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

TL;DR

This paper investigates the discrepancy between kinetic energy estimates from LAT and X-ray data in GRB afterglows, proposing models that reconcile observations and suggest lower prompt efficiencies, addressing the high efficiency paradox.

Contribution

It introduces self-consistent models explaining the high efficiency paradox by considering electron cooling regimes and magnetic field strengths in GRB afterglows.

Findings

LAT emission indicates higher kinetic energy than X-ray estimates.

Weak magnetic fields are required to fit the data.

Prompt efficiencies are around 15%, lower than previous estimates.

Abstract

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: $10^{-6}< \epsilon_B < 10^{-3}$ (where $\epsilon_B$ 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 of order 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".