GRB 090510: Early LAT emission is not from external shock

Long-lived high-energy (> 100MeV) 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 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 under significant Klein-Nishina effects. 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.