HARD X-RAY OBSERVATIONS OF THE X-RAY PULSAR A0535+26

We present the observations of three ''giant'' outbursts from the X-ra y pulsar A0535+26 in April 1989, November 1993, and February 1994 by t he HEXE instrument on board the Mir-Kvant Observatory. We measured the pulsation periods, constructed the pulse profiles in various energy b ands, and studied their variability with time. The power spectrum of t he fractional rms in the range 2 x 10(-3) to 1 Hz is given; its shape is typical of flicker noise, which is commonly observed in black-hole candidates and in some pulsars. The fractional rms slightly increases with increasing energy from similar to 20% at 20 keV to similar to 30% at 80 keV. We constructed the photon spectra in the range 15 to 200 k eV and studied their variability with pulse phase. The shape of each o f these spectra can be approximately described by a ''canonical'' cont inuum model for X-ray pulsars with a photon index gamma similar to 1.1 , a break at E(c)similar to 23 keV, and E(f)similar to 19 keV; the fit ting parameters depend only slightly on the luminosity. The greatest d eviation from the above continuum is observed for the main pulse at en ergy similar to 100 keV. A cyclotron absorption line is assumed to be responsible for this deviation. The HEXE data are compared with the CG RO data. The properties of the pulsar A0535+26 at luminosity similar t o 10(38) erg s(-1) are shown to differ qualitatively from its properti es at luminosity similar to 5 x 10(36) erg s(-1). At the same time, th e data are consistent with the assumption that the structure of the po lar cap at L similar to 10(38) erg s(-1) changes qualitatively, and th at accretion columns extended along the magnetic-field lines are forme d at the poles.