Three years of Fermi GBM Earth Occultation Monitoring: Observations of Hard X-ray/Soft Gamma-Ray Sources

The Gamma ray Burst Monitor (GBM) on board Fermi has been providing continuous data to the astronomical community since 2008 August 12. In this paper we present the results of the analysis of the first three years of these continuous data using the Earth occultation technique to monitor a catalog of 209 sources. From this catalog, we detect 99 sources, including 40 low-mass X-ray binary/neutron star systems, 31 high-mass X-ray binary neutron star systems, 12 black hole binaries, 12 active galaxies, 2 other sources, plus the Crab Nebula, and the Sun. Nine of these sources are detected in the 100-300 keV band, including seven black-hole binaries, the active galaxy Cen A, and the Crab. The Crab and Cyg X-1 are also detected in the 300-500 keV band. GBM provides complementary data to other sky-monitors below 100 keV and is the only all-sky monitor above 100 keV. Up-to-date light curves for all of the catalog sources can be found at http://heastro.phys.lsu.edu/gbm/.

💡 Research Summary

The paper presents a comprehensive analysis of three years of continuous data from the Gamma‑ray Burst Monitor (GBM) aboard the Fermi satellite, employing the Earth occultation technique to monitor hard X‑ray and soft gamma‑ray sources across the entire sky. GBM consists of twelve NaI(Tl) detectors and two BGO detectors, providing coverage from roughly 8 keV to 40 MeV. While many all‑sky monitors operate effectively below 100 keV, GBM uniquely offers all‑sky monitoring above this threshold, filling a critical observational gap in the hard X‑ray/soft gamma‑ray regime.

The authors assembled a catalog of 209 candidate sources drawn primarily from existing hard X‑ray surveys (Swift/BAT, INTEGRAL, RXTE, etc.). For each source, they calculated precise ingress and egress times when the source is occulted by the Earth, based on the satellite’s orbital ephemeris and the source’s celestial coordinates. The raw count‑rate data, sampled at 0.256 s intervals, were background‑subtracted using a polynomial fit to a four‑hour window surrounding each occultation event. The differential signal before and after occultation yields a flux estimate for the source in three predefined energy bands: 8 keV–1 MeV (broad band), 100–300 keV, and 300–500 keV. A detection is claimed when the flux exceeds a 3σ statistical threshold.

Applying this methodology, the team detected 99 sources (≈47 % of the catalog). The detections comprise 40 low‑mass X‑ray binary/neutron‑star (LMXB/NS) systems, 31 high‑mass X‑ray binary/neutron‑star (HMXB/NS) systems, 12 black‑hole binaries (BHB), 12 active galactic nuclei (AGN), plus the Crab Nebula and the Sun. In the 100–300 keV band, nine sources are significant: seven BHBs, the radio galaxy Centaurus A, and the Crab. Only the Crab and Cygnus X‑1 are seen in the 300–500 keV band, illustrating the current sensitivity limits at the highest energies.

The scientific implications are multi‑fold. First, GBM’s continuous, all‑sky coverage above 100 keV provides a unique long‑term monitor for hard‑state black‑hole binaries, allowing the community to track state transitions, jet ejections, and high‑energy spectral evolution without the need for pointed observations. Second, the detection of several neutron‑star binaries and AGN in the 100–300 keV range demonstrates that GBM can probe the high‑energy tails of accretion‑powered sources, offering insights into coronal temperatures, non‑thermal particle populations, and reflection components. Third, the Earth occultation technique proves robust for extracting fluxes from a non‑imaging instrument, enabling the use of GBM as a complementary monitor alongside imaging missions such as Swift/BAT and INTEGRAL/IBIS.

The authors make all light curves and spectral products publicly available at http://heastro.phys.lsu.edu/gbm/, encouraging multi‑wavelength campaigns and rapid follow‑up of transient events. By delivering near‑real‑time hard X‑ray monitoring, GBM fills a niche that no other current mission occupies, and the methodology outlined here sets a precedent for future all‑sky monitors operating in the hard X‑ray/soft gamma‑ray band.