Prompt Emission of High-energy Nonthermal Photons from a Radiation-dominated Relativistic Magnetic Reconnection

In astrophysics, relativistic magnetic reconnection, where particles can accelerate in a region of a strong electric field and weak magnetic field, is a key physical process for the explanation of high-energy photon synchrotron emission above 160 MeV, the limit given by the balance between the accelerating electric force and the radiation reaction force. However, the reconnection dynamics-more importantly, the particle acceleration and photon emission dynamics-in this radiation-dominated, relativistic regime have not been self-consistently investigated yet. In this paper, through theoretical derivation of the modified relativistic tearing instability (RTI) and kinetic particle-in-cell simulations, we find that, because of the radiation reaction, the compression of the reconnecting current sheet is significantly enhanced, leading to an increase in the RTI growth rate in the short-wavelength range. As a result, during reconnection, the current sheet is fragmented into a chain of many more magnetic null points separated by much smaller plasmoids, which eventually gives rise to significant improvement of particle acceleration efficiency and shortening of photon emission duration. In the simulations, prompt emission at duration omega (pe)Delta T similar or equal to 233 (reduced by a factor of 3) of high-energy nonthermal photons with a hard power law of index 2.11 for photon energies 100 MeV is observed. These characteristics are consistent with the observed emission properties of short gamma-ray bursts, particularly of GRB 090510, supporting the radiation-dominated reconnection scenario.