Swift observations of GRB 050712

We present the results of X-ray and optical observations of GRB 050712 performed by Swift . The X-ray light curve of this burst exhibits episodes of flares in the first 1000 s, the same epoch at which the UVOT detected an optical counterpart. A shallow X-ray decay, with a decay slope of  α=−0.73  , followed and lasted ∼70 ks. This behaviour can be explained in terms of activity of the gamma-ray burst 'inner engine', with the possibility that the last flare is caused by the interaction of the ejecta with the surrounding medium.
We also find interesting spectral parameters for the X-ray emission. In particular, data suggest the presence of an intrinsic absorption in the first 1000 s, which can be explained if circumburst medium clouds lie along the line of sight.     

The circumstellar environment of Wolf–Rayet stars and gamma-ray burst afterglows

We study the evolution of the circumstellar medium of massive stars. We pay particular attention to Wolf-Rayet stars that are thought to be the progenitors of some long gamma-ray bursts (GRBs). We detail the mass-loss rates we use in our stellar evolution models and how we estimate the stellar wind speeds during different phases. With these details we simulate the interactions between the wind and the interstellar medium to predict the circumstellar environment around the stars at the time of core-collapse. We then investigate how the structure of the environment might affect the GRB afterglow. We find that when the afterglow jet encounters the free-wind/stalled-wind interface, rebrightening occurs and a bump is seen in the afterglow light curve. However, our predicted positions of this interface are too distant from the site of the GRB to reach while the afterglow remains observable. The values of the final wind density,   A _*  , from our stellar models are of the same order (≲1) as some of the values inferred from observed afterglow light curves. We do not reproduce the lowest   A _*  values below 0.5 inferred from afterglow observations. For these cases, we suggest that the progenitors could have been a WO-type Wolf–Rayet (WR) star or a very low-metallicity star. Finally, we turn our attention to the matter of stellar wind material producing absorption lines in the afterglow spectra. We discuss the observational signatures of two WR stellar types, WC and WO, in the afterglow light curve and spectra. We also indicate how it may be possible to constrain the initial mass and metallicity of a GRB progenitor by using the inferred wind density and wind velocity.     

Effects of Magnetic Fields on Neutrino-dominated Accretion Model for Gamma-ray Bursts

Many models of gamma-ray bursts suggest a common central engine:a black hole of several solar masses accreting matter from a disk at an accretion rate from 0.01 to 10 M_⊙s~(-1),the inner region of the disk is cooled by neutrino emission and large amounts of its binding energy are liberated,which could trigger the fireball.We improve the neutrino- dominated accreting flows by including the effects of magnetic fields.We find that more than half of the liberated energy can be extracted directly by the large-scale magnetic fields in the disk,and it turns out that the temperature of the disk is a bit lower than the neutrino-dominated accreting flows without magnetic field.Therefore,the outflows are magnetically-dominated rather than neutrino dominated.In our model,the neutrino mechanism can fuel some GRBs (not the brightest ones),but cannot fuel X-ray flares.The magnetic processes(both BZ and electromagnetic luminosity from a disk)are viable mechanisms for most of GRBs and their following X-ray flares.     

Was GRB 990123 a unique optical flash?

GRB 990123 was a long, complex gamma-ray burst accompanied by an extremely bright optical flash. We find different constraints on the bulk Lorentz of this burst to be consistent with the speculation that the optical light is emission from the reverse shock component of the external shock. Motivated by this currently favoured idea, we compute the prompt reverse shock emission to be expected for bursts in which multiwavelength observations allow the physical parameters to be constrained. We find that for reasonable assumptions about the velocity of source expansion, a strong optical flash  m_V≈9  was expected from the reverse shocks, which were usually found to be mildly relativistic. The best observational prospects for detecting these prompt flashes are highlighted, along with the possible reasons for the absence of optical prompt detections in ongoing observations.