Prof. Adam Ingram, School of Mathematics Statistics and Physics, Newcastle University, United Kingdom — November 10, 2025

Stellar-mass black holes accreting gas from a binary partner (X-ray binaries) and supermassive black holes accreting gas from their host galaxy (active galactic nuclei, AGNs) can emit a huge X-ray flux from the vicinity of the black hole event horizon. This can be exploited to probe the strong field regime of General Relativity and measure the properties of the black hole: its mass and angular momentum. For all but two objects in the Universe, the vicinity of the accreting BH is far too small to directly image, necessitating the use of mapping techniques. I will talk about two novel mapping techniques: X-ray reverberation mapping and X-ray polarimetry. X-ray reverberation mapping utilises the light-crossing delay between X-rays that reach us directly from the central emitting region and those that take a longer path by first reflecting off the accretion disk. I will summarise our efforts to measure the mass of stellar and supermassive black holes with our X-ray reverberation mapping code RELTRANS. The main limitation of X-ray reverberation mapping is our continuing ignorance of the shape and nature of the hard X-ray emitting region -- the so-called X-ray corona. I will present observations of black hole X-ray binary systems by the Imaging X-ray Polarimetry Explorer (IXPE) that for the first time utilise the polarisation degree and angle of the X-rays. We find that, in all cases with good enough data, the polarisation angle aligns with the resolved radio jet, favouring an X-ray corona extended in the direction perpendicular to the jet. The polarisation degree is often larger than expected, particularly for Cygnus X-1, indicating either the presence of a warped accretion disk or a gap in our physical understanding.