A COMPREHENSIVE ANALYSIS OF FERMI GAMMA-RAY BURST DATA. I. SPECTRAL COMPONENTS AND THE POSSIBLE PHYSICAL ORIGINS OF LAT/GBM GRBs

Abstract

We present a systematic analysis of the spectral and temporal properties of 17 gamma-ray bursts (GRBs) codetected by the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT) onboard the Fermi satellite in 2010 May. We performed a time-resolved spectral analysis of all the bursts, with the finest temporal resolution allowed by statistics, to reduce temporal smearing of different spectral components. We found that the time-resolved spectra of 14 out of 17 GRBs are best modeled with the classical "Band" function over the entire Fermi spectral range, which may suggest a common origin for emissions detected by the LAT and GBM. GRB 090902B and GRB 090510 require the superposition of an MeV component and an extra power-law component, with the former having a sharp cutoff above E p . For GRB 090902B, this MeV component becomes progressively narrower as the time bin gets smaller, and can be fit with a Planck function as the time bin becomes small enough. In general, we speculate that, phenomenologically, there may be three elemental spectral components that shape the time-resolved GRB spectra: a Band-function component (e.g., in GRB 080916C) that extends over a wide energy range and does not narrow with decreasing time bins, which may be of non-thermal origin; a quasi-thermal component (e.g., in GRB 090902B), with spectra progressively narrowing with reducing time bins; and another non-thermal power-law component extending to high energies. The spectra of different bursts may be decomposed into one or more of these elemental components. We compare this sample with the Burst and Transient Source Experiment sample and investigate some correlations among spectral parameters. We discuss the physical implications of the data analysis results for GRB prompt emission, including jet composition (matter-dominated versus Poynting-flux-dominated outflow), emission sites (internal shock, external shock, or photosphere), as well as radiation mechanisms (synchrotron, synchrotron self-Compton, or thermal Compton upscattering).