On the High-energy Emission of the Short GRB 090510

Long-lived high-energy (>100 MeV) emission, a common feature of most Fermi-LAT-detected gamma-ray burst, is detected up to ~10² s in the short GRB 090510. We study the origin of this long-lived high-energy emission, using broadband 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 >~ 2 s) high-energy emission, but falls below the early-time (t < 2 s) 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 <~ 2 s) to the prompt component, and the long-lived high-energy emission (t >~ 2 s) to the adiabatic forward shock synchrotron afterglow radiation. This avoids the requirement for an extremely high initial Lorentz factor.