Y. F. Huang

Overall Evolution of Jetted Gamma-Ray Burst Ejecta

Whether gamma-ray bursts are highly beamed or not is a very difficult but important problem that we are confronted with. Some theorists suggest that beaming effect usually leads to a sharp break in the afterglow light curve during the ultrarelativistic phase, with the breaking point determined by γ = 1/θ0, where γ is the Lorentz factor of the blast wave and θ0 is the initial half-opening angle of the ejecta, but numerical studies tend to reject the suggestion. We note that previous studies are uniformly based on dynamics that is not proper for nonrelativistic blast waves. Here we investigate the problem in more detail, paying special attention to the transition from the ultrarelativistic phase to the nonrelativistic phase. Due to some crucial refinements in the dynamics, we can follow the overall evolution of a realistic jet until its velocity is as small as βc ~ 10-3c. We find no obvious break in the optical light curve during the relativistic phase itself. However, an obvious break does appear at the transition from the relativistic phase to the Newtonian phase if the physical parameters involved are properly assumed. Generally speaking, the Newtonian phase is characterized by a sharp decay of optical afterglows, with the power-law timing index α ~ 1.8-2.1. This is due to the quick lateral expansion at this stage. The quick decay of optical afterglows from GRB 970228, 980326, and 980519 and the breaks in the optical light curves of GRB 990123 and 990510 may indicate the presence of highly collimated gamma-ray burst ejecta.

A generic dynamical model of gamma-ray burst remnants

The conventional generic model is deemed to explain the dynamics of γ-ray burst remnants very well, no matter whether they are adiabatic or highly radiative. However, we find that for adiabatic expansion, the model could not reproduce the Sedov solution in the non-relativistic phase, thus the model needs to be revised. In the present paper, a new differential equation is derived. The generic model based on this equation has been shown to be correct for both radiative and adiabatic fireballs, and in both ultra-relativistic and non-relativistic phase.

Rebrightening of XRF 030723: Further Evidence for a Two-Component Jet in a Gamma-Ray Burst

We numerically investigate optical afterglows from two-component jets under various configurations. Generally, the light curve is characterized by a rapid rebrightening when the observer is off-axis with respect to the narrow component, with the amplitude and peak time depending on detailed parameters. We further show that the optical afterglow of XRF 030723, especially its notable and rapid rebrightening, can be well explained by a typical two-component jet. This X-ray flash, together with GRB 030329, strongly hints toward the two-component jet model as a unified picture for X-ray flashes and gamma-ray bursts. With a narrow but ultra-relativistic inner outflow and a wide but less energetic outer ejecta, a two-component jet will be observed as a typical gamma-ray burst if our line of sight is within the angular scope of the narrow outflow. Otherwise, if the line of sight is within or slightly beyond the cone of the wide component, an X-ray flash will be detected.

Failed gamma-ray bursts and orphan afterglows

It is believed that orphan afterglow searches can help to measure the beaming angle in gamma-ray bursts (GRBs). Great expectations have been put on this method. We point out that the method is in fact not as simple as we originally expected. As a result of the baryon-rich environment that is common to almost all popular progenitor models, there should be many failed gamma-ray bursts, i.e. fireballs with Lorentz factor much less than 100-1000, but still much larger than unity. In fact, the number of failed gamma-ray bursts may even be much larger than that of successful bursts. Owing to the existence of these failed gamma-ray bursts, there should be many orphan afterglows even if GRBs are due to isotropic fireballs, then the simple discovery of orphan afterglows never means that GRBs are collimated. Unfortunately, to distinguish between a failed-GRB orphan and a jetted but off-axis GRB orphan is not an easy task. The major problem is that the trigger time is unknown. Some possible solutions to the problem are suggested.

Optical flashes and very early afterglows in wind environments

The interaction of a relativistic fireball with its ambient medium is described through two shocks: a reverse shock that propagates into the fireball, and a forward shock that propagates into the medium. The observed optical flash of GRB 990123 has been considered to be the emission from such a reverse shock. The observational properties of afterglows suggest that the progenitors of some γ -ray bursts (GRBs) may be massive stars and their surrounding media may be stellar winds. We here study very early afterglows from the reverse and forward shocks in winds. An optical flash mainly arises from the relativistic reverse shock, while a radio flare is produced by the forward shock. The peak flux densities of optical flashes are larger than 1 Jy for typical parameters, if we do not take into account some appropriate dust obscuration along the line of sight. The radio flare always has a long-lasting constant flux, which will not be covered up by interstellar scintillation. The non-detections of optical flashes brighter than about ninth magnitude may constrain the GRB isotropic energies to be no more than a few 10 52 erg and wind intensities to be relatively weak.

