First Results from Fermi GBM Earth Occultation Monitoring: Observations of Soft Gamma-Ray Sources Above 100 keV

摘要

The NaI and BGO detectors on the Gamma-ray Burst Monitor (GBM) on Fermi arenow being used for long-term monitoring of the hard X-ray/low energy gamma-raysky. Using the Earth occultation technique as demonstrated previously by theBATSE instrument on the Compton Gamma-Ray Observatory, GBM can be used toproduce multiband light curves and spectra for known sources and transientoutbursts in the 8 keV to 1 MeV energy range with its NaI detectors and up to40 MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, andthe precession of the Fermi orbit allows the entire sky to be viewed every ~26days with sensitivity exceeding that of BATSE at energies below ~25 keV andabove ~1.5 MeV. We briefly describe the technique and present preliminaryresults using the NaI detectors after the first two years of observations atenergies above 100 keV. Eight sources are detected with a significance greaterthan 7 sigma: the Crab, Cyg X-1, SWIFT J1753.5-0127, 1E 1740-29, Cen A, GRS1915+105, and the transient sources XTE J1752-223 and GX 339-4. Two of thesources, the Crab and Cyg X-1, have also been detected above 300 keV.