AFTERGLOW OBSERVATIONS OFFERMILARGE AREA TELESCOPE GAMMA-RAY BURSTS AND THE EMERGING CLASS OF HYPER-ENERGETIC EVENTS

S Bradley Cenko ,D B Fox,B E Cobb,A Cucchiara,A N Morgan,M M Kasliwal,J Xavier Prochaska ,D Reichart ,Nathaniel R Butler ,Karl Glazebrook ,E Berger,J Haislip ,K Ivarsen ,Sebastián López ,D Perley ,Fiona A Harrison ,S R Kulkarni,Poonam Chandra ,Max Pettini ,J Bloom ,A V Filippenko,D A Frail,A Lacluyzé ,V Rana

doi:10.1088/0004-637x/732/1/29

Abstract

We present broadband (radio, optical, and X-ray) light curves and spectra of the afterglows of four long-duration gamma-ray bursts (GRBs 090323, 090328, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) 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 (10e54 erg). Although rare, these events are particularly important for testing GRB central-engine models. When combined with spectroscopic redshifts, our afterglow data for these four events are able to constrain jet collimation angles, the density structure of the circumburst medium, and both the true radiated energy release and the kinetic energy of the outflows. In agreement with our earlier work, we find that the relativistic energy budget of at least one of these events (GRB 090926A) exceeds the canonical value of 10e51 erg by an order of magnitude. Such energies pose a severe challenge for models in which the GRB is powered by a magnetar or neutrino-driven collapsar, but remain compatible with theoretical expectations for magneto-hydrodynamical (MHD) collapsar models. Our jet opening angles (theta) are similar to those found for pre-Fermi GRBs, but the large initial Lorentz factors (Gamma_0) inferred from the detection of GeV photons imply theta Gamma_0 ~ 70-90, values which are above those predicted in MHD models of jet acceleration. Finally, we find that these Fermi-LAT events preferentially occur in a low-density circumburst environment, and we speculate that this might result from the lower mass-loss rates of their lower-metallicity progenitor stars. Future studies of Fermi-LAT afterglows in the radio with the order-of-magnitude improvement in sensitivity offered by the EVLA should definitively establish the relativistic energy budgets of these events.

Highlights

Long-duration gamma-ray bursts (GRBs15), like hydrogendeficient Type Ib/c supernovae (SNe Ib/c; e.g., Filippenko 1997), result from the gravitational collapse of the evolved core of a massive star
With these constraints in hand, we have modeled the afterglow of GRB 090323 with the software described above
The inferred density is slightly lower than usual (A∗ = 0.10 g cm−1), though smaller values have been reported in the literature (e.g., GRB 020405; Chevalier et al 2004)

Summary

Introduction

Long-duration gamma-ray bursts (GRBs15), like hydrogendeficient Type Ib/c supernovae (SNe Ib/c; e.g., Filippenko 1997), result from the gravitational collapse of the evolved core of a massive star. A compact central engine is responsible for accelerating and collimating these jet-like outflows and driving the SN explosions (Woosley & Bloom 2006; Gehrels et al 2009; Soderberg et al 2010). All viable central-engine models for long-duration GRBs share some common characteristics (e.g., Piran 2005). They must produce a collimated outflow with an initial Lorentz factor (Γ0) of a few hundred on observed time scales of 10–100 s, with luminosities and kinetic energies of order 1050 erg s−1 and 1051 erg, respectively. Leading models include the “collapsar” model in which a relativistic jet is produced from a rotating black hole/accretion-disk system (Woosley 1993; MacFadyen & Woosley 1999) and the “magnetar” model in which the rapid energy loss from a newly born millisecond neutron star with a 1015 G magnetic field drives a Poynting-flux-dominated relativistic outflow (Usov 1992)

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Results

Discussion

Conclusion