English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We present broadband (radio, optical, and X-ray) light curves and spectra ofthe afterglows of four long-duration gamma-ray bursts (GRBs 090323, 090328,090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and LargeArea Telescope (LAT) instruments on the Fermi satellite. With its wide spectralbandpass, extending to GeV energies, Fermi is sensitive to GRBs with very largeisotropic energy releases (10e54 erg). Although rare, these events areparticularly important for testing GRB central-engine models. When combinedwith spectroscopic redshifts, our afterglow data for these four events are ableto constrain jet collimation angles, the density structure of the circumburstmedium, and both the true radiated energy release and the kinetic energy of theoutflows. In agreement with our earlier work, we find that the relativisticenergy budget of at least one of these events (GRB 090926A) exceeds thecanonical value of 10e51 erg by an order of magnitude. Such energies pose asevere challenge for models in which the GRB is powered by a magnetar orneutrino-driven collapsar, but remain compatible with theoretical expectationsfor magneto-hydrodynamical (MHD) collapsar models. Our jet opening angles(theta) are similar to those found for pre-Fermi GRBs, but the large initialLorentz factors (Gamma_0) inferred from the detection of GeV photons implytheta Gamma_0 ~ 70-90, values which are above those predicted in MHD models ofjet acceleration. Finally, we find that these Fermi-LAT events preferentiallyoccur in a low-density circumburst environment, and we speculate that thismight result from the lower mass-loss rates of their lower-metallicityprogenitor stars. Future studies of Fermi-LAT afterglows in the radio with theorder-of-magnitude improvement in sensitivity offered by the EVLA shoulddefinitively establish the relativistic energy budgets of these events.

Afterglow Observations of Fermi-LAT Gamma-Ray Bursts and the Emerging Class of Hyper-Energetic Events