Hard X-ray observations of the X-ray pulsar A0535+26

We present the observations of three "giant" outbursts from the X-ray pulsar A0535+26 in April 1989, November 1993, and February 1994 by the HEXE instrument on board the Mir-Kvant Observatory. We measured the pulsation periods, constructed the pulse profiles in various energy bands, and studied their variability with time. The power spectrum of the 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 keV and studied their variability with pulse phase. The shape of each of these spectra can be approximately described by a "canonical" continuum 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 fitting parameters depend only slightly on the luminosity. The greatest deviation from the above continuum is observed for the main pulse at energy similar to 100 keV. A cyclotron absorption line is assumed to be responsible for this deviation. The HEXE data are compared with the CGRO data. The properties of the pulsar A0535+26 at luminosity similar to 10(38) erg s(-1) are shown to differ qualitatively from its properties at luminosity similar to 5 x 10(36) erg s(-1). At the same time, the data are consistent with the assumption that the structure of the polar cap at L similar to 10(38) erg s(-1) changes qualitatively, and that accretion columns extended along the magnetic-field lines are formed at the poles.