AFTERGLOW OBSERVATIONS OF FERMI LARGE AREA TELESCOPE GAMMA-RAY BURSTS AND THE EMERGING CLASS OF HYPER-ENERGETIC EVENTSReportar como inadecuado




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Resumen

We present broadband radio, optical, and X-ray light curves and spectra of the afterglows of four long-durationgamma-ray bursts GRBs; GRBs 090323, 090328, 090902B, and 090926A detected by the Gamma-Ray BurstMonitor and Large Area Telescope LAT instruments on the Fermi satellite. With its wide spectral bandpass,extending to GeV energies, Fermi is sensitive to GRBs with very large isotropic energy releases 1054 erg.Although rare, these events are particularly important for testing GRB central-engine models. When combined withspectroscopic redshifts, our afterglow data for these four events are able to constrain jet collimation angles, thedensity structure of the circumburst medium, and both the true radiated energy release and the kinetic energy ofthe outflows. In agreement with our earlier work, we find that the relativistic energy budget of at least one of theseevents GRB 090926A exceeds the canonical value of 1051 erg by an order of magnitude. Such energies pose asevere challenge for models in which the GRB is powered by a magnetar or a neutrino-driven collapsar, but remaincompatible with theoretical expectations for magnetohydrodynamical collapsar models e.g., the Blandford–Znajekmechanism. Our jet opening angles θ are similar to those found for pre-Fermi GRBs, but the large initial Lorentzfactors Γ0 inferred from the detection of GeV photons imply θΓ0 ≈ 70–90, values which are above those predictedin magnetohydrodynamic models of jet acceleration. Finally, we find that these Fermi-LAT events preferentiallyoccur in a low-density circumburst environment, and we speculate that this might result from the lower mass-lossrates of their lower-metallicity progenitor stars. Future studies of Fermi-LAT afterglows at radio wavelengths withthe order-of-magnitude improvement in sensitivity offered by the Extended Very Large Array should definitivelyestablish the relativistic energy budgets of these events.Nota general

Artículo de publicación ISI



Autor: Cenko, S. B.; - Frail, D. A.; - Harrison, F. A.; - Haislip, J. B.; - Reichart, Daniel E.; - Butler, N. R.; - Cobb, B. E.; - Cucch

Fuente: http://repositorio.uchile.cl/



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