2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1 Observations of Soft Gamma Ray Sources > 100 keV Using Earth Occultation with GBM G.L. Case, M.L. Cherry, J. Rodi Dept. of Physics & Astronomy, Louisiana State Univ., Baton Rouge, LA 70803, USA A. Camero-Arranz Fundaci´ on Espa˜ nola de Ciencia y Tecnolog´ıa (MICINN), C/Rosario Pino,14-16, 28020-Madrid, Spain E. Beklen Middle East Technical University (METU), 06531, Ankara, Turkey C. A. Wilson-Hodge NASA Marshall Space Flight Center, Huntsville, AL 35812 P. Jenke NASA Postdoctoral Program Fellow, NASA Marshall Space Flight Center, Huntsville, AL 35812 P.N. Bhat, M.S. Briggs, V. Chaplin, V. Connaughton, R. Preece University of Alabama in Huntsville, Huntsville, AL 35899 M.H. Finger USRA, National Space Science and Technology Center, Huntsville, AL 35899 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. The Gamma ray Burst Monitor (GBM) on Fermi is currently the only instrument in orbit providing nearly continuous full sky coverage in the hard X-ray/low energy gamma ray energy range. The Earth occul- tation technique, used very successfully on BATSE, has been adapted to GBM. An initial catalog of 64 sources is currently being monitored and continuously augmented. At energies above 100 keV, six steady sources (the Crab, Cyg X-1, Swift J1753.5-0127, 1E 1740-29, Cen A, GRS 1915+105) and one transient source (XTE J1752-223) have been detected in the first year of observation. We describe the instrument, outline the technique, and present light curves for the seven sources. sists of 12 NaI detectors 5 ′′ in diameter by 0.5 ′′ thick mounted on the corners of the spacecraft and oriented such that they view the entire sky not occulted by the Earth. GBM also contains 2 BGO detectors 5 ′′ in di- ameter by 5 ′′ thick located on opposite sides of the spacecraft. None of the GBM detectors have direct imaging capability. The Gamma ray Burst Monitor is the secondary instrument onboard the Fermi satellite [1, 2]. It con- Known sources of gamma ray emission can be mon- itored with non-imaging detectors using the Earth oc- cultation technique, as was successfully demonstrated with BATSE [3, 4]. When a source of gamma rays is occulted by the Earth, the count rate measured by the detector will drop, producing a step-like feature. When the source reappears from behind the Earths limb, the count rate will increase, producing another step. The diameter of the Earth seen from Fermi is ∼ 140 $^{◦}$, so roughly 30% of the sky is occulted by the Earth at any one time. Coupled with the ± 35 ◦ slew- ing of the pointing direction every orbit, this means that the entire sky is occulted every two orbits. With an altitude of 565 km, a period of 96 minutes, and an orbital inclination of 26 . 5 $^{◦}$, individual occultation steps last for ∼ 10 seconds (Fig. 1). eConf C091122 II. GBM AND THE EARTH OCCULTATION OBSERVATIONAL TECHNIQUE I. INTRODUCTION arXiv:1001.0955v2 [astro-ph.HE] 6 Jan 2010