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Register a new userOptical Flashes from Internal Pairs Formed in Gamma-Ray Burst Afterglows
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A. Panaitescu
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We develop a numerical formalism for calculating the distribution with energy of the (internal) pairs formed in a relativistic source from unscattered MeV--TeV photons. For GRB afterglows, this formalism is more suitable if the relativistic reverse-shock that energizes the ejecta is the source of the GeV photons. The number of pairs formed is set by the source GeV output (calculated from the Fermi-LAT fluence), the unknown source Lorentz factor, and the unmeasured peak energy of the LAT spectral component. We show synchrotron and inverse-Compton light-curves expected from pairs formed in the shocked medium and identify some criteria for testing a pair origin of GRB optical counterparts. Pairs formed in bright LAT afterglows with a Lorentz factor in the few hundreds may produce bright optical counterparts (R < 10) lasting for up to one hundred seconds. The number of internal pairs formed from unscattered seed photons decreases very strongly with the source Lorentz factor, thus bright GRB optical counterparts cannot arise from internal pairs if the afterglow Lorentz factor is above several hundreds.
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We study thermal emission from circumstellar structures heated by gamma-ray burst (GRB) radiation and ejecta and calculate its contribution to GRB optical and X-ray afterglows using the modified radiation hydro-code small STELLA. It is shown that thermal emission originating in heated dense shells around the GRB progenitor star can reproduce X-ray plateaus (like observed in GRB 050904, 070110) as well as deviations from a power law fading observed in optical afterglows of some GRBs (e.g. 020124, 030328, 030429X, 050904). Thermal radiation pressure in the heated circumburst shell dominates the gas pressure, producing rapid expansion of matter similar to supenova-like explosions close to opacity or radiation flux density jumps in the circumburst medium. This phenomenon can be responsible for so-called supernova bumps in optical afterglows of several GRBs. Such a `quasi-supernova suggests interpretation of the GRB-SN connection which does not directly involve the explosion of the GRB progenitor star.
Spectropolarimetric measurements of gamma-ray burst (GRB) optical afterglows contain polarization information for both continuum and absorption lines. Based on the Zeeman effect, an absorption line in a strong magnetic field is polarized and split into a triplet. In this paper, we solve the polarization radiative transfer equations of the absorption lines, and obtain the degree of linear polarization of the absorption lines as a function of the optical depth. In order to effectively measure the degree of linear polarization for the absorption lines, a magnetic field strength of at least $10^3$ G is required. The metal elements that produce the polarized absorption lines should be sufficiently abundant and have large oscillation strengths or Einstein absorption coefficients. We encourage both polarization measurements and high-dispersion observations of the absorption lines in order to detect the triplet structure in early GRB optical afterglows.
We present a study of the intermediate regime between ultra-relativistic and nonrelativistic flow for gamma-ray burst afterglows. The hydrodynamics of spherically symmetric blast waves is numerically calculated using the AMRVAC adaptive mesh refinement code. Spectra and light curves are calculated using a separate radiation code that, for the first time, links a parametrisation of the microphysics of shock acceleration, synchrotron self-absorption and electron cooling to a high-performance hydrodynamics simulation.
Aims: Drawing an analogy with Active Galactic Nuclei, we investigate the one-zone SSC model of Gamma Ray Bursts afterglows in the presence of electron injection and cooling both by synchrotron and SSC losses. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. We back up our numerical calculations with analytical solutions of the equations using various profiles of the magnetic field evolution under certain simplifying assumptions. Results: We apply the model to the afterglow evolution of GRBs in a uniform density environment and examine the impact various parameters have on the multiwavelength spectra. We find that in cases where the electron injection and/or the ambient density is high, the losses are dominated by SSC and the solutions depart significantly from the ones derived in the synchrotron standard cases.
Bright X-ray flares are routinely detected by the Swift satellite during the early afterglow of gamma-ray bursts, when the explosion ejecta drives a blast wave into the external medium. We suggest that the flares are produced as the reverse shock propagates into the tail of the ejecta. The ejecta is expected to contain a few dense shells formed at an earlier stage of the explosion. We show an example of how such dense shells form and describe how the reverse shock interacts with them. A new reflected shock is generated in this interaction, which produces a short-lived X-ray flare. The model provides a natural explanation for the main observed features of the X-ray flares --- the fast rise, the steep power-law decline, and the characteristic peak duration Delta t /t= (0.1-0.3).
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