Improved Measurements of the IXPE Crab Polarization

Abstract

Abstract X-ray polarization from the Imaging X-ray Polarimetry Explorer (IXPE) provides an important new probe of the geometry of the pulsar emission zone and of particle acceleration in the surrounding pulsar wind nebula. However, with IXPE's modest ∼20″–30″ spatial resolution, separation of the pulsar signal from the nebula is a challenge. Conventional analysis defines an “off” phase window as pure nebular emission and subtracts its polarization to isolate the phase-varying pulsar (on-off fitting). We present a more sensitive scheme that uses external measurements of the nebula structure and the pulsar’s light curve to isolate their contributions to the phase- and spatially-varying polarization via least-squares regression (simultaneous fitting). Tests with simulation data show ∼30% improvement in pulse phase polarization uncertainties, decreased background systematics, and substantially improved nebular polarization maps. Applying simultaneous fitting to early IXPE Crab data extracts additional phase bins with significant polarization. These bins show interesting departures from the well-known optical polarization sweeps, although additional exposure will be needed for precise model confrontation.