COMPTON POLARIMETRY IN GAMMA-RAY ASTRONOMY

The analysis of compact astronomical objects has generally dealt with the physical properties of the source within a two-parameter space, wh ich is defined by the spectral characteristics and time variability. T his approach often leads to the situation whereby two or-more very dif fer ent models can explain the observations successfully. Polarimetric observations have the diagnostic potential to discriminate between th e different compact source models and can offer a unique insight into the geometrical nature of the emission zones. To date, however, no pol arization observation in the gamma-ray energy domain has been successf ully performed, due to the difficulties in making polarimetric measure ments in this high-energy region of the spectrum. In this piper the po larized gamma-ray emission mechanisms are reviewed with the emphasis o n their detectable characteristics. Potential astronomical sites in wh ich these emission mechanisms may be at work are discussed. Observatio nal results obtained in other wavebands and theoretical predications m ade for some of the most likely astronomical sources of polarization a re reviewed. Compton polarimetry has long been used in the field of nu clear gamma-ray spectroscopy in the laboratory. The operational princi ple behind all generations of nuclear gamma-ray polarimeters has been to measure the asymmetry in the azimuthal distribution of the scattere d photons. However none of the polarimeters designed for laboratory ex periments will be sensitive enough to observe even the strongest astro nomical source. In the past few years there have been a number of inno vative developments aimed at the construction of astronomical gamma-ra y polarimeters, either as dedicated experiments or in missions with po larimetric capability. The designs of all the polarimeters are based o n either discrete or continuous position sensitive detector planes. In this paper the data analysis techniques associated with this type of polarimeter are discussed as well as methods of removing some of the s ystematic effects introduced by a non-ideal detector response function and observation conditions. Laboratory tests of these new polarimetri c techniques are reviewed. They demonstrate the feasibility of buildin g a suitably sensitive astronomical gamma-ray polarimeter. Optimizatio n of the design of pixellated detector an ay based polarimeters is als o addressed. The INTEGRAL mission, which is to be launched by ESA in t he year 2001, is the most likely telescope to perform the first succes sful gamma-ray polarization observation. The polarimetric characterist ics of the two main instruments on board INTEGRAL are evaluated and th eir sensitivities to a wide range of potentially polarized gamma-ray s ources are estimated.