HIGH-ENERGY EMISSION COMPONENTS IN THE SHORT GRB 090510

We investigate the origin of the prompt and delayed emission observed in the short GRB 090510. We use the broadband data to test whether the most popular theoretical models for gamma-ray burst emission can accommodate the observations for this burst. We first attempt to explain the soft-to-hard spectral evolution associated with the delayed onset of a GeV tail with the hypothesis that the prompt burst and the high-energy tail both originate from a single process, namely, synchrotron emission from internal shocks (IS). Considerations on the compactness of the source imply that the high-energy tail should be produced in a late-emitted shell, characterized by a Lorentz factor greater than the one generating the prompt burst. However, in this hypothesis, the predicted evolution of the synchrotron peak frequency does not agree with the observed soft-to-hard evolution. Given the difficulties of a single-mechanism hypothesis, we test two alternative double-component scenarios. In the first, the prompt burst is explained as synchrotron radiation from IS and the high-energy emission (up to about 1 s following the trigger) as IS synchrotron-self-Compton. In the second scenario, in view of its long duration (∼100 s), the high-energy tail is decoupled from the prompt burst and has an external shock origin. In this case, we show that a reasonable choice of parameters does indeed exist to accommodate the optical-to-GeV data, provided the Lorentz factor of the shocked shell is sufficiently high. Finally, we attempt to explain the chromatic break observed around ∼10³ s with a structured jet model. We find that this might be a viable explanation and that it lowers the high value of the burst energy derived by assuming isotropy, ∼10⁵³ erg, below ∼10⁴⁹ erg, which is more compatible with the energetics from a binary merger progenitor.