Linear polarisation in gamma-ray burst prompt emission is an important diagnostic with the potential to significantly constrain models .
The spectrometer aboard INTEGRAL , SPI , has the capability to detect the signature of polarised emission from a bright \gamma –ray source .
GRB 041219a is the most intense burst localised by INTEGRAL with a fluence of 5.7 \times 10 ^ { -4 } ergs cm ^ { -2 } over the energy range 20 keV–8 MeV and is an ideal candidate for such a study .
Polarisation can be measured using multiple events scattered into adjacent detectors because the Compton scatter angle depends on the polarisation of the incoming photon .
A search for linear polarisation in the most intense pulse of duration 66 seconds and in the brightest 12 seconds of GRB 041219a was performed in the 100–350 keV , 100–500 keV and 100 keV–1 MeV energy ranges .
It was possible to divide the events into six directions in the energy ranges of 100–350 keV and 100–500 keV using the kinematics of the Compton scatter interactions .
The multiple event data from the spectrometer was analysed and compared with the predicted instrument response obtained from Monte–Carlo simulations using the GEANT 4 INTEGRAL mass model .
The \chi ^ { 2 } distribution between the real and simulated data as a function of the percentage polarisation and polarisation angle was calculated for all three energy ranges .
The degree and angle of polarisation were obtained from the best–fit value of \chi ^ { 2 } .
A weak signal consistent with polarisation was found throughout the analyses .
The degree of linear polarisation in the brightest pulse of duration 66 s was found to be 63 ^ { +31 } _ { -30 } % at an angle of 70 ^ { +14 } _ { -11 } degrees in the 100–350 keV energy range .
The degree of polarisation was also constrained in the brightest 12 s of the GRB and a polarisation fraction of 96 ^ { +39 } _ { -40 } % at an angle of 60 ^ { +12 } _ { -14 } degrees was determined over the same energy range .
However , despite extensive analysis and simulations , a systematic effect that could mimic the weak polarisation signal could not be definitively excluded .
Our results over several energy ranges and time intervals are consistent with a polarisation signal of about 60 % but at a low level of significance ( \sim 2 \sigma ) .
The polarisation results are compared with predictions from the synchrotron and Compton drag processes .
The spectrum of this GRB can also be well fit by a combined black body and power law model which could arise from a combination of the Compton and synchrotron processes , with different degrees of polarisation .
We therefore conclude that the procedure described here demonstrates the effectiveness of using SPI as a polarimeter , and is a viable method of measuring polarisation levels in intense gamma–ray bursts .