Energies of GRB blast waves and prompt efficiencies as implied by modeling of X-ray and GeV afterglows

Paz Beniamini; Lara Nava; Rodolfo Barniol Duran; Tsvi Piran

doi:10.1093/mnras/stv2033

paper

Energies of GRB blast waves and prompt efficiencies as implied by modeling of X-ray and GeV afterglows

arXiv:1504.04833 · doi:10.1093/mnras/stv2033

Abstract

We consider a sample of ten GRBs with long lasting ( $≳ 1 0^{2} sec$ ) emission detected by Fermi/LAT and for which X-ray data around $1$ day are also available. We assume that both the X-rays and the GeV emission are produced by electrons accelerated at the external forward shock, and show that the X-ray and the GeV fluxes lead to very different estimates of the initial kinetic energy of the blast wave. The energy estimated from GeV is on average $\sim 50$ times larger than the one estimated from X-rays. We model the data (accounting also for optical detections around $1$ day, if available) to unveil the reason for this discrepancy and find that good modelling within the forward shock model is always possible and leads to two possibilities: either the X-ray emitting electrons (unlike the GeV emitting electrons) are in the slow cooling regime or ii) the X-ray synchrotron flux is strongly suppressed by Compton cooling, whereas, due to the Klein-Nishina suppression, this effect is much smaller at GeV energies. In both cases the X-ray flux is no longer a robust proxy for the blast wave kinetic energy. On average, both cases require weak magnetic fields ( $1 0^{- 6} ≲ ε_{B} ≲ 1 0^{- 3}$ ) and relatively large isotropic kinetic blast wave energies $1 0^{53} erg < E_{0, kin} < 1 0^{55} erg$ corresponding to large lower limits on the collimated energies, in the range $1 0^{52} erg < E_{θ, kin} < 5 \times 1 0^{52} erg$ for an ISM environment with $n\sim 1\mbox{cm}^{-3}$ and $1 0^{52} erg < E_{θ, kin} < 1 0^{53} erg$ for a wind environment with $A_{*} \sim 1$ . These energies are larger than those estimated from the X-ray flux alone, and imply smaller inferred values of the prompt efficiency mechanism, reducing the efficiency requirements on the still uncertain mechanism responsible for prompt emission.

14 pages, 7 figures. Accepted for publication in MNRAS

Energies of GRB blast waves and prompt efficiencies as implied by modeling of X-ray and GeV afterglows

Abstract

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