A simple test of the external shock model for the prompt emission in gamma-ray bursts

It is demonstrated here that if the prompt GRB emission is produced by the simplest version of the external shock model, a specific relation should prevail between the observed duration, isotropic equivalent energy, and photon peak energy. In essence, this relation arises because both the burst duration and the typical energy of the emitted synchrotron photons depend on the same combination of the, usually poorly constrained, external density at the deceleration radius, n_dec, and initial bulk Lorentz factor, Γ₀. This has the fortunate consequence of making the relation independent of both Γ₀ and n_dec. Unless the efficiency of electron acceleration is very low, synchrotron gamma-rays from the external shock would fail to meet the current observational constraints for the vast majority of GRBs, including those with a smooth, single peak temporal profile. This argues either against an external shock origin for the prompt emission in GRBs or for changes in our understanding of the microphysical and radiation processes occurring within the shocked region.     

A mini-supernova model for optical afterglows of gamma-ray bursts

An energy deposition of ∼10⁵⁰ erg into the exterior 10⁻³ M⊙ layers of a red giant is calculated to produce an optical phenomenon similar to afterglows of gamma-ray bursts (GRB) recently observed. This mechanism can be realized if a GRB is generated by some mechanism in a close binary system. In contrast to a 'hypernova' scenario for GRB recently proposed by Paczyński, this model does not require huge kinetic energy in the expanding shell to explain optical afterglows of GRB.     

Relativistic beaming in the central components of double radio quasars

Using a large sample of 78 well-observed double quasars, we have investigated several consequences of the relativistic beaming model. In this model the ratio of the strengths of the central component and outer lobes of a double source depends on whether the jet axis lies close to or away from the line of sight, If this is the actual situation, the fraction of emission from the core,f _c, may be used as a statistical measure of the orientation of the source and should be correlated with other source parameters which also depend on the inclination of the jet axis to the line of sight. We findf _c to be anticorrelated with the overall projected linear size of the extended emission but to exhibit a positive correlation with both the observed degree of misalignment from a collinear double structure, and the ratio of separations of the outer hotspots from the central component. As might be expected from these relationships, we also find sources of smaller projected linear sizes to appear more misaligned and the degree of misalignment to be correlated with the ratio of separations of the outer hotspots. All these correlations are consistent with the predictions of the relativistic beaming model.     

Constraints on the luminosities of gamma-ray bursts

Constraints are found on the gamma-ray burst luminosity function from an analysis of the combined BATSE/PVO intensity distribution. If bursts originate in an extended Galactic halo, then the intrinsic luminosity range is narrow, with bursts spanning only a factor of five or less in luminosity. If bursts originate in a simple Friedmann cosmology with = 1 and = 0, then very few luminosity constraints exist.National Research Council Fellow at NASA/MSFC     

Constraints on gen4eralized Chaplygin gas model including gamma-ray bursts

Generalized Chaplygin gas (whose equation of state is PGCG = -A/PGCGα) was proposed as a candidate for unification of dark energy and dark matter. We inves-tigate constraints on this model with the latest observed data. We test the model with type-Ia supernovae (SNe Ia), cosmic microwave background (CMB) anisotropy, X-ray gas mass fractions in clusters, and gamma-ray bursts (GRBs). We calibrate the GRB lu-minosity relations without assuming any cosmological models using SNe Ia. We show that GRBs can extend the Hubble diagram to higher redshifts (z> 6). The GRB Hubble diagram is well behaved and delineates the shape of the Hubble diagram well. We mea-sure As≡A/PGCG,0α+1=0.68-0.08+0.04 (where PGCG,0 is the energy density today) and α=-0.22-0.13+0.15at the 1σconfidence level using all the datasets. Our results rule out the standard Chaplygin gas model (α = 1) at the 3a confidence level. The ACDM is allowed at the 2or confidence level. We find that acceleration could have started at a redshift of z ～ 0.70. The concordance of the generalized Chaplygin gas model with the age estimate of an old high redshift quasar is found. In addition, we show that GRBs can break the degeneracy between the generalized Chaplygin gas model and the XCDM model.     

Influence of the pulsar parameters on γ-ray burst afterglows

By applying the fireball model of γ-ray burst with a central pulsar, the radiation fluxes of the afterglows of two γ-ray bursts, GRB970228 and GRB000301c, are calculated. The results of the calculation agree very well with the observations. The differing characters of the light curves with a “break” in the optical waveband R of the afterglows of two bursts are interpreted, in terms of differing pulsar parameter values.     

