FIG. 1: Single Crab occultation step in a single GBM NaI detector. Horizontal scale is in seconds centered on the occultation time. Vertical scale is in measured counts. The shape of the individual occultation steps de- pends on energy and occultation angle. Transmis- sion as a function of time is modeled as T ( t ) = exp [ − µ ( E ) A ( h )], where µ ( E ) is the mass attenuation coefficient of gamma rays at energy E in air and A ( h ) is the air mass along the line of sight at a given alti- tude h ( t ). Account is taken of the detector response as it changes as a function of angle across the fit win- dow. For each source, occultation times are predicted. Each step is fit over a 4-minute window along with a quadratic background and using an assumed spectrum to determine the detector count rate due to the source. The instrument response is used to convert the count rate to a flux. Up to 31 steps are possible for a given source in a day, and these steps are summed to get a single daily average flux. The GBM occultation sensi- tivity exceeds that of BATSE at energies below ∼ 25 keV and above ∼ 1 . 5 MeV [5]. FIG. 2: Crab light curve. Horizontal scale is in modified Julian days over the 490 day GBM exposure period. Ver- tical scale is in photons/cm $^{2}$/sec/keV averaged over daily intervals. Horizontal lines show the average flux in each of five energy bands increasing from top to bottom eral sources over the same time intervals in ref. [2], where it is seen that the results measured by the two instruments compare well. At energies above the up- per energy limit of ∼ 195 keV of the Swift 22-month catalog [6], however, the GBM observations provide the only wide-field monitor available of the low en- ergy gamma ray sky. The sources Crab, Cyg X-1, Swift J1753.5-0127, 1E 1740-29, Cen A, and GRS 1915+105 are detected by GBM at energies above 100 keV. We show GBM light curves generated from the Earth occultation analysis in several energy bands with one day resolution for these six sources in Figures 2 - 7. Table I gives the fluxes and significances averaged over all the days from Aug. 12, 2008 (the beginning of science operations) to Dec. 15, 2009, approximately 490 days. The Crab (Fig. 2) spectrum in the hard x-ray/low energy gamma-ray region can be described by a bro- ken power law, with the spectrum steepening at 100 keV and then hardening at 650 keV [7, 8]. While the GBM CTIME data do not have the spectral resolution The results presented here are preliminary. We have not completed the fine tuning of our algorithms, though the average fluxes are not expected to change much. Future work will include using the GBM CSPEC data, with its finer energy binning, to exam- ine the detailed spectra for these sources. The measured 20 - 50 keV GBM light curves are compared to Swift’s 15 - 50 keV light curves for sev- III. RESULTS A. Steady Sources This work uses the GBM CTIME data, with its 8 broad energy channels and 0.256-second resolution, rebinned to 2-second resolution. The occultation tech- nique relies on an input catalog of known sources. Currently, we are monitoring 64 sources. Of these 64 sources, 6 steady sources are detected above 100 keV with a significance of at least 5 σ after ∼ 490 days of observations, and one transient source. eConf C091122 2 2009 Fermi Symposium, Washington, D.C., Nov. 2-5