Energy-resolved polarization study of the Crab nebula with <i>IXPE</i>

ABSTRACT This work presents a new detailed study on the energy-dependent variation in the X-ray polarization of the Crab pulsar wind nebula (PWN), obtained using data from the Imaging X-ray Polarimetry Explorer (IXPE). For the entire PWN, we observed a linear variation in polarization degree (PD), and detected the rotation of the polarization angle (PA) with the energy at higher than 99.9999 per cent of the confidence level. This energy-dependent polarization variation is in line with the indication found in Vela PWN by IXPE, and it can be interpreted that the emitting region of the polarized photons shrinks with increasing energy, leading to higher PD because they are less influenced by the turbulence of the magnetic field. We compared the IXPE polarization results with those of other hard X-ray/gamma observatories (PoGO+, Intregral, AstroSat) for the PWN, finding the same trend from soft-X to hard-X with the PD increasing with the energy and the PA approaching the pulsar’s spin axis. In fact, in this wide energy band, the fitting results show an energy trend for the PA compatible with the estimated pulsar’s spin axis within 3$\sigma$ of confidence level.