Hard X-ray recurrent transients

Abstract

Hard X‐ray recurrent transients are intriguing constituents of X‐ray sources. Some of them exhibit flares periodically. Many hard X‐ray transient sources are associated with Be star binaries, which are typically known to have long orbital periods and relatively large eccentricities. The recurrent period seems to be equal to the binary period and the outburst is considered to occur near the periastron passage of the neutron star.Spherically symmetric stellar wind for the case of eccentric binary orbit has been investigated by many authors to explain the transient behaviors. This model requires the extremely large eccentricity (∼0.9) which is inconsistent with observations (0.1∼0.5). Therefore, the simple spherically symmetric wind must be abandoned.If the equatorial plane of the Be star is not coplanar with the binary orbital plane, X‐rays are emitted when the neutron star penetrates through the gas disk around the Be star. In this model the outbursts occur twice in an orbital period and in general there will be two different time intervals between the outbursts if the binary orbit is eccentric. These properties, however, do not seem to be supported by observational evidence.Here the stellar wind with the presence of magnetic field has been studied. The ejected gas can obtain the angular momentum from the rotating primary star through the magnetic field. Hence, the gas will rotate around the star as it moves much slower outwards than the case without the magnetic field. The radial velocity is suppressed this way up to the Alfven radius where the ram pressure of the gas equals the magnetic pressure. Beyond the Alfven radius, the gas will no longer be influenced by the magnetic pressure and escape freely.Now consider the case where the binary orbit is eccentric, and the periastron and the apoastron are inside and outside the Alfven radius, respectively. The mass accretion onto the neutron star is enhanced inside the Alfven radius because of the slower velocity and higher density of the stellar wind in contrast to those outside the Alfven radius. Hence, the outburst occurs near the periastron passage of the neutron star. The required eccentricity is ∼0.3 7 which is consistent with the observations. In this model, furthermore, the change of the light curves of outbursts as observed in 4U0538‐66 can be explained in terms of the variation in the mass loss rate from the primary star.The detail for this study is seen in the Proceedings of a Workshop on Accreting Neutron Stars held at the Max‐Planck‐Institute in Munich (1982).