Hadronic Models for the Extra Spectral Component in the short GRB 090510

A short gamma-ray burst GRB 090510 detected by {\it Fermi} shows an extra spectral component between 10 MeV and 30 GeV, an addition to a more usual low-energy ($<10$ MeV) Band component. In general, such an extra component could originate from accelerated protons. In particular, inverse Compton emission from secondary electron-positron pairs and proton synchrotron emission are competitive models for reproducing the hard spectrum of the extra component in GRB 090510. Here, using Monte Carlo simulations, we test the hadronic scenarios against the observed properties. To reproduce the extra component around GeV with these models, the proton injection isotropic-equivalent luminosity is required to be larger than $10^{55}$ erg/s. Such large proton luminosities are a challenge for the hadronic models.

💡 Research Summary

The paper investigates the origin of an additional high‑energy spectral component observed in the short gamma‑ray burst GRB 090510, as detected by the Fermi satellite. While the bulk of the prompt emission below ~10 MeV is well described by the conventional Band function, the LAT instrument revealed a distinct power‑law extending from roughly 10 MeV up to 30 GeV with a photon index of about –1.6. The authors explore whether this component can be produced by hadronic processes involving ultra‑relativistic protons accelerated in the internal dissipation region of the burst. Two specific mechanisms are examined: (1) inverse‑Compton (IC) scattering by secondary electron‑positron pairs generated in photomeson (pγ) interactions, and (2) synchrotron radiation directly from the accelerated protons themselves.

To test these scenarios, the authors construct a Monte Carlo simulation that follows the full cascade of particle interactions. The model inputs include the variability timescale (Δt ≈ 0.01 s), emission radius (R ≈ 10¹³ cm), magnetic field strength (B), and the isotropic‑equivalent luminosity of the injected protons (Lₚ). The simulation tracks pγ collisions, pion production, pion decay into muons and neutrinos, subsequent muon decay into secondary e⁺e⁻ pairs, and the radiative cooling of all charged particles via synchrotron and IC processes. Photon–photon (γγ) annihilation is also accounted for, ensuring that the emergent spectrum respects internal opacity constraints.

The results show that reproducing the observed GeV flux with either mechanism demands an extremely large proton power. For the IC‑dominated case, the secondary pairs must up‑scatter the abundant MeV photons efficiently, which requires a proton injection luminosity Lₚ ≥ 10⁵⁵ erg s⁻¹. The magnetic field needed to keep the pairs in the fast‑cooling regime is modest (B ≈ 10⁴ G), but the required proton energy budget remains prohibitive. In the proton‑synchrotron scenario, achieving synchrotron photons in the GeV range demands both a very high magnetic field (B ≈ 10⁵–10⁶ G) and proton Lorentz factors extending to PeV energies. Even under these extreme conditions, the model still needs Lₚ ≈ 10⁵⁵ erg s⁻¹ to match the data.

These luminosities exceed the total isotropic gamma‑ray energy inferred for GRB 090510 (≈10⁵³ erg) by two orders of magnitude, implying an unrealistically high conversion efficiency from the central engine to relativistic protons. Moreover, such intense hadronic activity would inevitably produce a detectable flux of high‑energy neutrinos, yet no neutrino counterpart was reported by IceCube for this burst. The required magnetic fields also lie above typical values expected in internal shock or magnetic reconnection models. Consequently, the authors conclude that while hadronic processes can in principle generate a hard GeV component, the energetic demands render them implausible for GRB 090510 under standard GRB fireball assumptions. They suggest that alternative explanations—such as leptonic synchrotron self‑Compton, external shock emission, or more exotic acceleration sites—are more viable, and they emphasize that future observations with next‑generation γ‑ray telescopes (e.g., CTA) and neutrino detectors (IceCube‑Gen2) will be crucial for discriminating between these models.