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

¹ UCD School of Physics, University College Dublin, Dublin 4, Ireland
² Department of Physics, Royal Institute of Technology (KTH), AlbaNova University Centre, 10691 Stockholm, Sweden e-mail: smcglynn@particle.kth.se
³ Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
⁴ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Received: 5 September 2008
Accepted: 24 March 2009

Abstract

Context. GRB 061122 is one of the brightest GRBs detected within INTEGRAL's field of view to date, with a peak flux (20–200 keV) of 32 photons cm^-2 s^-1 and fluence of 2 10^-5 erg cm^-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 γ-ray detectors on INTEGRAL were used to investigate the spectral characteristics of GRB 061122. A search for linear polarisation in the prompt emission was carried out on GRB 061122 using the SPI multiple event data in the energy range 100 keV–1 MeV. The X-ray properties were examined using data from the X-Ray Telescope (XRT) on Swift.

Methods. The γ-ray spectral and temporal properties of GRB 061122 were determined using IBIS and SPI. The afterglow properties were obtained using XRT. The multiple event data of GRB 061122 from SPI were analysed and compared with the predicted instrument response obtained from Monte-Carlo simulations using the GEANT 4 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 8 MeV. A pseudo-redshift value of pz = 0.95 ± 0.18 was determined using the spectral fit parameters. The isotropic energy at this pseudo-redshift is 8.510⁵² erg. The jet opening angle was estimated to be smaller than 2.8° or larger than 11.9° from the X-ray lightcurve. An upper limit of 60% polarisation was determined for the prompt emission of GRB 061122, 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 GRB ejecta 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 2.8°, but further constraints on the level of polarisation are not possible.