Magnetic jet model for GRBs and the delayed arrival of >100 MeV photons

TL;DR

This paper proposes a magnetic jet model to explain the observed delay of >100 MeV photons in long gamma-ray bursts, attributing it to the slower acceleration of magnetic jets compared to baryonic outflows.

Contribution

It introduces a magnetic jet framework that naturally accounts for the delayed arrival of high-energy photons and predicts a specific energy dependence of this delay.

Findings

Delay increases with photon energy as E^{0.17} for E>100 MeV.

Delay is proportional to burst redshift and launch radius R_0.

Model matches observed delays from Fermi satellite data.

Abstract

Photons of energy larger than 100 MeV from long-GRBs arrive a few seconds after <10 MeV photons do. We show that this delay is a natural consequence of a magnetic dominated relativistic jet. The much slower acceleration of a magnetic jet with radius (compared with a hot baryonic outflow) results in high energy gamma-ray photons to be converted to electron-positron pairs out to a larger radius whereas lower energy gamma-rays of energy less than ~10 MeV can escape when the jet crosses the Thomson-photosphere. The resulting delay for the arrival of high energy photons is found to be similar to the value observed by the Fermi satellite for a number of GRBs. A prediction of this model is that the delay should increase with photon energy (E) as E^{0.17} for E>100 MeV. The delay depends almost linearly on burst redshift, and on the distance from the central compact object where the jet is launched (R_0). Therefore, the delay in arrival of >10^2 MeV photons can be used to estimate burst redshift if the magnetic jet model for gamma-ray generation is correct and R_0 is roughly the same for long-GRBs.