Fast radio bursts （FRBs） are a type of newly-discovered transient astronomical phenomenon. They have short durations, high dispersion measures and a high event rate. However, due to unknown dis- tances and undetected electromagnetic counterparts at other wavebands, it is difficult to further investigate FRBs. Here we propose a method to study their environmental magnetic field using an indirect method. Starting withdispersion measures and rotation measures （RMs）, we try to obtain the parallel magnetic field component ^-B ││ which is the average value along the line of sight in the host galaxy. Because both RMs and redshifls are now unavailable, we demonstrate the dependence of ^-B ││ on these two separate quantities. This result, if the RM and redshift of an FRB are measured, would be expected to provide a clue towards understanding an environmental magnetic field of an FRB.
Multiple rebrightenings have been observed in the multiband afterglow of GRB 030329. In particular, a marked and quick rebrightening occurred at about t 1.2 × 10^5 s. Energy injection from late and slow shells seems to be the best interpretation for these rebrightenings. Usually it is assumed that the energy is injected into the whole external shock. However, in the case of GRB 030329, the rebrightenings are so quick that the usual consideration fails to give a satisfactory fit to the observed light curves. Actually, since these late/slow shells freely coast in the wake of the external shock, they should be cold and may not expand laterally. The energy injection then should only occur at the central region of the external shock. Considering this effect, we numerically re-fit the quick rebrightenings observed in GRB 030329. By doing this, we were able to derive the beaming angle of the energy injection process. Our result, with a relative residual of only 5% - 10% during the major rebrightening, is bet- ter than any previous modeling. The derived energy injection angle is about 0.035. We assume that these late shells are ejected by the central engine via the same mechanism as those early shells that produce the prompt gamma-ray burst. The main difference is that their velocities are much slower, so that they catch up with the external shock relatively late and are manifested as the observed quick rebrightenings. If this were true, then the derived energy injection angle can give a good measure of the beaming angle of the prompt γ-ray emission. Our study may hopefully provide a novel method to measure the beaming angle of gamma-ray bursts.
We present broadband （radio, optical, X-ray and GeV） fits to the afterglow light curves and spectra of three long-duration gamma-ray bursts （GRBs 080916C, 090902B, and 090926A） detected by the Gamma-Ray Burst Monitor and Large Area Telescope （LAT） instruments on the Fermi satellite. Using the observed broadband data, we study the origin of the high energy emission, and suggest that the early-time GeV emission and the late-time radio, optical, and X-ray afterglows can be under- stood as being due to synchrotron emission from an external forward shock caused by structured ejecta propagating in a wind bubble jumping to a homogeneous density medium. If the ceasing time for a majority of the energy injection is assumed to be close to the deceleration time of the forward shock, the structured ejecta with con- tinuous energy injection towards the forward shock can well explain the early rising feature of the GeV mission from these bursts, and the density-jump medium can ac- count for some particular plateaus or flares in the late afterglows. From our fits, we find that, on one hand, the external shock origin of the GeV photons will make the optical depth not have a significant contribution to the early LAT rising part, which will loosen the strong constraint of lower limits of the Lorentz factor. On the other hand, these Fermi-LAT events preferentially occur in a low-density circumburst environment, in which case the Klein-Nishina cutoff will significantly suppress the Self- Synchrotron Compton radiation. Such an environment might result from superbubbles or low-metallicity progenitor stars （which have a low mass-loss rate at late times of stellar evolution） of type Ib/c supernovae.
We investigate in detail the influence of parametrizations of the dark energy equation of state on reconstructing dark energy geometrical parameters, such as the deceleration parameter q（z） and Om diagnostic. We use a type Ia supernova sample, baryon acoustic oscillation data, cosmic microwave background information along with twelve observational Hubble data points to constrain cosmological parameters. With the joint analysis of these current datasets, we find that the parametrizations of w（z） have little influence on the reconstruction result of q（z） and Ore. The same is true for the transition （cosmic deceleration to acceleration） redshift zt, for which we find that for different parametrizations of w（z）, the best fitted values of zt are very close to each other （about 0.65）. All of our results are in good agreement with the ACDM model. Furthermore, using the combination of datasets, we do not find any signal of decreasing cosmic acceleration as suggested in some recent papers. The results suggest that the influence of the prior w（z） is not as severe as one may anticipate, and thus we can, to some extent, safely use a reasonable parametrization of w（z） to reconstruct some other dark energy parameters （e.g. q（z）, Ore） with a combination of datasets.
