The study of galactic nuclei has advanced rapidly during the past few years. Observations carried out with space-borne telescopes, such as the Hubble Space Telescope, or from the ground, using adaptive optics, have allowed us to study the kinematics of stars and gas in regions reaching down to sub-parsec scales for external galaxies, and to the milliparsec range for the Milky Way. An outstanding conclusion is that dark compact objects, very probably massive black holes (MBH), with masses ranging between a million and a thousand million solar masses, occupy the centres of most galaxies for which such observations can be made.
We have discovered that there exists a deep link between the central MBH and its host galaxy. Claims of detection of “intermediate-mass” black holes (IMBHs, with masses between 100 and 10,000 solar masses) raise the possibility that these correlations extend to much smaller systems, but the strongest -if not totally conclusive- observational evidences for the existence of IMBHs are ultra-luminous X-ray sources. The origins of these IMBH are still shrouded in mystery, and many aspects of their interplay with the surrounding stellar cluster remain to be elucidated. The particularly important modes of interaction between stars and the MBH in galactic nuclei are, firstly, that stars can produce gases which are accreted onto the MBH through normal stellar evolution, collisions or disruptions of stars by the strong central tidal field. Secondly, compact stars can be swallowed whole if they gradually inspiral due to the emission of GWs. The latter process, is known as an “Extreme Mass Ratio Inspiral”. Indeed, in the past few years, several galaxies have exhibited X-ray/UV flares consistent with the tidal disruption of a star. Tidal disruptions trigger phases of bright accretion that may reveal the presence of a MBH in an otherwise quiescent, possibly very distant, galaxy, with important implications for extra-galactic UV/X-ray astronomy. Partial disruptions, for instance stripping the envelope of a giant star, may also have tell-tale observational consequences. Such outbursts may have been detected already and space missions such as SWIFT, the Advanced Telescope for High Energy Astrophysics or the Energetic X-ray Imaging Survey Telescope (EXIST) and the extended ROentgen Survey with an Imaging Telescope Array (eRosita) should yield a rich harvest of data.
The workshop will bring experts from rather different fields: stellar dynamicists, observers from optical and X/UV-ray, astrometry, data analysts, general relativity and numerical modeling of different regimes (from kiloparsec distances to parsecs down to the horizon of the massive black hole). The workshop is born from the need for a place where the distinct communities involved in X-ray astronomy, gravitation wave including electromagnetic counterparts might gather. While these communities – theoretical and observational astrophysics, general relativity, cosmology and data analysis – have made significant collaborative progress over recent years, we believe that it is indispensable to future advancement that they draw closer, and that they speak a common idiom.