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Title: Upper Limits on High Energy emissions from GRB
Authors: Barbiellini Amidei, Guido
Longo, Francesco
Moretti, Elena
Keywords: High energy astrophysics, GRB, upper limits
Issue Date: 12-Mar-2010
Publisher: Università degli studi di Trieste
Abstract: The intense and unpredictable flashes of gamma rays in the energy band (10 keV – 1 MeV), called Gamma-Ray Bursts (GRB), were discovered in the late 60's. Since then several experiments were dedicated to detect and understand these phenomena. Up to now, we do not have yet a complete explanation for the GRB progenitors and their emission mechanism. In the first phase, the so-called prompt phase, lasting from few ms to tens of seconds, these bursts emit mainly in the band from hard-X to soft gamma. In a longer second phase, called afterglow, the GRB emission ranges from the radio frequencies to the X-ray band. The hard gamma band (>50 MeV), both in the prompt and in the afterglow phase, was poorly explored until the gamma-ray experiment EGRET flown on the Compton Gamma-ray Observatory (CGRO). Nevertheless EGRET detected only 5 GRBs in the band >200 MeV in 7 years of operation. Nowadays 2 gamma-ray experiments AGILE and Fermi/LAT are currently in operation. The number of detected burst with emitted energy >50 MeV is already more than duplicated by these two missions. The two experiments are based on the same high energy gamma-ray detection technique so these two experiments are similar: their core is made of a silicon tracker with tungsten conversion layers, surrounded by a plastic scintillator to veto cosmic-ray particles events. Below the tracker, a calorimeter provides the measure of the energy of the produced pairs. The main differences between the experiments are the larger effective area of the Fermi/LAT (~10 times larger) and its deeper calorimeter. On board of the satellites that host LAT and AGILE there are other 2 experiments respectively: the Fermi/GBM dedicated to the GRB science in the 8 keV-40 MeV band and the SuperAGILE that is a X-ray detector operational in the 18-60 keV band. Fermi/GBM, SuperAGILE and the Mini Calorimeter in the AGILE mission can independently trigger on a burst event respectively in the energy band (8 keV – 40 MeV), (18-60 keV) and (0.3-100 MeV). Their FoV is quite different however, ranging from 2 sr for SuperAGILE to almost 4 sr for MiniCalorimeter and 6 sr for FermiGBM. If the burst, triggered by these instruments or by other missions, is in the field of view of one of the two gamma-ray detectors a high energy signal is searched. In the AGILE pipeline the GRB signal is searched in the burst prompt time interval. During this time interval both background and signal are supposed to follow a Poisson distribution and the signal to be non-negative. The background average rate is computed before the burst trigger, in the same signal extraction region (15deg from the GRB position), with the same analysis cuts and in a time interval at least 10 times longer than the signal duration. Instead in the Fermi/LAT pipeline a map of the test statistic variable is computed. The test statistic distribution indicates how much the data differ from the background model used. In this thesis the non-detection cases are considered: a methodology for the computation of the upper limit on the signal is proposed. This method is based on the Bayesian statistics and was elaborated from Helene in 1984 (Helene, O. 1984, Nuclear Instruments and Methods 228, 120), it considers a Poisson fluctuation of the known background mean and of the estimated signal in the region of interest. The applications of this upper limit computing method to the AGILE and the Fermi/LAT data are also showed deriving upper limit on GRB flux. The AGILE energy coverage is smaller but starts from lower energy with respect the actual Fermi/LAT energy band. In the AGILE energy range above 30 MeV and till 2 GeV, the estimated GRB flux upper limits range between 1x10-3 and 1x10-2 ph s-1cm-2. Instead the Fermi/LAT flux upper limit is roughly 5x10-5 ph s-1cm-2 in the energy range from 100 MeV to 100 GeV. The studies of the upper limits help to understand the GRB emission mechanisms: most of these bursts are not detected in the highest energy band even if the extrapolation of their spectra from the low energy band predicts a detectable flux from those two instruments. On the other case there are some GRBs with low energy spectra predicting a non detectable H.E. flux but with high energy photons clearly detected. These photons indicate the existence of a new component above 100 MeV in the GRB photon spectrum extending up to the GeV region. This thesis gives a new contribution on the computation of the upper limits on the GRB flux in both the gamma-ray experiments operating nowadays. The thesis will concentrate in particular on the study of the upper limits in the interesting cases, when a high energy signal is predicted but not detected, giving some interesting hints on the GRB source physics.
Description: 2008/2009
NBN: urn:nbn:it:units-7513
Appears in Collections:Scienze fisiche

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