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.
I. INTRODUCTION
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.
II. GBM AND THE EARTH OCCULTATION
OBSERVATIONAL TECHNIQUE
The Gamma ray Burst Monitor is the secondary
instrument onboard the Fermi satellite [1, 2]. It con-
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.
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
arXiv:1001.0955v2  [astro-ph.HE]  6 Jan 2010