Gamma-Ray Astrophysics NSSTC Fermi GBM

The Fermi Gamma-Ray Burst Monitor (GBM) is not a pointed or imaging instrument. To determine fluxes for known sources, we measure the change in the count rate observed in the NaI (or BGO) detectors when the source enters or exits Earth occultation. The measured counts in each energy channel are converted to fluxes using an assumed spectrum for each source. For these measurements we currently use CTIME data, with 0.256-s resolution and 8 energy channels covering 8 keV to 1 MeV. Our technique uses all 8 energy channels, but for most sources the majority of the signal is in the 12-25 keV and 25-50 keV bands. The fluxes listed in the table are for the 12-25 keV band and are the mission long average, the 5-day average flux for the most recent 5 days, and the 2-day average flux for the most recent 2 days. The fluxes are normalized to units of mCrab based on the Crab flux over the period from MJD 54690 to 54790, during which the Crab flux is relatively constant as measured by GBM.