LISA will be able to detect compact objects that spiral into a MBH by GW emission from up to a distance of a Gpc. The signal is expected to be weak. To detect it, it is necessary to know in advance the shape of the wave trains, and to do that, it is necessary to know the eccentricity of the inspiral orbits. High eccentricity can change the nature of the signal drastically. The eccentricity also determines how much time a star spends within the LISA band before its final plunge. We used dynamical Monte Carlo simulations to study how two-body scattering and energy dissipation by GW emission, acting together, modify the inspiral orbits. We find that INSPIRALING STARS TEND TO HAVE HIGH ECCENTRICITIES. We also find that the event rate is: - Independent of the relaxation time. - Lower than previously estimated: only a few detectable sources. - Strongly depends on mass segregation. The Swansong of Stars Orbiting Massive Black Holes Clovis Hopman Weizmann Institute of Science Ultraluminous X-ray Sources as Intermediate Mass Black Holes Fed by Tidally Captured Stars Hopman, Portegies Zwart & Alexander, ApJL 2004 The nature of ULXs, with X-ray luminosities higher than the Eddington luminosity of a stellar mass object, is not known. We argue that some are powered by INTERMEDIATE MASS BLACK HOLES (IBHs) of ~1000 Msun, which tidally capture a star. We apply this scenario to the X-ray source in cluster MGG-11 (see image). Our main results are: - Tidal capture can be followed by circularization of the star without destroying it during in-spiral for IBHs. This is NOT possible for MBHs of millions of solar masses, where tidal heating destroys the star! - Roche Lobe overflow provides enough gas to feed the IBH and power the ULX (see below) - Life-time and capture rate are large enough so that there is ~10% probability of presence of a ULX in a given IBH-cluster - The capture rate is approximately independent of the relaxation time - If the Roche Lobe overflow phase starts AFTER the cluster had evaporated, the ULX will appear host-less. Such ULXs are indeed observed! (Zezas et al. 2002) Chandra image of the ULX in MGG-11 Orbital Inspiral into a Massive Black Hole in a Galactic Center Alexander & Hopman, ApJL 2003 Gravitational Waves: The Event Rates and Orbital Statistics of LISA Sources Hopman & Alexander, in preparation From the LISA website Eccentricity histograms for several compact objects for a MBH of a million solar masses (left figure) and an IBH of a thousand solar masses (right figure). Hopman & Alexander, in preparation. Luminosity as a function of time for a star filling its Roche Lobe on a circular orbit around a 1000 solar mass black hole. Hopman, Portegies Zwart & Alexander, ApJL 2004 Stars may spiral into a MBH if they can dissipate their orbital energy. Examples of dissipative processes are tidal heating, or gravitational wave emission. These processes are effective only when the star passes very close to the MBH. Inspiral starts when the star is scattered into a highly eccentric orbit, which brings it close to the MBH. Every orbit it dissipates some orbital energy. To dissipate all the energy and achieve tidal circularization (left panel) or become a short-period LISA source (right panel), the star needs to AVOID being scattered again at all times. INSPIRAL IS A RARE PHENOMENON. -Only a few percent of all stars spiral into a MBH. The rest fall directly in (or are tidally destroyed) -Tidal capture has a negligible contribution to the growth of a MBH by eating stars. -Stars that are strongly perturbed by a close tidal interaction (tidal scattering) are scattered back to the ambient population. Many "weird stars" probably exist around MBHs. Alexander & Hopman, ApJL 2003