Accretion mechanisms in the Be/X-ray transient system A0535 + 26
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Abstract
We present four EXOSAT observations of the Be/X-ray transient pulsar HDE 245770/A0535+26 obtained at various phases of the 111d period. On all occasions, a faint X-ray flux is detected from the source (Fx = 1.5-3.0 × 10-10 ergs cm-2 s-1) which corresponds to Lx (1-20 keV) = 2.5 × 103 ergs s-1 at 2.6 kpc. Pulsations at 103.36±0.08 s and 103.29±0.07 s are present during two of the four observations. The detection of a weak pulsed X-ray emission sets a limit of less than 2.5 × 1031 G cm3to the magnetic dipole moment of the neutron star. Previous X-ray studies have shown that the transient source may exhibit either no outburst, a "normal" or a "giant" outburst during a given 111d cycle. "Giant" events appear delayed in phase with respect to "normal" outburst. The UV line and flux histories extracted from the lUE data bank are not clearly correlated with the X-ray behavior while stronger evidences for a correlation may exist at optical wavelength. Using optical spectroscopic data obtained in part simultaneously with our EXOSAT observation, we confirm the existence of a 111d periodicity in the equivalent width of the Hβ emission line. This modulation is likely to be caused by tidal rather than X-ray ionization effects. We argue that there probably exist a two-way interaction between the X-ray source and the Be star circumstellar envelope and that high envelope densities seem to be associated with X-ray active phases. We discuss the constraints that the variety of X-ray time behaviors imply on the physical conditions in the Be circumstellar envelope. The modeling of "normal" and "giant" X-ray outbursts including both subsonic and supersonic accretion regimes shows that these different behaviors reflect strongly different velocity structures of the envelope, "giant" events being caused by slowly expanding (≈ 20-80 km s-1) envelopes and "normal" events implying envelope radial velocities of the order of ≈200 km s-1 or more. Eccentricities derived from the modeling of the light curve of the 1983 June "normal" outburst are in the range 0. O.7. We find that the steep dependency of the specific angular momentum of the accreted material with envelope expansion velocity implies that a transient accretion disk is probably formed around the neutron star during "giant" outbursts while no disc should build up during "normal" outbursts. This picture is independently supported by the X-ray pulse history.