29 October: Melissa Ewing

Speaker: Melissa Ewing (University of Newcastle)

Date: Wednesday 29 October 2025

Time: 15:00

Location: Physics 3.21

The Impact of Polarimetry on hard-state X-ray Binary Studies: Opening a New Avenue in X-ray Polarimetry Timing

The Imaging X-ray Polarimetry Explorer (IXPE) enables, for the first time, sensitivity to X-ray polarisation in the 2–8 keV band, providing new diagnostics - polarisation degree (PD) and angle (PA) - to probe the geometry of accreting matter around compact objects. X-ray binaries (XRBs) are prime targets for this mission, as brand new insights into the geometry of the spatially unresolved X-ray corona, the cloud of hot electrons within the inner accretion flow, can be made. I will present the first polarimetric studies of hard state X-ray binaries Cygnus X-1, Swift J1727.8-1613 and IGRJ17091-3624 which have already broken degeneracies in spectral models, as well as brought other models into question. X-ray polarimetry timing, the study of rapid PD and PA variability, offers a unique probe of mechanisms driving variability, in particular quasi-periodic oscillations (QPOs). Type-C QPOs are predicted to arise from Lense-Thirring precession of the corona, producing characteristic modulations in PD and PA. I will present a Fourier-based method which is able to recover polarisation variability on fast timescales, circumventing issues with current methods, validated on pulsations from X-ray pulsars, and applied to the Type-C QPO in IXPE observations of the black hole XRB Swift J1727.8−1613.