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INTEGRAL measures a polarized signal from a Gamma-Ray Burst
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INTEGRAL measures a polarized signal from a Gamma-Ray Burst

January 8, 2013
On 19 December 2004, the INTEGRAL Burst Alert system discovered GRB 041219A. This is one of the brightest and longest Gamma-Ray Bursts (GRBs) ever detected. Thanks to its brightness IBIS could be used as a Compton polarimeter, by analyzing the events that interacted both in the upper detection layer, ISGRI, and the lower layer, PICsIT. This has allowed a team lead by Diego Götz (CEA Saclay) to measure a time variable polarization signal in the GRB. Indeed, integrating over the first peak the net polarization signal is null, but the time resolved analysis, slicing the burst in 10 s interval bins shows that the polarization fraction can vary from 20% to 90%. The variation of polarization angle over the whole first peak produces the null average value. The second peak shows less variation and a polarized signal can be detected over its entire duration. The image shows the IBIS light curve of the Compton events, together with the IBIS Compton significance image of the whole event, and the polarigram (the flux distribution as a function of the azimuthal scattering angle) corresponding to the second peak of the GRB where the measured polarization fraction is 43% ± 25%. The level and variability of the polarization signal observed by INTEGRAL in the 200-800 keV energy band favour models involving synchrotron emission from a relativistic outflow with a magnetic field coherent on an angular scale of 1/Gamma, where Gamma is the the bulk Lorentz factor of the outflow. Related publication: D. Götz, P. Laurent, F. Lebrun, F. Daigne, Z. Bosnjak, Variable polarization measured in the prompt emission of GRB 041219A using IBIS on board INTEGRAL, Astrophysical Journal Letters 695 (2009) L208-L212, DOI: 10.1088/0004-637X/695/2/L208 Date: 05 May 2009 Satellite: INTEGRAL Depicts: IBIS lightcurve and polarigram of GRB 041219A Copyright: D. Götz et al. 2009