Abstract
A hard X-ray (30-190 keV) observation of A0535+26 in outburst was performed during a balloon flight on October 4, 1980. Detailed spectral analysis shows that the average photon spectrum of A0535+26 can be well fitted by a Wein law with kT = 7.68 + or - 0.035 keV. This spectral law is in good agreement with theoretical model spectra of X-ray pulsars. The energy spectra over 51 phase bins are also well fitted by a Wein law with a kT variable with pulse phase from 7.3 to about 8.5 keV. Apart from this spectral variability, photon spectra averaged over a pulse period strongly suggest that chaotic spectral variability is also present. A simple geometrical model for the X-ray emission and modulation with pulse phase from A0535+26 is presented. This model gives good qualitative agreement with the present observations. 29 references.
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