The Fermi GBM Gamma-Ray Burst Catalog: The First Two Years

The Fermi Gamma-ray Burst Monitor (GBM) is designed to enhance the scientific return from Fermi in studying gamma-ray bursts (GRBs). In its first two years of operation GBM triggered on 491 GRBs. We summarize the criteria used for triggering and quantify the general characteristics of the triggered GRBs, including their locations, durations, peak flux, and fluence. This catalog is an official product of the Fermi GBM science team, and the data files containing the complete results are available from the High-Energy Astrophysics Science Archive Research Center (HEASARC).

💡 Research Summary

The paper presents the first two‑year catalog of gamma‑ray bursts (GRBs) detected by the Fermi Gamma‑ray Burst Monitor (GBM). GBM, mounted on the Fermi satellite, continuously surveys the entire sky in the 8 keV–40 MeV energy range using twelve NaI(Tl) detectors and two BGO detectors. During the period from July 2008 to June 2010 the instrument triggered on 491 GRBs. The authors describe in detail the on‑board trigger logic, which evaluates 28 different time‑energy combinations (Δt ranging from 16 ms to 4 s and various energy bands) and requires a significance of at least 4.5σ above background. Once a trigger occurs, automated checks are performed and a human operator validates the event.

For each burst the catalog provides sky location, duration (T₉₀), peak flux, and fluence. Locations are derived from relative count rates among the NaI detectors using a maximum‑likelihood algorithm, yielding typical uncertainties of 3–5 degrees. The duration distribution clearly separates short bursts (T₉₀ ≤ 2 s), which constitute roughly 70 % of the sample, from long bursts (T₉₀ > 2 s). Peak fluxes and fluences are measured on multiple timescales (64 ms, 256 ms, 1 s) and span 10⁻⁸–10⁻⁴ erg cm⁻². Compared with the BATSE catalog, GBM’s lower energy threshold (≈8 keV) results in a higher detection rate of soft, low‑energy GRBs, while the overall sky distribution remains isotropic, showing no significant bias with respect to the Galactic plane.

All data products—including time‑energy spectra, probability sky maps, and a comprehensive set of metadata—are archived in FITS format at the High‑Energy Astrophysics Science Archive Research Center (HEASARC) and are freely accessible. The authors emphasize that this catalog serves as a foundational resource for multi‑wavelength follow‑up, joint analyses with gravitational‑wave detectors, and theoretical modeling of GRB emission mechanisms (internal shocks, external shocks, jet composition). The paper also outlines future improvements to the trigger algorithms, plans for extending the catalog beyond the initial two years, and prospects for coordinated observations with other high‑energy missions.