Tracing Polarization Angle Evolution in Accreting Systems with IXPE using Bayesian Nested Sampling

Abstract: X-ray polarimetry offers a powerful and unprecedented window into the magnetic field configurations and emission geometries of a wide range of astrophysical systems. In this talk, I will present results from a new Bayesian nested sampling framework developed to analyze time and energy-dependent evolution of the electric vector position angle (EVPA) of several sources using data from the Imaging X-ray Polarimetry Explorer (IXPE). This method extends the unbinned maximum likelihood approach previously applied to Mrk 421 and enables simultaneous optimization of Stokes Q and U parameters, offering statistically robust model selection and parameter estimation even in low-count regimes. We apply this approach to three IXPE targets: the neutron star X-ray binary GX 13+1, the magnetic cataclysmic variable EX Hydrae, and the blazar BL Lacertae. For GX 13+1, we constrain variability in the PA rotation rate across multiple observations providing insight into dynamic behavior of the source’s winds. In EX Hydrae, we find marginal evidence for energy-dependent PA rotation, suggesting possible constraints on the source's geometry and behavior of the accretion column. And for BL Lac, we observe marginal yet coherent EVPA rotation exceeding 180° in the q-u plane, even as the polarization degree remains low, challenging simple jet models that predict high intrinsic polarization once vector cancellation in Stokes space is disentangled. These results demonstrate the power of time-resolved X-ray polarimetry to reveal subtle polarization signatures that are lost in time-averaged measurements and highlight the versatility of Bayesian nested sampling for probing polarization variability in compact objects across a wide range of physical environments.