Speaker
Description
Observations of accreting X-ray pulsars (XRPs) carried out by the Imaging X-ray Polarimetry Explorer (IXPE) have provided completely new insight into these fascinating objects, thanks to the indispensable information embedded in their linear X-ray polarization. X-ray polarimetry enables us to measure the polarization angle (PA) and degree (PD) as a function of pulse phase for XRPs, which allows for the system geometry to be determined. Additionally, the radiation from XRPs was expected to be highly polarized, with estimates for a PD of up to 80% for favorable orientations.
About a dozen XRPs have been observed by IXPE so far, and pulsar geometries have largely been determined by successful modeling of the pulse phase dependence of the PA with the Rotating Vector Model (RVM). Interestingly, IXPE observations of XRPs have unveiled surprisingly low polarization degrees consistently across the board, significantly lower than theoretical expectations.
Several individual XRPs have demonstrated particularly peculiar polarization properties, which adds to the pool of new questions raised as a result of the polarimetric information. Overall, the XRPs observed with IXPE show little energy-dependence of their polarization properties. However, a couple of XRPs display clear energy dependence of their polarization, with unique swings in the PA between low and high energies, and complex energy dependencies over pulse phase as well. Some XRPs require additional polarized components in order to explain their energy-dependent polarization properties, with possible origins outside the adiabatic radius.
I will discuss the energy dependent polarization properties of these sources, which vary in complexity and require different approaches in order to properly explain their behavior. I will also address unresolved questions and discuss the proposed solutions.