Shedding light on the prompt high efficiency paradox - Self consistent modeling of GRB afterglows

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

Research output: Contribution to journalConference articlepeer-review


We examine GRBs with both Fermi-LAT and X-ray afterglow data. Assuming that the 100MeV (LAT) emission is radiation from cooled electrons accelerated by external shocks, we show that the kinetic energy of the blast wave estimated from the 100MeV flux is ∼ 50 times larger than the one estimated from the X-ray flux. This can be explained if either: i) electrons radiating at X-rays are significantly cooled by SSC (suppressing the synchrotron flux above the cooling frequency) or ii) if the X-ray emitting electrons, unlike those emitting at 100MeV energies, are in the slow cooling regime. In both cases the X-ray flux is no longer an immediate proxy of the blast wave kinetic energy. We model the LAT, X-ray and optical data and show that in general these possibilities are consistent with the data, and explain the apparent disagreement between X-ray and LAT observations. All possible solutions require weak magnetic fields: 10-6 ≲ϵB ≲ 10-3 (where eB is the fraction of shocked plasma energy in magnetic fields). Using the LAT emission as a proxy for the blast wave kinetic energy we find that the derived prompt efficiencies are ∼ 15%. This is considerably lower compared with previous estimates (87% and higher for the same bursts). This provides at least a partial solution to the "prompt high efficiency paradox".

Original languageEnglish
Article number064
JournalProceedings of Science
StatePublished - 2014
Externally publishedYes
Event10th Conference on Swift, SWIFT 2014 - Rome, Italy
Duration: 2 Dec 20145 Dec 2014

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