On the High Energy Emission of the Short GRB 090510

TL;DR

This paper investigates the origin of long-lived high-energy emission in short GRB 090510, attributing late emission to forward shock synchrotron afterglow and early emission to prompt processes, challenging previous reverse shock models.

Contribution

It demonstrates that the late high-energy emission can be explained by forward shock synchrotron radiation, and proposes a new interpretation for the early high-energy component in short GRBs.

Findings

Late > 100 MeV emission is consistent with forward shock synchrotron.

Early high-energy emission likely originates from the prompt component.

Reverse shock models are inconsistent with the observed early emission.

Abstract

Long-lived high-energy (>100MeV) emission, a common feature of most Fermi-LAT detected gamma-ray burst, is detected up to \sim 10^2 s in the short GRB 090510. We study the origin of this long-lived high-energy emission, using broad-band observations including X-ray and optical data. We confirm that the late > 100 MeV, X-ray and optical emission can be naturally explained via synchrotron emission from an adiabatic forward shock propagating into a homogeneous ambient medium with low number density. The Klein-Nishina effects are found to be significant, and effects due to jet spreading and magnetic field amplification in the shock appear to be required. Under the constraints from the low-energy observations, the adiabatic forward shock synchrotron emission is consistent with the later-time (t>2s) high-energy emission, but falls below the early-time (t < 2s) high energy emission. Thus we argue that an extra high energy component is needed at early times. A standard reverse shock origin is found to be inconsistent with this extra component. Therefore, we attribute the early part of the high-energy emission (t< 2s) to the prompt component, and the long-lived high energy emission (t>2s) to the adiabatic forward shock synchrotron afterglow radiation. This avoids the requirement for an extremely high initial Lorentz factor.