Model for bursting X-ray sources: time-dependent accretion by magnetic neutron stars and degenerate dwarfs

We consider spherically symmetric accretion flow onto a strongly magnetic compact star. We show that under certain conditions such flow is intermittent, and that the resultant time-dependent X-ray emission from the stellar surface is akin to that observed in the bursting X-ray sources. The stability properties of the magnetosphere of a compact star provide a natural gating mechanism that can produce burst rise times (approx.0.1--1 s) and durations (approx.1--100 s) which are much shorter than the time (approx.10--105 s) between bursts. A key feature of high-luminosity flows is the interaction of the burst radiation with the accreting matter, which determines the burst shape and the time between bursts.Four distinct types of burstlike flows are possible, depending on the luminosity and temperature of the burst radiation. In each type of flow the bursts are terminated by a different physical mechanism: compressional heating of matter at the magnetospheric boundary, Compton heating of matter at the boundary, Compton heating of matter far from the boundary, or radiation pressure. The possible burst luminosities produced by such flows range from 9 or approx. =1036 erg s-1 for the low-luminosity flow up to 1038-1039 ergs s-1 for the high-luminosity flows.Flows of the type studied here may be able to account both for behavior like that of the rapid burster, MXB 1730--335, and like that of the high-luminosity, long-period bursters, such as 3U1820--30=NGC 6624 and MXB 1728--34