THE SOFT GAMMA-RAY SPECTRUM OF A0535- DETECTION OF AN ABSORPTION FEATURE AT 110-KEV BY OSSE(26 )

We present soft gamma-ray observations by the Oriented Scintillation S pectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory (GR O) of the transient X-ray binary pulsar A0535+26. The observations wer e made 1994 February 8-17, immediately prior to the peak of a giant ou tburst. The phase averaged spectrum is complex and cannot be described by a single-component model. We find that structure in the spectrum a bove 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 OSSE's 45 keV threshold, we are unable to make a definiti ve 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 opti cal depth than the line at 110 keV. A first harmonic (=fundamental) cy clotron resonance at 110 keV corresponds to a magnetic field strength at the surface of the neutron star of similar to 1 x 10(13) G (similar to 5 x 10(12) G if the first harmonic is at 55 keV).