Extracting polarization properties of resolved extended sources to assess various pulsar radiation model classes

Since its discovery over fifty years ago, the pulsar has been the subject of intensive research yet many puzzles remain. A major unresolved question is the geometry of the radiation zone. Several model classes have been proposed (e.g. polar cap, outer gap, striped wind), each of which leads to a different polarization curve – polarization properties can thus be an important discriminant. In December 2021, NASA launched IXPE, the first mission dedicated to imaging X-ray polarization. It contains three telescope/detector assemblies with 25” resolution and a nominal energy range of 2-8 keV. It has already observed the Crab and its PWN, along with magnetar, blazar, and SNR sources. To study the pulsar, we need to isolate its polarization and light curve from the surrounding nebula. The accepted method is to approximate the off-pulse as pure nebula and subtract it from the overall light curve to find the pulsar contribution. However, for resolved extended sources such as the Crab, we can take advantage of the known pulsar light curve and nebula spatial image, taken from previous observations with Chandra, which have much finer spatial and temporal resolution, and use these to constrain a simultaneous fit of the polarization image and light curve structure. Here, we apply this novel fitting method to simulated IXPE Crab data and compare it with that of the on-off method. We see that it provides a more accurate nebula map and modest improvements to the pulsar polarization curve that can allow for more precise assessment of the emission models.