| The Anti-Coincidence Shield SPI-ACS
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The anti-coincidence system of SPI,
developed in the Gamma-ray department of the Max-Planck
Institute for Extraterrestrial Physics, consists of 91 Bismuth
Germanate (BGO) and provides a quasi-omnidirectional field of view
with a large (0.3 square meter) effective area for the detection of
gamma-ray events between ~80 keV and ~10 MeV. Unfortunately, the
electronics impedes a spatial resolution because only the total event
rate of all crystals with 50 ms time resolution is telemetered down to
Earth. The SPI-ACS data do not includes spectral information because
the energy of an interacting gamma-ray photon is not measured. Since
December 2002, SPI-ACS has been an important member of the 3rd
Interplanetary Network of Gamma-ray burst detectors , which
provides burst localizations using the triangulation method.
Herewith, the lack of spatial resolution of the ACS can at least
partly be countervailed. |
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| The Gamm-Ray Burst Selection |
The base sample was constructed from all triggered events in the
SPI-ACS overall rate light curve. Each trigger in this preliminary
sample was subsequently be checked for particle or solar origin using
the INTEGRAL Radiation Monitor (IREM) and the Geostationary
Operational Environmental Satellites (GOES), respectively.
Triangulated events with a localization consistent with the position
of a Soft Gamma-ray Repeater (SGR) were removed from the sample. A
significance threshold was then applied to each event with possible
cosmic origin, such that a trigger was included in the sample of GRB
candidates when its significance, S, above the background, B, exceeds
S=12 sigma in any time interval during the event. Here, 1 sigma
corresponds to 1.57 sqrt(B), where the factor 1.57 takes into account
the measured deviation of the background noise from a Poissonian
distribution. The conservative threshold of S=12 sigma has been
chosen in order to minimize the contamination by weak events of
non-GRB origin. | |
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| The Catalogue |
According to the selection criteria described above, a total of 374
GRB candidates were detected from Oct. 27, 2002 to Jan. 12, 2005. In
addition, 14 GRBs with a significance below the selection threshold
but confirmed by other gamma-ray missions, were detected. From the
total 388 GRB candidates the cosmic origins of 179 have been
confirmed. More information can be found in Rau, von Kienlin,
Hurley & Lichti 2005, A&A, 438, 1175.
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| Events with
T90<0.25s |
The sample shows the known duration bimodality with a strong excess at
the very short end of the distribution (<0.25 s). The origin of this
population of events (~40 % of the total number) is significantly
different from the normal GRB sample, as shown both by the
logN-logCmax distribution as well as by a V/Vmax
test. Observations of simultaneous saturations in the spectrometer Ge
detectors and very short events in the SPI-ACS overall rate suggest a
cosmic ray origin for a significant fraction of these events.
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