INTEGRAL mesures a polarized signal from a Gamma-Ray Burst
On December 19 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 Goetz (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.
For more details, see:
D. Goetz, 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
(ADS)
