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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 as demonstrated previously by the BATSE instrument on the Compton Gamma-Ray Observatory, GBM can be used to produce multiband light curves and spectra for known sources and transient outbursts in the 8keV to 1MeV energy range with its NaI detectors and up to 40MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, and the precession of the Fermi orbit allows the entire sky to be viewed every 26 days with sensitivity exceeding that of BATSE at energies below 25keV and above 1.5MeV. We briefly describe the technique and present preliminary results using the NaI detectors after the first two years of observations at energies above 100keV. Eight sources are detected with a significance greater than 7σ: the Crab, Cyg X-1, SWIFT J1753.5-0127, 1E 1740-29, Cen A, GRS 1915+105, and the transient sources XTE J1752-223 and GX 339-4. Two of the sources, the Crab and Cyg X-1, have also been detected above 300keV. © 2011. The American Astronomical Society.

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Astrophysical Journal