Compact Objects in 3D -- Steps towards X-ray Polarimetric-Spectral-Timing

The history of X-ray timing is rich; since the advent of X-ray astronomy, variability techniques have been used to study many types of accreting compact objects, including black hole X-ray binaries, neutron stars, ultra-luminous X-ray sources, active galactic nuclei, and tidal disruption events.

All of these systems efficiently convert the gravitational potential energy of accreted matter into radiation. As a result, they emit strong X-ray radiation from an accretion disc and a hot electron ‘corona’. These structures are coupled to other observed phenomena of the systems seen across the electromagnetic spectrum, such as radio jets and disk winds. While much progress has been made to understand these processes, the geometrical configuration of the inner accretion flow and the origin of the observed variability is still debated.

Two major observatories facilitating X-ray timing studies are RXTE and NICER. These allowed the use of sophisticated analysis methods, focusing on an energy-resolved view on the short-term variability, which we call spectral-timing analysis. While these analysis techniques have become established over the last two decades, with the launch of IXPE the polarisation of the observed X-rays has become a key avenue to understand the geometry of systems.

Both analysis techniques – spectral-timing and polarimetry – are used to study the same objects independently, and often find conflicting results while it is clear that the underlying physics must be the same. During this workshop, we will work to fully expand X-ray analysis into the 3D world of polarimetric-spectral-timing, whereby X-ray timing methods are used to study variations across both X-ray energy and polarisation.