse origin, 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 XTE or 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 ∼ 10 9 K.

RXTEMeasurement of the Diffuse X‐Ray Emission from the Galactic Ridge: Implications for the Energetics of the Interstellar Medium

“…The presence of a hard power law tail above 10 keV has been reported from combined observations with Ginga and Welcome-1 and OSSE (Skibo et al 1997). Here, we search for this component by fitting the RXTE data in the 10 − 35 keV energy range with a single power law component.…”

“…Recent diffuse emission measurement of the Galactic plane at longitude 95 • with OSSE in the 50 −500 keV band also reveals that the spectrum can be fitted with a power law model (Γ = 2.6 ± 0.3; Skibo et al [1997]). If the spectrum extends smoothly from the hard X-ray to the soft γ-ray regime, it means that the same physical processes are dominating the emission in a wide bandpass.…”

“…Unfortunately, the two-temperature model does not produce a good fit to the data above 10 keV indicating the presence of additional component(s) at higher energies. Indeed, a hard power law tail has been detected in the hard X-ray/soft γ-ray band from observations of the ridge with Ginga and the balloon experiment Welcome-1 , and OSSE (Skibo et al 1997).…”

Dark matter candidates: a ten-point test

“…In particular observations in the soft gamma-ray energy band, between roughly 50 keV and 1 MeV, have been performed by the Osse experiments [310] and more recently by INTEGRAL in the range 20-8000 keV (see [311,312]). All-sky observations have been performed by COMPTEL in the energy range 3 MeV -30 • measured by different experiments for energies ranging from sub MeV up to tens of GeV.…”

Hard X-ray emission from the Galactic ridge

“…The GRXE spectrum above >20 keV is not yet accurately measured (e.g., Purcell et al 1996;Skibo et al 1997;Kinzer et al 1999). The most recent results on GRXE were obtained with the instruments aboard INTEGRAL observatory (Winkler et al 2003).…”