First-year Results of Broadband Spectroscopy of the Brightest Fermi-GBM Gamma-Ray Bursts

TL;DR

This study analyzes the temporal and spectral properties of 52 bright gamma-ray bursts observed by Fermi-GBM in its first year, revealing a higher fraction of short GRBs and expanding the duration-energy relationship to high energies.

Contribution

It introduces a new selection methodology for GRB detection thresholds and extends the duration-energy analysis to energies up to 10MeV, comparing results with BATSE data.

Findings

Higher fraction of short GRBs in the GBM sample compared to BATSE.

Confirmed that GRB durations decrease with energy as a power law (~-0.4).

Short GRBs are generally spectrally harder than long GRBs.

Abstract

We present here our results of the temporal and spectral analysis of a sample of 52 bright and hard gamma-ray bursts (GRBs) observed with the Fermi Gamma-ray Burst Monitor (GBM) during its first year of operation (July 2008-July 2009). Our sample was selected from a total of 253 GBM GRBs based on each event peak count rate measured between 0.2 and 40MeV. The final sample comprised 34 long and 18 short GRBs. These numbers show that the GBM sample contains a much larger fraction of short GRBs, than the CGRO/BATSE data set, which we explain as the result of our (different) selection criteria and the improved GBM trigger algorithms, which favor collection of short, bright GRBs over BATSE. A first by-product of our selection methodology is the determination of a detection threshold from the GBM data alone, above which GRBs most likely will be detected in the MeV/GeV range with the Large Area Telescope (LAT) onboard Fermi. This predictor will be very useful for future multiwavelength GRB follow ups with ground and space based observatories. Further we have estimated the burst durations up to 10MeV and for the first time expanded the duration-energy relationship in the GRB light curves to high energies. We confirm that GRB durations decline with energy as a power law with index approximately -0.4, as was found earlier with the BATSE data and we also notice evidence of a possible cutoff or break at higher energies. Finally, we performed time-integrated spectral analysis of all 52 bursts and compared their spectral parameters with those obtained with the larger data sample of the BATSE data. We find that the two parameter data sets are similar and confirm that short GRBs are in general harder than longer ones.