Detection of Soft Gamma-Ray Emission from the Seyfert II Galaxy NGC 4507 by the OSSE Telescope (Preprint)

We report the first soft gamma-ray observation by the OSSE experiment onboard the CGRO of the optically selected Seyfert II galaxy, NGC 4507. The source was observed on two separate weeks in 1993 and detected between 50 and 200 keV at a confidence level of 7 . There is no evidence of flux variability within or between the two observation periods which sample timescales from days to months. The source spectrum obtained from combining the two sets of data is best described by spectral forms including some exponential cutoff. The observed shape is steeper than the one observed by GINGA in the X-ray band in 1990. The joint GINGA/OSSE spectrum is best described by an absorbed power law exponentially cutoff at high energies and having an iron K alpha emission line. Comparison between NGC 4507 and other Seyfert galaxies indicate that although at low energies a difference between type I and II objects is possible (but still weak in this specific case) at high energies the observed shapes are indeed similar. We interpret this as evidence that the primary source emission (i.e. not reprocessed by material in the source) is the same for both types; this finding is in agreement with the unified scheme of Seyfert galaxies. Apart from its own interest, the OSSE detection of hard X-ray emission from a Seyfert II galaxy is also important for the implications it has on the contribution of Seyfert galaxies of both types to the cosmic diffuse background, as type II objects are expected to outnumber type I sources.
Prepared in collaboration with the Centre National de la Recherche Scientifique, Toulouse, France and the E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC. Preprint. Accepted for publication in the Astrophysical Journal; published Astrophysical Journal, Part 2-Letters, v444, n2, pL73-L76, May 1995.