105 research outputs found
Activation in the COMPTEL double-scattering gamma-ray telescope
Abstract-The COMPTEL gamma-ray telescope has been operating in low Earth orbit for six years, since the launch of the Compton Gamma-Ray Observatory in April 1991. Comparisons of data for different orbits and epochs show evidence of activation on time scales from minutes (27Mg, q,2=9.5 min) to years C2Na, q&.58 yr). The activation is correlated with both the orbital altitude and solar cosmic-ray modulation. Because it requires coincident measurements in two different detectors, COMPTEL is most susceptible to instrumental background events in which two or more photons are produced simultaneously
COMPTEL measurements of MeV gamma-ray burst spectra
We present results from the on-going spectral analysis of gamma-ray bursts measured by the COMPTEL instrument in its main Compton âTelescopeâ observing mode (0.75â30 MeV). Thus far, 18 bursts have been analyzed from three years (April 1991âApril 1994) of observations. The time-averaged spectra of these events above 1 MeV are all consistent with a simple power law model with spectral index in the range 1.5â3.5. Exponential, thermal bremsstrahlung and thermal synchrotron models are statistically inconsistent with the burst sample, although they can adequately describe some of the individual burst spectra. We find good agreement between burst spectra measured simultaneously by BATSE, COMPTEL and EGRET, which typically show a spectral transition or âbreakâ in the BATSE energy range around a few hundred keV followed by simple power law emission extending to hundreds of MeV. However, the temporal relation between MeV and GeV (e.g., as measured by EGRET) burst emission is still unclear. Measurement of rapid variability at MeV energies in the stronger bursts provides evidence that either the sources are nearby (within the Galaxy) or the gamma-ray emission is relativistically beamed
The angular distribution of COMPTEL Gamma-Ray bursts
The superior burst location capability of the COMPTEL instrument aboard the Compton Gamma-Ray Observatory allows us to study the small-scale angular distribution of burst sources with good sensitivity even though the number of burst detections is small. We accumulate four years (April 1991âApril 1995) of observations to form a catalog of 27 burst locations whose mean 1Ï uncertainty is âŒ1°. We find that the COMPTEL bursts are consistent with an isotropic distribution of sources, yet the spatial coincidence of two of the bursts within COMPTELâs angular resolution indicates the possibility of repetition. This possibility is studied using the two-point angular correlation function and the nearest neighbor statistic. Model dependent upper limits on the fraction of repeating sources are derived
A time dependent model for the activation of COMPTEL
The structure of the CGRO satellite is irradiated by cosmic rays and trapped particles fromradiation belts. These incident particles produce radioactive nuclei in nuclear reactions with the satellite structure. Most of the radiation dose can be attributed to the passages through the South Atlantic Anomaly. The incident particle flux on the COMPTEL instrument is estimated from the event rate of a plastic scintillation detector. This event rate is modeled with a Neural Network simulation. The increase of the event rate during SAA passages is taken as a measure for the amount of induced radioactivity. A Neural Network Model is used to derive the buildup ofradioactive nuclei in the instrument over the first five years of the mission. Measurements of the internal 22Na - and 24Na-activity are used to estimate the proton flux in the SAA. The result is consistent with earlier measurements and models
Observations of the 1991 June 11 solar flare with COMPTEL
The COMPTEL instrument onboard of the Compton GammaâRay Observatory (CGRO) is sensitive to Îłârays in the energy range from 0.75 to 30 MeV and to neutrons in the energy range from 10 to 100 MeV.
During the period of unexpectedly high solar activity in June 1991, several flares from active region 6659 were observed by COMPTEL. For the flare on June 11, we have analyzed the COMPTEL telescope data, finding strong 2.223 MeV line emission, that declines with a time constant of 11.8 minutes during the satellite orbit in which the flare occurs. It remains visible for at least 4 hours. We obtained preliminary values for the 2.2 MeV and 4â7 MeV fluences. Neutrons with energies above 20 MeV have been detected and their arrival time at the Earth is consistent with the Îłâray emission during the impulsive phase
COMPTEL observations of gammaâray flares in October 1991
The COMPTEL experiment on GRO images 0.75â30 MeV celestial gammaâradiation that falls within its 1 steradian field of view. During observation 12 (primary target Cen A) in October 1991 the sun had been in the fov and several solar flares associated with the active region 6891 had been observed. Time profile and energy spectra had been produced, using COMPTELâs primary mode of operation (the telescope mode). Additionally the number of counts received in the D2âsingle burst detector (the secondary mode of operation) are given. We summarize the preliminary results on all of these flares
MeV measurements of gamma-ray bursts by CGRO-COMPTEL
Since the launch of the Compton Gamma-Ray Observatory in April 1991, the imaging COMPTEL telescope has accumulated positions and 0.75â30 MeV spectra of more than thirty gamma-ray bursts within its âŒÏ sr field of view. In an ongoing collaboration with BACODINE/GCN, COMPTEL positions are relayed to a global network of multiwavelength observers in near real time (âŒ10 minutes). Here we summarize the MeV properties, and present spatial, spectral, and temporal data for the latest of these events, GRB 970807. In concurrence with earlier SMM and current BATSE, OSSE, and EGRET measurements, COMPTEL data add to the accumulating evidence that GRB spectra do seem to have a characteristic shape: a peak (inE2F(E) ) around several hundred keV; and a power law above (spectral index 1.5â3.5) extending beyond the COMPTEL energy range
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
COMPTELâs solar flare catalog
COMPTEL, the imaging gammaâray telescope, capable of detecting gamma rays in the range of 0.1â30 MeV, is one of four instruments aboard NASAâs Compton GammaâRay Observatory. The Comptel burst detectors (single Defector Mode) have a field of view of âŒ2.5 Ï sr. These detectors of COMPTEL permit measurements of energy spectra and time histories of solar flare gammaâray emission. A search through the Single Detector Modeâs data is being conducted. We summarize the preliminary results of this search
First results of the BATSE/COMPTEL/NMSU rapid burst response campaign
The Imaging Compton Telescope (COMPTEL) on board the Compton Gamma Ray Observatory regularly observes gammaâray bursts which occur inside the instrumentâs âŒ1 sr fieldâofâview. COMPTEL images bursts in the 0.75â30 MeV energy range with a typical location accuracy of 1â3 degrees, depending on burst strength, position, duration, and spectrum. COMPTELâs imaging capability has been exploited in order to search for fading gammaâray burst counterparts at other wavelengths through the establishment of a BATSE/COMPTEL/NMSU rapid burst response campaign. This campaign utilizes near realâtime identification and preliminary burst location by BATSE, accelerated COMPTEL imaging, and a worldâwide network of observers to search COMPTEL error boxes as quickly as possible. Timely, deep searches for lingering counterpart emission of several bursts per year are the realized goal of this campaign. During its first year of operation, the rapid response program has been successfully applied to two strong bursts: GRB 930131 and GRB 930309. These bursts were imaged in record time only hours after their occurrence. Subsequently, several observations were made at radio and optical observatories worldâwide
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