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The spectrometer aboard INTEGRAL, SPI, has the capability to detect thesignature of polarised emission from a bright gamma-ray source. GRB 041219a isthe most intense burst localised by INTEGRAL and is an ideal candidate for sucha study. Polarisation can be measured using multiple events scattered intoadjacent detectors because the Compton scatter angle depends on thepolarisation of the incoming photon. A search for linear polarisation in themost intense pulse of duration 66 seconds and in the brightest 12 seconds ofGRB 041219a was performed in the 100-350keV, 100-500keV and 100keV-1MeV energyranges. The multiple event data from the spectrometer was analysed and comparedwith the predicted instrument response obtained from Monte-Carlo simulationsusing the GEANT 4 INTEGRAL mass model. The chi^2 distribution between the realand simulated data as a function of the percentage polarisation andpolarisation angle was calculated for all three energy ranges. The degree oflinear polarisation in the brightest pulse of duration 66s was found to be63+/-31% at an angle of 70+/-14 degrees in the 100-350keV energy range. Thedegree of polarisation was also constrained in the brightest 12s of the GRB anda polarisation fraction of 96+/-40% at an angle of 60+/-14 degrees wasdetermined over the same energy range. However, despite extensive analysis andsimulations, a systematic effect that could mimic the weak polarisation signalcould not be definitively excluded. Our results over several energy ranges andtime intervals are consistent with a polarisation signal of about 60% at a lowlevel of significance (2 sigma). We conclude that the procedure described heredemonstrates the effectiveness of using SPI as a polarimeter, and is a viablemethod of measuring polarisation levels in intense gamma--ray bursts.Comment: 11 pages, 10 figures, accepted by A&A, 1 reference added, typo  correcte

Polarisation studies of the prompt gamma-ray emission from GRB 041219a using the spectrometer aboard INTEGRAL