Search

Hyperaccreting disks around neutron stars or magnetars cooled via neutrino emission can be the potential central engine of GRBs. The neutron-star disk can cool more efficiently, produce much higher neutrino luminosity and neutrino annihilation luminosity than its black hole counterpart with the same accretion rate. The neutron star surface boundary layer could increase the annihilation luminosity as well. An ultra relativistic jet via neutrino annihilation can be produced along the stellar poles. Moreover, we investigate the effects of strong fields on the disks around magnetars. In general, stronger fields give higher disk densities, pressures, temperatures and neutrino luminosity; the neutrino annihilation mechanism and the magnetically-driven pulsar wind which extracts the stellar rotational energy can work together to generate and feed an even stronger ultra-relativistic jet along the stellar magnetic poles. [ABSTRACT FROM AUTHOR]

Hyperaccretion disks around neutron stars seem to exist in some GRBs. Such disks may also occur in type-II supernovae. We study the structure of a hyperaccretion disk around a neutron star. We divide the disk into two regions, which are called inner and outer disks. The outer disk is similar to that of a black hole and the inner disk has a self-similar structure. We first adopt a simple model, in which some microphysical processes in the disk are simplified, and then consider an elaborate model with more physical considerations about the thermodynamics and microphysics in the neutron star disk. Different from the disk around a black hole, the disk around a neutron star should eventually release the gravitational binding energy of accreted matter more efficiently. [ABSTRACT FROM AUTHOR]

In the framework of the popular internal shock model, paired forward and reverse shocks are generated simultaneously by collisions among relativistic shells. If the Lorentz factors of these two types of shocks are very different, the synchrotron emissions produced by them could peak at two different energy bands. We show that such a two-component synchrotron scenario can account for the prompt optical and MeV γ-ray emissions of GRB 080319B. The same dynamical origin of the forward and reverse shocks is consistent with the temporal coincidence between the prompt γ-ray and optical emission components. [ABSTRACT FROM AUTHOR]

Radio afterglows have been detected following two giant flares from soft gamma repeaters (i.e. SGR1900+14, SGR 1806-20). Recent follow-up observations of the December 27 giant flare of SGR 1806-20 have detected a multi-frequency radio afterglow from 240 MHz to 8.46 GHz, extending in time from one week to about one month after the flare. The angular size of the source was also measured for the first time. Here we show that this radio afterglow gives the first piece of clear evidence that an energetic blast wave sweeps up its surrounding medium and produces a synchrotron afterglow, the same mechanism as established for GRB afterglows. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are currently believed to originate from highly magnetized, rapidly rotating compact objects. After the GRB, such an object may directly lose its rotational energy through some magnetically-driven processes, which produce an ultrarelativistic wind dominated possibly by the energy flux of electron-positron pairs. The interaction of this wind with an outward-expanding fireball leads to a relativistic wind bubble. Here we discuss the effects of this wind bubble. We find that when the wind energy significantly exceeds the initial energy of the fireball, the bulk Lorentz factor of the wind bubble decays more slowly than before, and more importantly, the reverse-shock emission could dominate the afterglow emission, which yields a bump in afterglow light curves. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

In many GRBs, afterglows have been observed for months or even years. It deserves noting that at such late stages, the remnants should have entered the deep Newtonian phase, during which the majority of shock-accelerated electrons will no longer be highly relativistic. However, a small portion of electrons are still ultra-relativistic and capable of emitting synchrotron radiation. Under the assumption that the electrons obey a power-law distribution according to their kinetic energy (not simply the Lorentz factor), we calculate optical afterglows from both isotropic fireballs and beamed ejecta, paying special attention to the late stages. In the beamed cases, it is found that the light curves are universally characterized by a flattening during the deep Newtonian phase. Implication of our results on orphan afterglows is also addressed. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

There is growing evidence that some long gamma-ray bursts (GRBs) arise from the core collapse of massive stars, and thus it is inevitable that the environments of these GRBs are preburst winds or dense media. We studied, for the first time, the wind model for afterglows based on the Blandford-McKee self-similar solution of a relativistic shock, and suggested that GRB 970616 is an interactor with a stellar wind. We also proposed a dense medium model for some afterglows, e.g., the steepening in the light curve of the R-band afterglow of GRB 990123 may be caused by the adiabatic shock which has evolved from an ultrarelativistic phase to a nonrelativistic phase in a dense medium. We further discussed the dense medium model in more details, and investigated the effects of synchrotron self absorption and energy injection. A shock in a dense medium becomes nonrelativistic rapidly after a short relativistic phase. The afterglow from the shock at the nonrelativistic stage decays more rapidly than at the relativistic stage. Since some models for GRB energy sources predicted that a strongly magnetic millisecond pulsar may be born during GRB formation, we discussed the effect of such a pulsar on the evolution of the nonrelativistic shock through magnetic dipole radiation. We found that in the pulsar energy injection case, the dense medium model fits very well all the observational data of GRB 980519. Recently, we combined the dense medium model with the pulsar energy injection effect to provide a good fit to the optical afterglow data of GRB 000301C. [ABSTRACT FROM AUTHOR]