Spectral-Timing Observations of Disk-Jet Coupling in Black Hole X-ray Binaries

Abstract

Accreting black holes are a fundamental tool to understand accretion and ejection physics, and are ideal laboratories to ultimately test Einstein's general relativity (GR) in the strongest gravity regime in the Universe. High-fidelity GR tests require a precise knowledge of the physical environments in which particles move. Two biggest challenges there are how close to the event horizon inspiraling gases reach, and how the relativistic jets are launched. The puzzle piece linking these two challenges together is the nature and geometry of the hot (hundreds of keV) X-ray emitting plasma called the “corona". X-ray Reverberation Mapping, where X-rays produced close to the BH reverberate off inspiraling gas, allows us to map out scales close to the event horizon -- orders of magnitude better than the resolution of our telescopes. Black hole X-ray binaries (BHXBs) are binary systems with a stellar-mass black hole and a companion star. They are usually transients as they cycle through phases of quiescence and outburst in which they exhibit different accretion states with distinct spectral-timing features, allowing us to study the accretion-ejection physics or disk-jet coupling in a single source on a human timescale. In MAXI J1820+070, I discover that the soft reverberation lag becomes longer during the hard-to-soft state transition, several days before the transient radio jet is observed. Together with the discovery that the reverberation lag gets shorter in the hard state while the compact jet becomes weaker, this result suggests a close relationship between the X-ray corona and the radio jet. The corona might be the base of the jet that expands and/or gets ejected during the state transition. In the "NICER reverberation machine", I expand the sample size of BHXBs where reverberation is detected from 3 to 11, and find the evolution of reverberation lag in the hard and intermediate states is a generic feature of BHXBs, and should be explained with state transition models. I explore simultaneous modeling of the flux-energy spectrum and cross spectra and a proof-of-concept to apply machine learning to fitting the cross spectra. I also study a BHXB IGR J17091--3624 that exhibits “heartbeat"-like variabilities in its 2022 outburst and find the source began in traditional hard and intermediate states and transitioned into an exotic soft state. I also discover one of the most coherent quasi-periodic oscillations, and find an interplay between heartbeats and iron emission/absorption line. These results lead to new insights into the physical nature of exotic variabilities and accretion disk instability.