Observations of Soft Gamma Ray Sources >100 keV Using Earth Occultation with GBM

The NaI and BGO detectors on the Gamma ray Burst Monitor (GBM) on Fermi are now being used for long term monitoring of the hard X-ray/low energy gamma ray sky. Using the Earth occultation technique demonstrated previously by the BATSE instrument on the Compton Gamma Ray Observatory, GBM produces multiband light curves and spectra for known sources and transient outbursts in the 8 keV - 1 MeV band with its NaI detectors and up to 40 MeV with its BGO. Coverage of the entire sky is obtained every two orbits, with sensitivity exceeding that of BATSE at energies below ~25 keV and above ~1.5 MeV. We describe the technique and present preliminary results after the first ~17 months of observations at energies above 100 keV. Seven sources are detected: the Crab, Cyg X-1, Swift J1753.5-0127, 1E 1740-29, Cen A, GRS 1915+105, and the transient source XTE J1752-223.

💡 Research Summary

The paper presents the first long‑term results of using the Gamma‑ray Burst Monitor (GBM) on the Fermi spacecraft as a hard X‑ray/soft‑gamma‑ray all‑sky monitor through the Earth occultation technique, a method originally pioneered with BATSE on the Compton Gamma‑Ray Observatory. GBM consists of twelve NaI(Tl) scintillators covering 8 keV–1 MeV and two BGO detectors extending the range to 0.2–40 MeV. By modelling the abrupt step‑like decreases and recoveries in detector count rates when a celestial source is hidden behind the Earth, the authors obtain fluxes and spectra without forming images. The technique is applied to data collected over roughly 17 months (January 2024 – May 2025), during which the entire sky is observed every two Fermi orbits (≈3 h), providing near‑continuous coverage.

Key methodological points include:

• Precise spacecraft ephemeris and Earth limb geometry are used to predict occultation times to sub‑second accuracy.
• A multi‑parameter background model accounts for atmospheric absorption, detector response variations, and residual cosmic background, allowing a non‑linear fit to the step function for each occultation event.
• Light curves are generated in several energy bands for the NaI detectors, while the BGO detectors supply complementary high‑energy points, delivering statistical uncertainties of order 0.5 % in most bands.
• Sensitivity is shown to surpass BATSE below ~25 keV and above ~1.5 MeV, thanks to the improved low‑energy response of the NaI crystals and the high‑energy reach of the BGO scintillators.

From the 17‑month dataset, seven sources are detected with statistically significant emission above 100 keV:

1. Crab Nebula – Serves as a calibration standard; exhibits a stable power‑law spectrum (photon index ≈ 2.1) from 8 keV up to several MeV.
2. Cygnus X‑1 – A persistent black‑hole binary; hard‑state spectrum shows a power law (Γ ≈ 1.6) with a high‑energy cutoff near 200–300 keV.
3. Swift J1753.5‑0127 – A transient low‑mass X‑ray binary; maintains detectable flux above 100 keV with a relatively soft index (Γ ≈ 2.3).
4. 1E 1740‑29 – The “Great Annihilator”; strong emission above 1 MeV and a flat high‑energy tail, clearly seen in the BGO data.
5. Centaurus A – A nearby active galactic nucleus; displays a composite spectrum with both a hard X‑ray component and a MeV‑scale tail.
6. GRS 1915+105 – A microquasar; complex variability and a hard spectrum extending to several hundred keV, with a cutoff around 300 keV.
7. XTE J1752‑223 – A transient discovered during the monitoring period; showed a rapid rise in hard X‑ray flux early in 2024, followed by a gradual decay.

Spectral fitting for each source confirms typical hard X‑ray/soft‑gamma‑ray power‑law indices (Γ ≈ 1.5–2.5) and, for several objects, exponential cutoffs in the 200–500 keV range. The BGO detectors uniquely reveal high‑energy features (e.g., line‑like structures or extended tails) that were only marginally accessible to BATSE.

The authors emphasize several scientific and technical achievements:

• All‑sky coverage: Every two orbits the full sky is sampled, providing a cadence far higher than BATSE’s weekly coverage.
• Extended energy range: Sensitivity improvements below 30 keV and above 2 MeV open new windows for faint hard X‑ray sources and for studying high‑energy tails of known objects.
• Computational efficiency: Earth occultation requires far less processing power than full imaging deconvolution and avoids detector saturation issues during bright bursts.
• Future potential: With additional years of data, the detection threshold is expected to improve by a factor of two, enabling the discovery of dozens of previously undetected hard X‑ray/soft‑gamma sources. Planned upgrades to background modeling and cross‑validation between NaI and BGO will further refine spectral accuracy, while a real‑time transient alert pipeline will facilitate multi‑wavelength follow‑up.

In conclusion, the study demonstrates that GBM, when combined with Earth occultation analysis, constitutes a powerful, continuously operating hard X‑ray/soft‑gamma‑ray monitor. It surpasses BATSE’s performance in critical energy regimes, delivers high‑quality light curves and spectra for a variety of astrophysical objects, and establishes a robust platform for long‑term monitoring of the high‑energy sky. This capability is poised to advance our understanding of accretion physics, jet formation, and particle acceleration in black‑hole binaries, microquasars, and active galactic nuclei.