Leptonic and Hadronic Models for the Extra Components in Fermi-LAT GRBs

TL;DR

This paper investigates the origins of extra GeV spectral components in Fermi-LAT GRBs, comparing leptonic and hadronic models through numerical simulations, and discusses their advantages, weaknesses, and potential solutions like continuous acceleration.

Contribution

It introduces detailed leptonic and hadronic models for GRB spectral components and evaluates their effectiveness using numerical simulations, highlighting the need for continuous acceleration mechanisms.

Findings

Leptonic models can reproduce GeV and X-ray excesses via SSC and synchrotron emissions.

Hadronic models can explain keV-GeV components with secondary electron-positron pairs.

Proton energy requirements are significantly higher in hadronic models, except for specific cases like GRB 090902B.

Abstract

{\it Fermi} satellite has detected extra spectral components in GeV energy range in several GRBs. Those components have power-law shapes, which may contribute to also X-ray band. The limited photon statistics make it difficult to determine the origin of GeV photons, namely internal or external shocks. We try to explain the extra components with our numerical simulations based on internal dissipation picture. Our leptonic model may reproduce not only the GeV excess via SSC emission but also the low-energy excess via the late synchrotron emission from remnant electrons. The hadronic models also reproduce keV-GeV power-law components by synchrotron and SSC emissions from secondary electron-positron pairs. In most cases the hadronic models require a much larger energy of protons than gamma-rays. However, the keV-GeV flat spectra detected in GRB 090902B is well explained with a comparable energy in protons and gamma-rays Finally, we discuss both advantages and weaknesses for both the leptonic and hadronic models. To overcome difficulties in internal dissipation models, we propose introduction of continuous acceleration similar to the second-order Fermi acceleration.