The Detection of Gamma-ray Polarization Using the INTEGRAL Satellite

Throughout the universe we observe powerful engines which accelerate particles to immense energies. The precise details of how these engines function are still poorly understood, but polarisation measurements of the high energy radiation are pivotal to gain a deeper insight into the physical environment in these systems. Interaction processes usually dominated by strong magnetic fields and highly ordered geometries are required to produce a net polarisation signal in the emission from a source. The strength and direction of a polarisation signal will lead back to an understanding of the configuration of the fields at the emission site.

There have been few attempts at measuring gamma-ray polarisation since measurements are hampered by large backgrounds and systematic effects within the detectors. However, the advent of modern fast computing clusters has made large scale simulation of an instrument's response now possible. By combining instrument data and results from detailed Monte-Carlo Mass-Model simulations using GEANT 4, it is possible to put constraints on the polarisation characteristics of the gamma-ray flux emitted by a source. Using the Compton scattered events in the SPI instrument on INTEGRAL, constraints have been put on the percentage polarisation in the prompt gamma-ray flux of GRB041219a to be 96±40% at 68% confidence and greater than 5% at 99.7% confidence. The method has also been extended to allow the summation of multiple data sets to obtain a measurement of the off-pulse polarisation of the Crab pulsar of 46% ± 10% at an angle of 123° ± 11°, showing a remarkable alignment to the rotational axis of the spinning Neutron star.