GAMMA-RAY BURST AFTERGLOWS FROM REALISTIC FIREBALLS

A gamma-ray burst (GRB) afterglow has been commonly thought to be due to continuous deceleration of a postburst fireball. Many analytical models have made simplifications for deceleration dynamics of the fireball and its radiation property, although they are successful at explaining the overall features of the observed afterglows. We here propose a model for a GRB afterglow in which the evolution of a postburst fireball is in an intermediate case between the adiabatic and highly radiative expansion. In our model, the afterglow is both due to the contribution of the adiabatic electrons behind the external blast wave of the fireball and due to the contribution of the radiative electrons. In addition, this model can describe evolution of the fireball from the extremely relativistic phase to the nonrelativistic phase. Our calculations show that the fireball will go to the adiabatic expansion phase after about a day if the accelerated electrons are assumed to occupy the total internal energy. In all cases considered, the fireball will go to the mildly relativistic phase about 10(5) s later, and to the nonrelativistic phase after several days. These results imply that the relativistic adiabatic model cannot describe the deceleration dynamics of the several-days-later fireball. The comparison of the calculated light curves with the observed results at late times may imply the presence of impulsive events or energy injection with much longer durations.

On the optical light curves of afterglows from jetted gamma‐ray burst ejecta: effects of parameters

Due to some refinements in the dynamics, we can follow the overall evolution of a realistic jet numerically till its bulk velocity being as small asc � 10 3 c. We find no obvious break in the optical light curve during the relativistic phase itself. However, an obvious break does exist at the transition from the relativistic phase to the non- relativistic phase, which typically occurs at time t � 10 6 — 10 6.5 s (i.e., 10 — 30 d). The break is affected by many parameters, such as the electron energy fractione, the magnetic energy fraction � 2 B , the initial half opening angle �0, and the medium number density n. Increase of any of them to a large enough value will make the break disappear. Although the break itself is parameter-dependent, afterglows from jetted GRB remnants are uniformly characterized by a quick decay during the non- relativistic phase, with power law timing index � � 2.1. This is quite different from that of isotropic fireballs, and may be of fundamental importance for determining the degree of beaming in -ray bursts observationally.

MODELING THE MULTIWAVELENGTH LIGHT CURVES OF PSR B1259-63/LS 2883. II. THE EFFECTS OF ANISOTROPIC PULSAR WIND AND DOPPLER BOOSTING

PSR B1259-63/LS 2883 is a binary system in which a 48 ms pulsar orbits around a Be star in a high eccentric orbit with a long orbital period of about 3.4 yr. It is special for having asymmetric two-peak profiles in both the X-ray and TeV light curves. Recently, an unexpected GeV flare has been detected by the Fermi gamma-ray observatory several weeks after the last periastron passage. In this paper, we show that this observed GeV flare could be produced by the Doppler-boosted synchrotron emission in the bow-shock tail. An anisotropic pulsar wind model, which mainly affects the energy flux injection into the termination shock in a different orbital phase, is also used in this paper, and we find that the anisotropy in the pulsar wind can play a significant role in producing the asymmetric two-peak profiles in both X-ray and TeV light curves. The X-ray and TeV photons before periastron are mainly produced by the shocked electrons around the shock apex, and the light curves after periastron are contributed by the emission from the shock apex and the shock tail together, which result in asymmetric two-peak light curves.

REPEATING FAST RADIO BURSTS FROM HIGHLY MAGNETIZED PULSARS TRAVELING THROUGH ASTEROID BELTS

Very recently Spitler et al. (2016) and Scholz et al. (2016) reported their detections of sixteen additional bright bursts from the direction of the fast radio burst (FRB) 121102. This repeating FRB is inconsistent with all the catastrophic event models put forward previously for hypothetically non-repeating FRBs. Here we propose a different model, in which highly magnetized pulsars travel through asteroid belts of other stars. We show that a repeating FRB could originate from such a pulsar encountering lots of asteroids in the belt. During each pulsar-asteroid impact, an electric field induced outside the asteroid has such a large component parallel to the stellar magnetic field that electrons are torn off the asteroidal surface and accelerated to ultra-relativistic energies instantaneously. Subsequent movement of these electrons along magnetic field lines will cause coherent curvature radiation, which can account for all the properties of an FRB. In addition, this model can self-consistently explain the typical duration, luminosity, and repetitive rate of the seventeen bursts of FRB 121102. The predicted occurrence rate of repeating FRB sources may imply that our model would be testable in the next few years.

FAST RADIO BURSTS: COLLISIONS BETWEEN NEUTRON STARS AND ASTEROIDS/COMETS

Fast radio bursts (FRBs) are newly discovered radio transient sources. Their high dispersion measures indicate an extragalactic origin. But due to the lack of observational data in other wavelengths, their progenitors still remain unclear. Here we suggest the collisions between neutron stars and asteroids/comets as a promising mechanism for FRBs. During the impact process, a hot plasma fireball will form after the material of the small body penetrates into the neutron star surface. The ionized matter inside the fireball will then expand along the magnetic field lines. Coherent radiation from the thin shell at the top of the fireball will account for the observed FRBs. Our scenario can reasonably explain the main features of FRBs, such as their durations, luminosities, and the event rate. We argue that for a single neutron star, FRBs are not likely to happen repeatedly in a forseeable time span since such impacts are of low probability. We predict that faint remnant X-ray emissions should be associated with FRBs, but it may be too faint to be detected by detectors at work.