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Short gamma-ray bursts (GRBs) are believed to be produced by relativistic jets from mergers of neutron-stars (NS) or neutron-stars and black-holes (BH). If the Lorentz-factors $\Gamma$ of jets from compact-stellar-mergers follow a similar power-law distribution to those observed for other high-energy astrophysical phenomena (e.g. blazars, AGN), the population of jets would be dominated by low-$\Gamma$ outflows. These jets will not produce the prompt gamma-rays, but jet energy will be released as x-ray/optical/radio transients when they collide with the ambient medium. Using Monte Carlo simulations, we study the properties of such transients. Approximately $78\%$ of merger-jets $, Comment: 13 pages, 4 figures. Accepted for publication in ApJ

Compact binary mergers, with neutron stars or neutron star and black-hole components, are thought to produce various electromagnetic counterparts: short gamma-ray bursts (GRBs) from ultra-relativistic jets followed by broadband afterglow; semi-isotropic kilonova from radioactive decay of r-process elements; and late time radio flares; etc. If the jets from such mergers follow a similar power-law distribution of Lorentz factors as other astrophysical jets then the population of merger jets will be dominated by low-�� values. The prompt gamma-rays associated with short GRBs would be suppressed for a low-�� jet and the jet energy will be released as X-ray/optical/radio transients when a shock forms in the ambient medium. Using Monte Carlo simulations, we study the properties of such transients as candidate electromagnetic counterparts to gravitational wave sources detectable by LIGO/Virgo. Approximately 78% of merger-jets result in failed GRB with optical peaks 14-22 magnitude and an all-sky rate of 2-3 per year., 4 pages, 2 figures. Submitted to Proceedings of IAU Symposium 324 (New Frontiers in Black Hole Astrophysics)

We systematically analyze the prompt emission and the early afterglow data of a sample of 31 GRBs detected by {\em Swift} before September 2005, and estimate the GRB radiative efficiency. BAT's narrow band inhibits a precise determination of the GRB spectral parameters, and we have developed a method to estimate these parameters with the hardness ratio information. The shallow decay component commonly existing in early X-ray afterglows, if interpreted as continuous energy injection in the external shock, suggests that the GRB efficiency previously derived from the late-time X-ray data were not reliable. We calculate two radiative efficiencies using the afterglow kinetic energy E_K derived at the putative deceleration time t_{dec}) and at the break time (t_b) when the energy injection phase ends, respectively. At t_b XRFs appear to be less efficient than normal GRBs. However, when we analyze the data at t_{dec} XRFs are found to be as efficient as GRBs. Short GRBs have similar radiative efficiencies to long GRBs despite of their different progenitors. Twenty-two bursts in the sample are identified to have the afterglow cooling frequency below the X-ray band. Assuming \epsilon_e = 0.1, we find \eta_\gamma(t_b) usually 90%. Nine GRBs in the sample have the afterglow cooling frequency above the X-ray band for a very long time. This suggests a very small \epsilon_B and/or a very low ambient density n., Comment: 43 pages, 10 figures, ApJ, in press

We study synchrotron self-inverse Compton radiation from a reverse-shocked fireball. If the inverse Compton process dominates the cooling of shocked electrons, an X-ray flare produced by the first order Compton scattering would emerge in the very early afterglow phase, with the bulk of the shock energy radiated in the second order scattering component at 10-100 MeV. The dominance of inverse Compton cooling leads to the lack of prompt optical flashes. We show that for plausible parameters this scattering process can produce an X-ray flare with a relative amplitude change by a factor of several. Flares with a larger amplitude and multiple X-ray flares in a single event are likely to be produced by another mechanism (e.g. internal shocks)., 14 pages, 3 figures, accepted for publication in ApJ

Lyutikov (astro-ph/0503505) raised a valid point that for shock deceleration of a highly magnetized outflow, the fate of the magnetic fields after shock crossing should be considered. However, his comment that the deceleration radius should be defined by the total energy rather than by the baryonic kinetic energy is incorrect. As strictly derived from the shock jump conditions in Zhang & Kobayashi (2005), during the reverse shock crossing process the magnetic energy is not tapped. As a result, the fireball deceleration radius is defined by the baryonic energy only. The magnetic energy is expected to be transferred to the circumburst medium after the reverse shock disappears. The evolution of the system then mimicks a continuously-fed fireball. As a result, Lyutikov's naive conclusion that the forward shock dynamics is independent on the ejecta content is also incorrect. The shock deceleration dynamics and the reverse shock calculation presented in Zhang & Kobayashi (2005) are robust and correct., 4 pages

We review recent progress in understanding the hydrodynamics of compact binary mergers using relativistic smoothed particle hydrodynamics (SPH) codes. Recent results are discussed for both double neutron stars and black hole - neutron star binaries. The gravitational wave signals from these mergers contain a lot of information about the masses and radii of the stars and the equation of state of the neutron star fluid. For black hole - neutron star mergers, the final fate of the fluid and the gravitational wave emission depend strongly on the mass ratio and the black hole spin., Comment: 12 pages, to appear in proceedings of JGRG14 (Yukawa Institute for Theoretical Physics, Kyoto, Japan, Nov 29 - Dec 3, 2004)

