Polarization light curves and position angle variation of beamed gamma-ray bursts

The recently detected linear polarization in the optical light curve of GRB 990510 renewed interest in how polarization can be produced in gamma-ray burst fireballs. Here we present a model based on the assumption that we are seeing a collimated fireball, observed slightly off-axis. This introduces some degree of anisotropy, and makes it possible to observe a linearly polarized flux even if the magnetic field is completely tangled in the plane orthogonal to the line of sight. We construct the light curve of the polarization flux, showing that it is always characterized by two maxima, with the polarization position angle changing by 90° between the first and the second maximum. The very same geometry as assumed here implies that the total flux initially decays in time as a power law, but gradually steepens as the bulk Lorentz factor of the fireball decreases.     

Energetics–spectral correlations versus the BATSE gamma-ray bursts population

The proposed correlations between the energetics of gamma-ray bursts (GRBs) and their spectral properties, namely the peak energy of their prompt emission, can broadly account for the observed fluence distribution of all 'bright' BATSE GRBs, under the hypothesis that the GRB rate is proportional to the star formation rate and that the observed distribution in peak energy is independent of redshift. The correlations can also be broadly consistent with the properties of the whole BATSE long GRB population for a peak energy distribution smoothly extending towards lower energies, and in agreement with the properties of a sample at 'intermediate' fluences and with the luminosity functions inferred from the GRB number counts. We discuss the constraints that this analysis imposes on the shape of such peak energy distribution, the opening angle distribution and the tightness of the proposed correlations.     

Observational limits on inverse Compton processes in gamma-ray bursts

Inverse Compton (IC) scattering is one of two viable mechanisms that can produce prompt non-thermal soft gamma-ray emission in gamma-ray bursts. IC requires low-energy seed photons and a population of relativistic electrons that upscatter them. The same electrons will upscatter the gamma-ray photons to even higher energies in the TeV range. Using the current upper limits on the prompt optical emission, we show that under general conservative assumption the IC mechanism suffers from an 'energy crisis'. Namely, IC will overproduce a very high energy component that would carry much more energy than the observed prompt gamma-rays, or alternatively it will require a low-energy seed that is more energetic than the prompt gamma-rays. Our analysis is general, and it makes no assumptions on the specific mechanism that produces the relativistic electron population.     

Time-scales in long gamma-ray bursts

We analyse a sample of bright long bursts and find that the pulse durations have a lognormal distribution while the intervals between pulses have an excess of long intervals (relative to lognormal distribution). This excess can be explained by the existence of quiescent times , long periods with no signal above the background level. The lognormal distribution of the intervals (excluding the quiescent times ) is similar to the distribution of the pulse widths. This result suggests that the quiescent times are made by a different mechanism than the rest of the intervals. It also suggests that the intervals (excluding the quiescent times ) and the pulse width are connected to the same parameters of the source. We find that there is a correlation between a pulse width and the duration of the interval preceding it. There is a weaker, but still a significant, correlation between a pulse width and the interval following it. The significance of the correlation drops substantially when the intervals considered are not adjacent to the pulse.     

Gamma-ray bursts from unstable Poynting-dominated outflows

Poynting-flux driven outflows from magnetized rotators are a plausible explanation for gamma-ray burst engines. We suggest a new possibility for how such outflows might transfer energy into radiating particles. We argue that, in a region near the rotation axis, the Poynting flux drives non-linearly unstable large-amplitude electromagnetic waves (LAEMW) that 'break' at radii     where the MHD approximation becomes inapplicable. In the 'foaming' (relativistically reconnecting) regions formed during the wave breaks, the random electric fields stochastically accelerate particles to ultrarelativistic energies which then radiate in turbulent electromagnetic fields. The typical energy of the emitted photons is a fraction of the fundamental Compton energy     with     plus additional boosting due to the bulk motion of the medium. The emission properties are similar to synchrotron radiation, with a typical cooling time ∼10⁻³ s. During the wave break, the plasma is also bulk accelerated in the outward radial direction and at larger radii can produce afterglows due to interactions with the external medium. The near equipartition fields required by afterglow models may be due to magnetic field regeneration in the outflowing plasma (similar to field generation by LAEMW in laser–plasma interactions) and mixing with the upstream plasma.     

