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Register a new userThe spectral catalogue of INTEGRAL gamma-ray bursts: results of the joint IBIS/SPI spectral analysis
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We present the updated INTEGRAL catalogue of gamma-ray bursts (GRBs) observed between December 2002 and February 2012. The catalogue contains the spectral parameters for 59 GRBs localized by the INTEGRAL Burst Alert System (IBAS). We used the data from the two main instruments on board the INTEGRAL satellite: the spectrometer SPI (SPectrometer on INTEGRAL) nominally covering the energy range 18 keV - 8 MeV, and the imager IBIS (the Imager on Board the INTEGRAL Satellite) operating in the range from 15 keV to 10 MeV. For the spectral analysis we applied a new data extraction technique, developed in order to explore the energy regions of highest sensitivity for both instruments, SPI and IBIS. It allowed us to perform analysis of the GRB spectra over a broad energy range and to determine the bursts spectral peak energies. The spectral analysis was performed on the whole sample of GRBs triggered by IBAS, including all the events observed in period December 2002 - February 2012. The catalogue contains the trigger times, burst coordinates, positional errors, durations and peak fluxes for 28 unpublished GRBs observed between September 2008 and February 2012. The light curves in 20 - 200 keV energy band of these events were derived using IBIS data. We compare the prompt emission properties of the INTEGRAL GRB sample with the BATSE and Fermi samples.
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We present a catalogue with the properties of all the bursts detected and localized by the IBIS instrument onboard the INTEGRAL satellite from November 2002 to September 2008. The sample is composed of 56 bursts, corresponding to a rate of ~ 0.8 GRB per month. Thanks to the performances of the INTEGRAL Burst Alert System, 50% of the IBIS GRBs have detected afterglows, while 5% have redshift measurements. A spectral analysis of the 43 bursts in the INTEGRAL public archive has been carried out using the most recent software and calibration, deriving an updated, homogeneous and accurate catalogue with the spectral features of the sample. When possible also a time-resolved spectral analysis has been carried out. The GRBs in the sample have 20-200 keV fluences in the range 5 x 1E-8 --2.5 x 1E-4 erg cm-2, and peak fluxes in the range 0.11 - 56 ph cm-2 s-1. While most of the spectra are well fitted by a power law with photon index ~ 1.6, we found that 9 bursts are better described by a cut-off power law, resulting in Ep values in the range 35--190 keV. Altough these results are comparable with those obtained with BAT onboard Swift, there is a marginal evidence that ISGRI detects dimmer bursts than Swift/BAT. Using the revised spectral parameters and an updated sky exposure map that takes into account also the effects of the GRB trigger efficiency, we strengthen the evidence for a spatial correlation with the super galactic plane of the faint bursts with long spectral lag (Foley et al.,2008).
The emission process responsible for the so-called prompt emission of gamma-ray bursts is still unknown. A number of empirical models fitting the typical spectrum still lack a satisfactory interpretation. A few GRB spectral catalogues derived from past and present experiments are known in the literature and allow to tackle the issue of spectral properties of gamma-ray bursts on a statistical ground. We extracted and studied the time-integrated photon spectra of the 200 brightest GRBs observed with the Gamma-Ray Burst Monitor which flew aboard the BeppoSAX mission (1996-2002) to provide an independent statistical characterisation of GRB spectra. The spectra were fit with three models: a simple power-law, a cut-off power law or a Band function. The typical photon spectrum of a bright GRB consists of a low-energy index around 1.0 and a peak energy of the nuFnu spectrum E_p~240 keV in agreement with previous results on a sample of bright CGRO/BATSE bursts. Spectra of ~35% of GRBs can be fit with a power-law with a photon index around 2, indicative of peak energies either close to or outside the GRBM energy boundaries. We confirm the correlation between E_p and fluence, with a logarithmic dispersion of 0.13 around the power-law with index 0.21+-0.06. The low-energy and peak energy distributions are not yet explained in the current literature. The capability of measuring time-resolved spectra over a broadband energy range, ensuring precise measurements of parameters such as E_p, will be crucial for future experiments (abridged).
A Thesis Submitted to the Tata Institute of Fundamental Research, Mumbai for the degree of Doctor of Philosophy in Physics (supervisor: Prof. A. R. Rao)
We explain the results of Yu et al. (2015b) of the novel sharpness angle measurement to a large number of spectra obtained from the Fermi gamma-ray burst monitor. The sharpness angle is compared to the values obtained from various representative emission models: blackbody, single-electron synchrotron, synchrotron emission from a Maxwellian or power-law electron distribution. It is found that more than 91% of the high temporally and spectrally resolved spectra are inconsistent with any kind of optically thin synchrotron emission model alone. It is also found that the limiting case, a single temperature Maxwellian synchrotron function, can only contribute up to 58+23 -18% of the peak flux. These results show that even the sharpest but non-realistic case, the single-electron synchrotron function, cannot explain a large fraction of the observed spectra. Since any combination of physically possible synchrotron spectra added together will always further broaden the spectrum, emission mechanisms other than optically thin synchrotron radiation are likely required in a full explanation of the spectral peaks or breaks of the GRB prompt emission phase.
We present the sample of gamma-ray bursts detected with the anti-coincidence shield ACS of the spectrometer SPI on-board INTEGRAL for the first 26.5 months of mission operation (up to Jan 2005). SPI-ACS works as a nearly omnidirectional gamma-ray burst detector above ~80 keV but lacks spatial and spectral information. In this catalogue, the properties derived from the 50 ms light curves (e.g., T90, Cmax, Cint, variability, V/Vmax) are given for each candidate burst in the sample. A strong excess of very short events with durations <0.25 s is found. This population is shown to be significantly different from the short- and long-duration burst sample by means of the intensity distribution and V/Vmax test and is certainly connected with cosmic ray hits in the detector. A rate of 0.3 true gamma-ray bursts per day is observed.
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