New spectral-timing techniques across the full X-ray band Revealing the dynamic corona in black hole x-ray binaries

Black holes are the most extreme objects in the universe and the most exotic endpoints of stellar evolution. When a stellar-mass black hole shares a close orbit with a companion star, mass transfer from the star onto the black hole can occur in a process that is known as accretion. Most of the emission from accreting black holes is emitted in X-rays, which is why these systems are called black hole X-ray binaries. Lower-energy X-ray emission is mainly produced by the accretion disk, while higher energy X-rays originate from close to the black hole in a structure known as the corona. The nature and geometry of the corona has been subject to debate for decades.
In my thesis, I aim to put new constraints on the nature of the corona by studying the strong X-ray variability on short timescales, ranging from minutes to milliseconds, with Fourier techniques. This is known as spectral-timing and it has revealed lags between X-rays at different energies. By applying and testing new analysis techniques, I have found that lags themselves vary on short time-scales as well, which indicates that the coronal geometry is highly dynamic. I analysed data from several telescopes, notably NICER and Insight-HXMT, enabling comparison of different Fourier techniques across a broad range of X-ray energies, providing the sharpest view to date of the complex variability in black hole X-ray binaries. I also combined X-ray spectral-timing with X-ray polarization measurements to provide new constraints on the geometry and behaviour of the corona.