(Abridged) The Swift X-Ray Telescope (XRT) reveals some interesting features of early X-ray afterglows, including a distinct rapidly decaying component preceding the conventional afterglow component in many sources, a shallow decay component before the more ``normal'' decay component observed in a good fraction of GRBs (e.g. GRB 050128, GRB 050315, GRB 050319, and GRB 050401), and X-ray flares in nearly half of the afterglows (e.g. GRB 050406, GRB 050502B, GRB 050607, and GRB 050724). In this paper, we systematically analyze the possible physical processes that shape the properties of the early X-ray afterglow lightcurves, and use the data to constrain various models. We suggest that the steep decay component is consistent with the tail emission of the prompt gamma-ray bursts and/or of the X-ray flares. This provides clear evidence that the prompt emission and afterglow emission are two distinct components, supporting the internal origin of the GRB prompt emission. The shallow decay segment observed in a group of GRBs suggests that the forward shock keeps being refreshed for some time. This might be caused either by a long-lived central engine, or by a power law distribution of the shell Lorentz factors, or else by the deceleration of a Poynting flux dominated flow. X-ray flares suggest that the GRB central engine is still active after the prompt gamma-ray emission is over, but with a reduced activity at later times. In some cases, the central engine activity even extends days after the burst trigger. Analyses of early X-ray afterglow data reveal that GRBs are indeed highly relativistic events. Early afterglow data of many bursts, starting from the beginning of the XRT observations, are consistent with the afterglow emission from an interstellar medium (ISM) environment., Comment: emulateapj, 19 pages, ApJ, in press. Conclusions unchanged

We investigate the role of the bulk Compton-scattering process in the internal shock model. We numerically show that the radiation field created by internal shocks does not affect the efficiency of the energy conversion in the shocks even if fireball outflows have highly nonuniform velocity distributions. However, the bulk Compton-scattering produces an additional high energy component especially when variability in the outflows arises because of a modulation of the mass injected into a constant energy flow. We show that an isotropic energy of $10^{50-51}$ ergs could be radiated in the range of 100MeV-GeV through the scattering process, provided that the shell's Lorentz factor varies between 10 and $10^4$., Comment: 12 pages, 2 figures, 2 tables, accepted for publication in ApJL

We discuss the effects of synchrotron self-absorption in the prompt emission from the reverse shock of GRB afterglows occurring in a dense environment, such as the wind of a massive stellar progenitor or a dense ISM in early galaxies. We point out that, when synchrotron losses dominate over inverse Compton losses, the higher self-absorption frequency in a dense environment implies a bump in the reverse shock emission spectrum, which can result in a more complex optical/IR light curve than previously thought. This bump is prominent especially if the burst ejecta is highly magnetized. In the opposite case of low magnetization, inverse Compton losses lead to a prompt X-ray flare. These effects give a possible new diagnostic for the magnetic energy density in the fireball, and for the presence of a dense environment., 11 pages, 2 figures, slightly expanded version of ApJL accepted paper

The strong optical flash observed by ROTSE, as well as the radio flare associated with GRB 990123 can be attributed to the emission of the fireball ejecta, initially heated by the reverse shock. We numerically study the evolution of an adiabatic relativistic fireball interacting with an ambient uniform medium, both in the initial energy transfer stage and in its late evolution. It is shown that the Blandford-McKee solution adequately describes the evolution of the shocked shell quite early on and for as long as the fireball material has relativistic temperatures. In the case where the reverse shock is only mildly relativistic the shocked shell becomes cold almost immediately and the evolution deviates from the Blandford-McKee solution. We derive analytical expressions for the ejecta evolution in its cold regime. This solution gives a good approximation to the numerical results. We estimate the radiation from the fireball ejecta using the numerical hydrodynamic evolution in both cases: cold and hot shells. Surprisingly, we find that both evolutions give rather similar light curves, decaying approximately as $T^{-2}$ in the optical and peaking at about one day in the radio, even though the hydrodynamics is different., Comment: 11 pages, 13 figures, Submitted to ApJ

We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum luminosity is only about half as large as that of SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a GRB clearly associated with a massive star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be \Gamma ~ 200, while for GRB 090424 a lower limit of \Gamma > 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux dominated outflow or to a dissipative photosphere.

We discuss the evolution of a relativistic outflow responsible for producing the emission associated with GRBs. We investigate how afterglows are produced in the inter- action between the outflow and the ambient medium. Understanding the properties of the outflow from afterglow emission can be coupled with information obtained from the prompt component to constrain the magnetisation of the outflow. We analytically and numerically evaluate the relative strength of the reverse shock emission as the out- flow propagates into either a wind or ISM -type environment. We find that previous estimates of magnetisation based on the relative strength of forward and reverse shock emission had been underestimated by up to a factor of 100. We then apply our revised magnetisation estimate to a sample of 10 GRBs and find that 5 of the 10 events can be described by the ISM model. As recent studies have indicated that the fraction of en- ergy stored in the magnetic fields are small, our findings would suggest that the ejecta is driven by thermal pressure. Finally we consider how inhomogeneities present in the outflow can lead to variations in the very early afterglow. Considering small gradi- ent in the ejecta density profile modifies the rising index of the afterglow and can be equivalent to changing the dimensionless parameter ξ by a factor of 2. Uncertainties in determining the width of the ejecta present difficulties in understanding the distribution of GRBs afterglow rising index.