The PoGO+ view on Crab off-pulse hard X-ray polarisation

Abstract

The linear polarisation fraction and angle of the hard X-ray emission from\nthe Crab provide unique insight into high energy radiation mechanisms,\ncomplementing the usual imaging, timing and spectroscopic approaches. Results\nhave recently been presented by two missions operating in partially overlapping\nenergy bands, PoGO+ (18-160 keV) and AstroSat CZTI (100-380 keV). We previously\nreported PoGO+ results on the polarisation parameters integrated across the\nlight-curve and for the entire nebula-dominated off-pulse region. We now\nintroduce finer phase binning, in light of the AstroSat CZTI claim that the\npolarisation fraction varies across the off-pulse region. Since both missions\nare operating in a regime where errors on the reconstructed polarisation\nparameters are non-Gaussian, we adopt a Bayesian approach to compare results\nfrom each mission. We find no statistically significant variation in off-pulse\npolarisation parameters, neither when considering the mission data separately\nnor when they are combined. This supports expectations from standard\nhigh-energy emission models.\n