The Soft Gamma-Ray Spectrum of A0535+26: Detection of an Absorption Feature at 110 keV by OSSE
NAVAL RESEARCH LAB WASHINGTON DC E O HULBURT CENTER FOR SPACE RESEARCH
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We present soft gamma-ray observations by the Oriented Scintillation Spectrometer Experiment OSSE on the Compton Gamma Ray Observatory GRO of the transient X-ray binary pulsar A053526. The observations were made 1994 February 817, immediately prior to the peak of a giant outburst. The phase- averaged spectrum is complex and cannot be described by a single-component model. We find that structure in the spectrum above 100 keV can best be modeled by an absorption feature near 110 keV, which we interpret as the signature of cyclotron resonant scattering. Because of OSSEs 45-keV threshold, we are unable to make a definitive statement on the presence of a 55-keV absorption line however we can conclude that if this line does exist, it must have a smaller optical depth than the line at 110 keV. A first harmonic fundamental cyclotron resonance at 110 keV corresponds to a magnetic field strength at the surface of the neutron star of 1 x 10exp 13 G 5 x 10exp 12 G if the first harmonic is at 55 keV.