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Before 2004, nearly all GRB afterglow data could be understood in the context of the external shocks model. This situation has changed in the past two years, when it became clear that some afterglow components should be attributed to the activity of the central engine; i.e., the central engine afterglow. We review here the afterglow emission that is directly related to the GRB central engine. Such an interpretation proposed by Katz, Piran & Sari, peculiar in pre-Swift era, has become generally accepted now. [ABSTRACT FROM AUTHOR]

We compare the luminosity function and rate inferred from the BATSE short hard bursts (SHBs) peak flux distribution with the redshift and luminosity distributions of SHBs observed by Swift/HETE II. The Swift/HETE II SHB sample is incompatible with SHB population that follows the star formation rate. However, it is compatible with a distribution of delay times after the SFR. This would be the case if SHBs are associated with binary neutron star mergers. The implied SHB rates that we find range from ∼ 8 to ∼ 30h703Gpc-3yr-1. This rate is a much higher than what was previously estimated and it is comparable to the rate of neutron star mergers estimated from statistics of binary pulsars. If GRBs are produced in mergers the implied rate practically guarantees detection by LIGO II and possibly even by LIGO I, if we are lucky. Our analysis, which is based on observed short hard burst is limited to bursts with luminosities above 1049erg/sec. Weaker bursts may exist but if so they are hardly detected by BATSE or Swift and hence their rate is very weakly constrained by current observations. Thus the rate of mergers that lead to a detection of a gravitational radiation signal might be even higher. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

We consider some recent developments in GRB/afterglow observations: (i) the appearance of a very hard prompt component in GRB 941017, and (ii) variability in the early afterglow light curves of GRB 021004 and GRB 030329. We show that these observations fit nicely within the internal-external shocks model. The observed variability indicates that the activity of the inner engine is more complicated than was thought earlier and that it involves patchy shells and refreshed shocks. We refute the claims of Berger et al. and of Sheth et al. that the radio and mm observations of GRB 030329 are inconsistent with refreshed shocks. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Gamma-Ray Bursts (GRBs) are the brightest objects observed. They are also the most relativistic objects known so far. GRBs occur when an ultrarelativisitic ejecta is slowed down by internal shocks within the flow. Relativistic particles accelerated within these shocks emit the observed gamma-rays by a combination of synchrotron and inverse Compton emission. External shocks with the circumstellar matter slow down further the ejecta and produce the afterglow, which lasts for months. Comparison of the predictions of this fireball model with observations confirm a relativistic macroscopic motion with a Lorentz factor of Γ ≥ 100. Breaks in the light curves of the afterglow indicate that GRBs are beamed with typical opening angles of a few degrees. The temporal variability of the gamma-rays signal provide us with the best indirect evidence on the nature of the "internal engine" that powers the GRBs and accelerates the relativistic ejecta, suggesting accretion of a massive disk onto a newborn black hole: GRBs are the birth cries of these black holes. Two of the most promising models: Neutron Star Mergers and Collapars lead naturally to this scenario. [ABSTRACT FROM AUTHOR]

We argue from observed features of ultrahigh energy cosmic rays that whatever mechanism produces the events above the GZK energy, ≈ 10[SUP19.7] eV, must also account for events down to ≈ 10[SUP18.7] eV, including their isotropy and spectral smoothness. This rules out distributed sources such as topological defects and Z-bursts, and GRBs. We are lead to identify the powerful radio galaxy Cen A, at 3.4 Mpc, as a probable source of most ultrahigh energy CRs observed at Earth today, and to estimate the extragalactic magnetic field to be ~ 0.3μG. [ABSTRACT FROM AUTHOR]

We discuss the spectrum arising from synchrotron emission by fast cooling electrons when fresh electrons are continually accelerated by a strong blast wave into a power law distribution of energies. The fast cooling spectrum was so far described by four power law segments: F[sub ν]∝ν[sup 2], ν[sup 1/3], ν[sup -1/2] and ν[sup -p/2], divided by three break frequencies: ν[sub sa]<ν[sub c]<ν[sub m]. This is valid for a homogeneous electron distribution. However, hot electrons are located right after the shock, while most electrons are farther down stream and have cooled. We show that this spatial distribution introduces a new break frequency, ν[sub ac], in the optically thick regime: ν[sub ac]<ν[sub sa]<ν[sub c]<ν[sub m], and a new spectral slope: F[sub ν]∝ν[sup 11/8] for ν[sub ac]<ν<ν[sub sa], while the familiar F[sub ν]∝ν[sup 2] is obtained only for ν<ν[sub ac]. While the above ordering of the break frequencies is relevant for afterglows with typical parameters in an ISM environment, other possibilities may also be relevant for internal shocks or afterglows in dense circumstellar winds. We discuss possible applications for gamma-ray bursts (GRBs) and their afterglows, in the context of the internal-external shock model. This may explain spectral slopes steeper than 1/3 seen in the 1–10 keV range in some bursts, if ν[sub sa] reaches the X-ray. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose that the angular inhomogeneities within the relativistic flow cause the same GRB to look very different to different observers. Consequently the most energetic bursts do not correspond to an exceptional energy release but instead they correspond to accidental exceptionally bright spots along the line of sight on colliding shells. We describe the patchy shell model and calculate the distribution function of the observed fluence for bursts with random angular fluctuations of ejecta. We predict, according to this model, that the GRB luminosity function will be much wider than the afterglow X-ray luminosity function and only little correlation between the γ-ray fluence and the afterglow emission. These two predictions are confirmed by the GRB-afterglow data. We also predict that the early (minutes to hours) afterglow would depict large temporal fluctuations whose amplitude decreases with time. Finally we predict that there should be many weak bursts with average afterglow luminosity in this scenario. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We describe fully relativistic three dimensional calculations of the slowing down and spreading of a relativistic jet by an external medium like the ISM. We calculate the synchrotron spectra and light curves using the conditions determined by the hydrodynamic calculations. Preliminary results with a moderate resolution are presented here. Higher resolution calculations are in progress. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We analyze the distribution of total energy of bursts with optical afterglows. Our sample contains eleven bursts for which there are BATSE data allowing detailed fits to the spectra. Six of these bursts have measured redshifts while five have host galaxies whose redshifts are unknown. Using a new technique based on the distribution of magnitudes of observed high redshift galaxies, we bracket the expected redshifts of these host galaxies. We perform a maximum likelihood fit for the energy distribution of these GRBs. Assuming that the total energy emitted has a log-normal distribution, we find that the average isotropic energy emitted is E[sub γ]=1.5[sub -0.6][sup +1.5]×10[sup 53] ergs (for H[sub 0]=65 km s[sup -1] Mpc[sup -1], Ω[sub m]=0.3 and Ω[sub A]=0.7) with a standard deviation σ[sub γ]=1.2[sub -0.4][sup +0.6]. The corresponding distribution of X-ray afterglow energy (for seven bursts) is significantly narrower with σ[sub x]=0.5[sub -0.25][sup +0.4] and E[sub x]=3.8[sub -1.5][sup +2]×10[sup 51] ergs, in agreement with the prediction of the patchy shell model of Kumar and Piran [1]. We also give a table with the detailed fits to the spectra of the 11 GRBs with optical afterglow with BATSE data. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]