A statistical study of moving type IV bursts based on Culgoora radioheliograph observations

Thirty-one moving type IV (IV(M)) bursts recorded with the Culgoora radioheliograph are examined to deduce their characteristic features, such as spatial distribution, projected velocity, etc., and their relation to other phenomena. The distribution of the projected velocity suggests that less than 15% of the total IV(M) bursts have fast velocities (>1000 km s^–1), almost equal to MHD shock velocity, and that the remaining IV(M) bursts have slower velocities (400 km s^–1) and are probably not associated with MHD shock waves. Most of the slow IV(M) bursts (and 70% of the total IV(M) bursts) are of an isolated plasmoid type. Even if they are associated with minor H flares, IV(M) bursts of the isolated-plasmoid type have 10³¹ ergs in the form of magnetic energy. They are in many cases closely associated with extended flare-continuum sources; this seems plausible if the flare continuum is interpreted as an interaction of a plasmoid with a large-scale magnetic arch.The association of IV(M) bursts with energetic proton events seems to be poor - contrary to expectation.     

Observational constraints on the nature of very short gamma-ray bursts

We discuss a very peculiar subgroup of gamma-ray bursts among the BATSE sources. These bursts are very short (T₉₀ ? 0.1 s), hard, and came predominantly from a restricted direction of the sky (close to the Galactic anti-center). We analyze their arrival times and possible correlations, as well as the profiles of individual bursts. We find no peculiarities in the arrival times of Very Short Bursts (VSBs) despite their highly non-uniform spatial distribution. There is no dependence in the burst shapes on location. Bursts coming both from the burst-enhancement Galactic Anticenter region and from all other directions show considerable dispersion in their rise and fall times. Significant fraction of VSBs have multiple peaks despite their extremely short duration. Burst time properties are most likely to be consistent with two origin mechanisms: either with binary NS–NS mergers with low total masses passing through a phase of hypermassive neutron star, or with evaporation of the primordial black holes in the scenario of no photosphere formation.     

Investigation of gamma-ray bursts with known redshifts: Statistical analysis of parameters

Observational parameters of the optical and gamma-ray emissions from 58 gamma-ray bursts (GRBs) with discovered afterglows and known redshifts are analyzed. The distributions of these parameters and pair correlations between them are studied. Approximately half of the objects exhibit a relatively slow decrease in the optical flux at initial afterglow phases (with a power-law index in the decay law α < 1). Correlations have been found between the luminosities, energies, and durations of the optical and gamma-ray emissions, which can be explained by the presence of universal features in the light curves. A correlation of the peak luminosity for afterglows with the redshift and an anticorrelation of their durations with the redshift have been found for the first time. Against the background of a weak z dependence of the total afterglow energy, this effect can be explained by cosmological evolution of the GRB environment, which determines the rate of optical energy release.     

A comparison of temporal properties of short and long gamma-ray bursts

In this paper, we have performed a temporal analysis of single pulses from short (46) and long (51) gamma-ray bursts (GRBs) to investigate possible differences in their properties. In particular we pay detailed attention to the asymmetry of pulses. We find that the asymmetry ratio of short GRB pulses clusters around 0.81 and that these pulses are on average more symmetric than those from long GRBs, which have an average value of 0.47. In addition we note that the pulses in short GRBs display exponential rises and fast decays (ERFD) in comparison the fast rise exponential decays (FRED) pulses of long GRBs. The asymmetry ratio does not depend on the full width at half maximum (FWHM) and does not vary with energy channel. Moreover, there is a general trend for slower pulses to be more asymmetric. Finally, we deduce that the asymmetry could be used to probe the emission mechanisms of GRBs.     

A complete reference of the analytical synchrotron external shock models of gamma-ray bursts

Gamma-ray bursts are most luminous explosions in the universe. Their ejecta are believed to move towards Earth with a relativistic speed. The interaction between this “relativistic jet” and a circumburst medium drives a pair of (forward and reverse) shocks. The electrons accelerated in these shocks radiate synchrotron emission to power the broad-band afterglow of GRBs. The external shock theory is an elegant theory, since it invokes a limit number of model parameters, and has well predicted spectral and temporal properties. On the other hand, depending on many factors (e.g. the energy content, ambient density profile, collimation of the ejecta, forward vs. reverse shock dynamics, and synchrotron spectral regimes), there is a wide variety of the models. These models have distinct predictions on the afterglow decaying indices, the spectral indices, and the relations between them (the so-called “closure relations”), which have been widely used to interpret the rich multi-wavelength afterglow observations. This review article provides a complete reference of all the analytical synchrotron external shock afterglow models by deriving the temporal and spectral indices of all the models in all spectral regimes, including some regimes that have not been published before. The review article is designated to serve as a useful tool for afterglow observers to quickly identify relevant models to interpret their data. The limitations of the analytical models are reviewed, with a list of situations summarized when numerical treatments are needed.