High energy emission and polarisation limits for the INTEGRAL burst GRB?061122

摘要

Context. GRB061122 is one of the brightest GRBs detected within INTEGRAL's field of view to date, with a peak flux (20-200keV) of 32photonscm-2s-1 and fluence of ergcm-2. The Spectrometer aboard INTEGRAL, SPI, can measure linear polarisation in bright GRBs through the process of Compton scattering in the Germanium detectors. Polarisation measurements of the prompt emission are relatively rare. The spectral and polarisation results can be combined to provide vital information about the circumburst region. Aims. The two -ray detectors on INTEGRAL were used to investigate the spectral characteristics of GRB061122. A search for linear polarisation in the prompt emission was carried out on GRB061122 using the SPI multiple event data in the energy range 100keV-1MeV. The X-ray properties were examined using data from the X-Ray Telescope (XRT) on Swift. Methods. The -ray spectral and temporal properties of GRB061122 were determined using IBIS and SPI. The afterglow properties were obtained using XRT. The multiple event data of GRB061122 from SPI were analysed and compared with the predicted instrument response obtained from Monte-Carlo simulations using the GEANT4 INTEGRAL mass model. The distributions between the real and simulated data as a function of the percentage polarisation and polarisation angle were calculated and limits on the level and angle of polarisation were obtained from the best-fit value of.Results. The prompt spectrum was best fit by a combination of a blackbody and a power-law model (the quasithermal model), with evidence for high energy emission continuing above 8MeV. A pseudo-redshift value of pz=was determined using the spectral fit parameters. The isotropic energy at this pseudo-redshift is erg. The jet opening angle was estimated to be smaller thanor larger thanfrom the X-ray lightcurve. An upper limit of60% polarisation was determined for the prompt emission of GRB061122, using the multiple event data from the spectrometer on INTEGRAL. Conclusions. The high energy emission observed in the spectrum may be due to the reverse shock interacting with the GRBejecta when it is decelerated by the circumburst medium. This behaviour has been observed in a small fraction of GRBs to date, but is expected to be more commonly observed by the Fermi Gamma-ray Space Telescope. The conditions for polarisation are met if the jet opening angle is less than ,but further constraints on the level of polarisation are not possible. Key words: polarization - gamma rays: bursts - gamma rays: obsrevations