arXiv subjects
Publications and source records attributed to Robert Mochkovitch.
Heise et al. (2001) have reported the identification of a new class of sources by Beppo-SAX, the so-called X-ray flashes (XRFs), which have many common properties with gamma-ray bursts (GRBs) but are not detected in the gamma-ray range. In the framework of the internal shock model, we investigate the possibility that XRFs have the same origin than GRBs but are intrinsically softer due to different shock parameters.
The prompt emission of gamma-ray bursts probably comes from a highly relativistic wind which converts part of its kinetic energy into radiation via the formation of shocks within the wind itself. Such "internal shocks" can occur if the wind is generated with a highly non uniform distribution of the Lorentz factor. We estimate the expected photospheric emission of such a wind when it becomes transparent. We compare this thermal emission (temporal profile + spectrum) to the non-thermal emission produced by the internal shocks. In most cases, we predict a rather bright thermal emission that should already have been detected. This favors acceleration mechanisms for the wind where the initial energy input is under magnetic rather than thermal form. Such scenarios can produce thermal X-ray precursors comparable to those observed by GINGA and WATCH/GRANAT.
Most models for the central engine of gamma-ray bursts involve a stellar mass black hole surrounded by a thick disk formed after the merging of a system of compact objects or the collapse of a massive star. Energy released from the accretion of disk material by the black hole or from the rotation of the hole itself extracted by the Blandford-Znajek mechanism powers a relativistic wind along the system axis. Lorentz factors of several hundreds are needed to solve the compactness problem in the wind which implies the injection of a tremendous power into a very small amount of matter. The Blandford-Znajek mechanism, where the outflow follows magnetic field lines anchored to the black hole is probably the best way to prevent baryonic pollution and can even initially produce a purely leptonic wind. In this paper we rather study the wind emitted from the inner part of the disk where the risk of baryonic pollution is much larger since the outflow originates from high density regions. We show that the baryonic load of this wind sensitively depends on the disk temperature and magnetic field geometry and that the outflow can become ultra-relativistic (Lorentz factor Gamma>100) under quite restrictive conditions only. Conversely, if Gamma remains of the order of unity the dense wind emitted from the inner disk could help to confine the central jet but may also represent a source of baryon contamination for the Blandford-Znajek mechanism.
The source of cosmic gamma-ray bursts (hereafter GRBs) is usually believed to be a stellar mass black hole accreting material from a thick disk. The mechanism for the production of a relativistic wind by such a system is still unknown. We investigate here one of the proposal where the disk energy is extracted by a magnetic field amplified to very large values B \sim 10^15 G. Using some very simple assumptions we compute the mass loss rate along magnetic field lines and then estimate the Lorentz factor Γat infinity. We find that Γcan reach high values only if severe constraints on the field geometry and the conditions of energy injection are satisfied. We discuss the results in the context of different scenarios for GRBs.