Effects of Environment and Energy Injection on Gamma-Ray Burst Afterglows

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 stellar 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.Comment: 7 pages, To appear in the proceedings of the First KIAS International Workshop on Astrophysics: Explosive Phenomena in Astrophysical Compact Objects, Seoul, Korea; 24-27 May 200

Behavior of X-Ray Dust Scattering and Implications for X-Ray Afterglows of Gamma-Ray Bursts

The afterglows of gamma-ray bursts (GRBs) have commonly been assumed to be due to shocks sweeping up the circum-stellar medium. However, most GRBs have been found in dense star-forming regions where a significant fraction of the prompt X-ray emission can be scattered by dust grains. Here we revisit the behavior of dust scattering of X-rays in GRBs. We find that the features of some X-ray afterglows from minutes to days after the gamma-ray triggers are consistent with the scattering of prompt X-ray emission from GRBs off host dust grains. This implies that some of the observed X-ray afterglows (especially those without sharp rising and decaying flares) could be understood with a dust-scattering--driven emission model.Comment: ApJ, in pres

Gamma-Ray Burst Afterglows: Effects of Radiative Corrections and Nonuniformity of the Surrounding Medium

The afterglow of a gamma-ray burst (GRB) is commonly thought to be due to continuous deceleration of a relativistically expanding fireball in the surrounding medium. Assuming that the expansion of the fireball is adiabatic and that the density of the medium is a power-law function of shock radius, viz., $n_{ext}\propto R^{-k}$, we analytically study the effects of the first-order radiative correction and the nonuniformity of the medium on a GRB afterglow. We first derive a new relation among the observed time, the shock radius and the fireball's Lorentz factor: $t_\oplus=R/4(4-k)\gamma^2c$, and also derive a new relation among the comoving time, the shock radius and the fireball's Lorentz factor: $t_{co}=2R/(5-k)\gamma c$. We next study the evolution of the fireball by using the analytic solution of Blandford and McKee (1976). The radiation losses may not significantly influence this evolution. We further derive new scaling laws both between the X-ray flux and observed time and between the optical flux and observed time. We use these scaling laws to discuss the afterglows of GRB 970228 and GRB 970616, and find that if the spectral index of the electron distribution is $p=2.5$, implied from the spectra of GRBs, the X-ray afterglow of GRB970616 is well fitted by assuming $k=2$.Comment: 17 pages, no figures, Latex file, MNRAS in pres

Constraining Parameters in Pulsar Models of Repeating FRB 121102 with High-Energy Follow-up Observations

Recently, a precise (sub-arcsecond) localization of the repeating fast radio burst (FRB) 121102 has led to the discovery of persistent radio and optical counterparts, the identification of a host dwarf galaxy at a redshift of $z=0.193$, and several campaigns of searches for higher-frequency counterparts, which gave only upper limits on the emission flux. Although the origin of FRBs remains unknown, most of the existing theoretical models are associated with pulsars, or more specifically, magnetars. In this paper, we explore persistent high-energy emission from a rapidly rotating highly magnetized pulsar associated with FRB 121102 if internal gradual magnetic dissipation occurs in the pulsar wind. We find that the efficiency of converting the spin-down luminosity to the high-energy (e.g., X-ray) luminosity is generally much smaller than unity, even for a millisecond magnetar. This provides an explanation for the non-detection of high-energy counterparts to FRB 121102. We further constrain the spin period and surface magnetic field strength of the pulsar with the current high-energy observations. In addition, we compare our results with the constraints given by the other methods in previous works and would expect to apply our new method to some other open issues in the future.Comment: 6 pages, 5 figures, ApJ in press, minor changes due to proof correction