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

TL;DR

This paper discusses the use of the Fermi GBM instrument for long-term monitoring of soft gamma-ray sources above 100 keV using Earth occultation, detecting multiple sources over 17 months.

Contribution

It introduces the Earth occultation technique with GBM for high-energy gamma-ray monitoring and presents initial results from 17 months of observations.

Findings

Seven sources detected, including Crab and Cyg X-1.

Coverage exceeds BATSE sensitivity below 25 keV and above 1.5 MeV.

Technique enables continuous sky monitoring for transient sources.

Abstract

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.