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Register a new userAfterglow calculation in the electromagnetic model for gamma-ray bursts
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F. Genet
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We compute the afterglow of gamma-ray bursts produced by purely electromagnetic outflows to see if it shows characteristic signatures differing from those obtained with the standard internal/external shock model. Using a simple approach for the injection of electromagnetic energy to the forward shock we obtain the afterglow evolution both during the period of activity of the central source and after. Our method equally applies to a variable source. Afterglow light curves in the visible and X-ray bands are computed both for a uniform medium and a stellar wind environment. They are brighter at early times than afterglows obtained with the internal/external shock model but relying only on these differences to discriminate between models is not sufficient.
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The early X-ray afterglow of gamma-ray bursts revealed by Swift carried many surprises. We focus in this paper on the plateau phase whose origin remains highly debated. We confront several newly discovered correlations between prompt and afterglow quantities (isotropic emitted energy in gamma-rays, luminosity and duration of the plateau) to several models proposed for the origin of plateaus in order to check if they can account for these observed correlations. We first show that the scenario of plateau formation by energy injection into the forward shock leads to an efficiency crisis for the prompt phase and therefore study two possible alternatives: the first one still takes place within the framework of the standard forward shock model but allows for a variation of the microphysics parameters to reduce the radiative efficiency at early times; in the second scenario the early afterglow results from a long-lived reverse shock. Its shape then depends on the distribution of energy as a function of Lorentz factor in the ejecta. In both cases, we first present simple analytical estimates of the plateau luminosity and duration and then compute detailed light curves. In the two considered scenarios we find that plateaus following the observed correlations can be obtained under the condition that specific additional ingredients are included. In the forward shock scenario, the preferred model supposes a wind external medium and a microphysics parameter epsilon_e that first varies as n^{- u} (n being the external density), with u~1 to get a flat plateau, before staying constant below a critical density n_0. To produce a plateau in the reverse shock scenario the ejecta must contain a tail of low Lorentz factor with a peak of energy deposition at Gamma >~ 10.
(Abridged) We interpret gamma ray bursts as relativistic, electromagnetic explosions. Specifically, we propose that they are created when a rotating, relativistic, stellar-mass progenitor loses much of its rotational energy in the form of a Poynting flux during an active period lasting $sim 100$ s. Initially, a non-spherically symmetric, electromagnetically-dominated bubble expands non-relativistically inside the star, most rapidly along the rotational axis of the progenitor. After the bubble breaks out from the stellar surface and most of the electron-positron pairs annihilate, the bubble expansion becomes highly relativistic. After the end of the source activity most of the electromagnetic energy is concentrated in a thin shell inside the contact discontinuity between the ejecta and the shocked circumstellar material. This electromagnetic shell pushes a relativistic blast wave into the circumstellar medium. Current-driven instabilities develop in this shell at a radius $sim3times10^{16}$ cm and lead to dissipation of magnetic field and acceleration of pairs which are responsible for the $gamma$-ray burst. At larger radii, the energy contained in the electromagnetic shell is mostly transferred to the preceding blast wave. Particles accelerated at the forward shock may combine with electromagnetic field from the electromagnetic shell to produce the afterglow emission.
Aims: We characterize a sample of Gamma-Ray Bursts with low luminosity X-ray afterglows (LLA GRBs), and study their properties. Method: We select a sample consisting of the 12% faintest X-ray afterglows from the total population of long GRBs (lGRBs) with known redshift. We study their intrinsic properties (spectral index, decay index, distance, luminosity, isotropic radiated energy and peak energy) to assess whether they belong to the same population than the brighter afterglow events. Results: We present strong evidences that these events belong to a population of nearby events, different from that of the general population of lGRBs. These events are faint during their prompt phase, and include the few possible outliers of the Amati relation. Out of 14 GRB-SN associations, 9 are in LLA GRB sample, prompting for caution when using SN templates in observational and theoretical models for the general lGRBs population.
The synchrotron self-Compton (SSC) emission from Gamma-ray Burst (GRB) forward shock can extend to the very-high-energy (VHE; $E_gamma > $100 GeV) range. Such high energy photons are rare and are attenuated by the cosmic infrared background before reaching us. In this work, we discuss the prospect to detect these VHE photons using the current ground-based Cherenkov detectors. Our calculated results are consistent with the upper limits obtained with several Cherenkov detectors for GRB 030329, GRB 050509B, and GRB 060505 during the afterglow phase. For 5 bursts in our nearby GRB sample (except for GRB 030329), current ground-based Cherenkov detectors would not be expected to detect the modeled VHE signal. Only for those very bright and nearby bursts like GRB 030329, detection of VHE photons is possible under favorable observing conditions and a delayed observation time of $la$10 hours.
The energetics of the long duration GRB phenomenon is compared with models of a rotating Black Hole (BH) in a strong magnetic field generated by an accreting torus. A rough estimate of the energy extracted from a rotating BH with the Blandford-Znajek mechanism is obtained with a very simple assumption: an inelastic collision between the rotating BH and the torus. The GRB energy emission is attributed to an high magnetic field that breaks down the vacuum around the BH and gives origin to a e+- fireball. Its subsequent evolution is hypothesized, in analogy with the in-flight decay of an elementary particle, to evolve in two distinct phases. The first one occurs close to the engine and is responsible of energizing and collimating the shells. The second one consists of a radiation dominated expansion, which correspondingly accelerates the relativistic photon--particle fluid and ends at the transparency time. This mechanism simply predicts that the observed Lorentz factor is determined by the product of the Lorentz factor of the shell close to the engine and the Lorentz factor derived by the expansion. An anisotropy in the fireball propagation is thus naturally produced, whose degree depends on the bulk Lorentz factor at the end of the collimation phase.
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