Galactic and extragalactic X-ray transients, theory and observational perspectives

Theoretical studies have shown that the multi-wavelength emission of the nuclei of active galaxies, stellar mass black hole and neutron star binaries is fueled by their accretion disks. The accretion process may be not stationary, and the accretion history as well as geometric structure of the infalling matter are not well understood. Importantly, the magnetic fields must play a key role in sustaining the accretion flows. While we have progressed in understanding many of these phenomena, there are recent observations and new models that drive our understanding in new directions. They are related either to the  instabilities in the innermost part of the disk or to the sudden interaction of circumnuclear stars with the central black hole and the disk. Processes close to the black hole also allow testing of  General Relativity in a strong field regime.

• Regular quasi-periodic outbursts (QPO) with a period that depends on the mean flux were observed in Galactic microquasars, and have also been suggested to explain intermediate- and supermassive black holes  duty cycles. This regular variability can be well modeled by the radiation pressure instability in the central parts of the accretion disk or alternatively, by orbiting bodies that regularly perturb the accretion flow. 

• Quasi-periodic eruptions (QPEs) form a novel class of extragalactic X-ray transients that are known to repeat at roughly regular intervals of a few hours to days. Their underlying physical mechanism is a topic of heated debate, with most models proposing that they originate either from instabilities within the inner accretion flow or from orbiting objects. In case orbiting stars or compact objects are behind the quasiperiodic variability of at least some QPEs, they would provide a unique opportunity to study precursors of future gravitational-wave detections of extreme-mass ratio inspirals by LISA. 

• Changing-Look Active Galactic Nuclei (CL-AGN) forms a special and interesting class of objects. There, the change of the spectral type of an AGN happens on timescales of a few years, with extreme luminosity variations that considerably  exceed the usual stochastic variability. These changes may or may not be related to the radiation pressure instability. Tidal disruption events (TDE) traditionally form a separate class, and they were searched for and found in otherwise inactive galaxies. When a TDE event occurs in an active or weak nucleus, however, it can also lead to a CL AGN. 

• From the theoretical side, an accretion disk formed around a supermassive black hole, after it disrupts a star, is expected to be initially misaligned with respect to the black hole’s equatorial plane. This misalignment induces relativistic torques on the disk, causing the disk to precess at early times, while at later times, the disk aligns with the black hole and precession terminates. The resulting Lense-Thirring precession in the disk can produce X-ray variability consistent with observations, but other physical mechanisms, such as the radiation-pressure instability, are also possible. 

• Supermassive black holes also offer a unique way to test GR in a strong field regime when we can follow the motion of gas or stars in its vicinity. For example, high-resolution observations of the Sgr A* region offer such opportunities.