Polarisation studies of the prompt gamma-ray emission from GRB 041219a using the spectrometer aboard INTEGRAL*
1
School of Physics, University College Dublin, Dublin 4, Ireland e-mail: smcglynn@bermuda.ucd.ie
2
School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
3
Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
Received:
3
August
2006
Accepted:
20
February
2007
Context.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
γ-ray source. GRB 041219a is the most intense burst localised by INTEGRAL
with a fluence of 5.7 
erg cm-2 over the
energy range 20 keV–8 MeV and is an ideal candidate for such a study.
Aims.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.
Methods.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
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 .
Results.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 
% at an angle of 
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 
%
at an angle of 
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.
Conclusions.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 (~
). 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.
Key words: gamma rays: bursts / gamma rays: observations / polarization
© ESO, 2007