Can the early X-ray afterglow of gamma-ray bursts be explained by a contribution from the reverse shock?

We propose to explain the recent observations of gamma-ray burst early X-ray afterglows with SWIFT by the dissipation of energy in the reverse shock that crosses the ejecta as it is decelerated by the burst environment. We compute the evolution of the dissipated power and discuss the possibility that a fraction of it can be radiated in the X-ray range. We show that this reverse shock contribution behaves in a way very similar to the observed X-ray afterglows if the following two conditions are satisfied. (i) The Lorentz factor of the material which is ejected during the late stages of source activity decreases to small values  Γ < 10  and (ii) a large part of the shock-dissipated energy is transferred to a small fraction  (ζ≲ 10⁻²)  of the electron population. We also discuss how our results may help to solve some puzzling problems raised by multiwavelength early afterglow observations such as the presence of chromatic breaks.     

An external-shock origin of the relation for gamma-ray bursts

We investigate the possibility that the     relation between the peak energy E _p of the  ν F _ν  spectrum and energy output     for long-duration gamma-ray bursts (GRBs) arises from the external shock produced by the interaction of a relativistic outflow with the ambient medium. To that aim, we take into account the dependence of all parameters which determine E _p and     on the radial distribution of the ambient medium density and find that the     relation can be explained if the medium around GRBs has a universal radial stratification. For various combinations of GRB radiative process (synchrotron or inverse-Compton) and dissipation mechanism (reverse or forward shock), we find that the circumburst medium must have a particle density with a radial distribution different than the   R ⁻²  expected for the stellar wind corresponding to a constant mass-loss rate and terminal speed.     

GeV emission from gamma-ray burst afterglows

We calculate the GeV afterglow emission expected from a few mechanisms related to gamma-ray bursts (GRBs) and their afterglows. Given the brightness of the early X-ray afterglow emission measured by Swift /X-Ray Telescope, Gamma-ray Large Area Space Telescope (GLAST)/Large Area Telescope (LAT) should detect the self-Compton emission from the forward shock driven by the GRB ejecta into the circumburst medium. Novel features discovered by Swift in X-ray afterglows (plateaus and chromatic light-curve breaks) indicate the existence of a pair-enriched, relativistic outflow located behind the forward shock. Bulk and inverse-Compton upscattering of the prompt GRB emission by such outflows provide another source of GeV afterglow emission detectable by LAT. The large-angle burst emission and synchrotron forward-shock emission are, most likely, too dim at high photon energy to be observed by LAT. The spectral slope of the high-energy afterglow emission and its decay rate (if it can be measured) allow the identification of the mechanism producing the GeV transient emission following GRBs.     

A unifying view of gamma-ray burst afterglows

We selected a sample of 33 gamma-ray bursts detected by Swift , with known redshift and optical extinction at the host frame. For these, we constructed the de-absorbed and K -corrected X-ray and optical rest-frame light curves. These are modelled as the sum of two components: emission from the forward shock due to the interaction of a fireball with the circumburst medium and an additional component, treated in a completely phenomenological way. The latter can be identified, among other possibilities, as a 'late prompt' emission produced by a long-lived central engine with mechanisms similar to those responsible for the production of the 'standard' early prompt radiation. Apart from flares or re-brightenings, that we do not model, we find a good agreement with the data, despite of their complexity and diversity. Although based, in part, on a phenomenological model with a relatively large number of free parameters, we believe that our findings are a first step towards the construction of a more physical scenario. Our approach allows us to interpret the behaviour of the optical and X-ray afterglows in a coherent way, by a relatively simple scenario. Within this context, it is possible to explain why sometimes no jet break is observed; why, even if a jet break is observed, it is often chromatic and why the steepening after the jet break time is often shallower than predicted. Finally, the decay slope of the late prompt emission after the shallow phase is found to be remarkably similar to the time profile expected by the accretion rate of fall-back material (i.e.  ∝ t ^−5/3  ), suggesting that this can be the reason why the central engine can be active for a long time.