Description
The Imaging X-ray Polarimetry Explorer (IXPE) provides, for the first time, sensitivity to X-ray polarisation in the 2–8 keV band, offering two powerful diagnostics to probe the extreme physics of accreting matter around compact objects: polarisation degree (PD) and polarisation angle (PA). I will present results from the first IXPE observation of the black hole X-ray binary (BH-XRB) IGR J17091, a system renowned for its extraordinary variability, including ‘heartbeat’-like oscillations. Our spectro-polarimetric and model-independent analyses reveal an unexpectedly high PD of 9.1 ± 1.6% at a PA of 83° ± 5°— the highest ever measured for a BH-XRB in the corona-dominated hard state. I will discuss these results alongside our energy-resolved polarimetric analysis, which hints at a positive correlation between PD and energy, as well as the influence of the source’s structured variability on the polarisation properties. I will then explore potential models to explain this extreme PD, including simulations investigating the role of polarised disc winds and relativistic coronal outflows.
