Traditional accretion disk models have always had problems explaining a variety of observed features of AGN, particularly the short wavelength SED in the ultraviolet and beyond, and the rapid variability. I will review possible resolutions to these problems, including the effects of outflows, opacity-driven convection in the disk, and magnetically elevated disks.
Sgr A$^*$, located only 8 kpc away, allows us to study in detail the accretion process on to a super-massive black hole. Direct observations show that the black hole luminosity varies on different time-scales, but remains extremely dim, despite the (disputed) presence of a cold gaseous disc. However, indirect evidence reveals that it was several orders of magnitude brighter just a few...
Radio variability in some radio-quiet (RQ) active galactic nuclei suggests emission from regions close to the central engine, possibly the outer accretion disc corona. If the origins of the radio and the X-ray emission are physically related, their emission may be temporarily correlated, possibly with some time delays. We present the results of quasi-simultaneous radio and X-ray monitoring of...
Active Galactic Nuclei (AGNs) can collect stars and stellar remnants from the vicinity of the galactic center into the inner plane of the AGN disk. The dense population of stellar objects give rise to a wealth of interactions from stellar-mass black hole collisions to the tidal disruption of stars on stellar-mass black holes. These transients are promising multi-messenger sources from...
Apart from regular, low-level stochastic variability, some AGNs occasionally show exceptionally large changes in luminosity, spectral shape, and/or X-ray absorption. The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Thus a name was coined as 'Changing-Look AGN' (CL AGN). The origin of this phenomenon is...
