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

TL;DR

This study compares hard X-ray polarisation measurements of the Crab nebula's off-pulse emission from PoGO+ and AstroSat CZTI, finding no significant variation and supporting standard emission models.

Contribution

It introduces finer phase binning and employs a Bayesian approach to compare polarisation data from two missions, enhancing analysis precision.

Findings

No significant off-pulse polarisation variation detected

Bayesian analysis confirms consistency between missions

Supports standard high-energy emission models

Abstract

The linear polarisation fraction and angle of the hard X-ray emission from the Crab provide unique insight into high energy radiation mechanisms, complementing the usual imaging, timing and spectroscopic approaches. Results have recently been presented by two missions operating in partially overlapping energy bands, PoGO+ (18-160 keV) and AstroSat CZTI (100-380 keV). We previously reported PoGO+ results on the polarisation parameters integrated across the light-curve and for the entire nebula-dominated off-pulse region. We now introduce finer phase binning, in light of the AstroSat CZTI claim that the polarisation fraction varies across the off-pulse region. Since both missions are operating in a regime where errors on the reconstructed polarisation parameters are non-Gaussian, we adopt a Bayesian approach to compare results from each mission. We find no statistically significant variation in off-pulse polarisation parameters, neither when considering the mission data separately nor when they are combined. This supports expectations from standard high-energy emission models.