GBM Occultation Flux Histories

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.

Description of plots:

Four energy bands are plotted for each source. Daily, two-day, or four-day averages are plotted, depending upon the source intensity. Red diamonds denote > 5-sigma statistical significance (possible detections), blue diamonds denote < -3-sigma statistical significance (usually an indication of interference or background problems). Each plot shows the long-term average as a dashed line, which is shown in pink if the long-term average statistical significance is > 10 sigma. Long-term averages include all plotted data, usually for the full Fermi mission to date, except for recently added sources.

Caveats:

1. These light curves are quick-look data only for observation planning purposes. Please contact the occultation project PI colleen.wilson@nasa.gov for more detailed analysis.
2. The position of the Sun is calculated at the time of each occultation, so it is not listed in the main table.
3. Sources in the galactic center region suffer from source confusion. Please treat galactic center sources with caution and correlate with other instruments before relying on these light curves.
4. Because the position of the Earth's limb on the sky repeats at the precession period of the Fermi orbit, apparent periods in the range of 50-55 days (or multiples thereof) may be present in the data and are likely to be spurious.
5. Significant negative (blue points) and positive (red points) close in time are often indicative of source interference problems. For example, the very bright outburst of A0535+262 in December 2009 caused such problems in several sources.

We ask that when results from these analyses are used, they be referenced as "Fermi GBM Earth occultation results provided by the Fermi GBM Earth occultation Guest Investigation teams at NASA/MSFC and LSU."