2 2009 Fermi Symposium, Washington, D.C., Nov. 2-5
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].
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 5σafter ∼ 490 days
of observations, and one transient source.
III. RESULTS
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-
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.
A. Steady Sources
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
eConf C091122