LISA Observations of Supermassive Black Hole Growth

Based on a high resolution cosmological n-body simulation, we track the hierarchical growth of black holes in galaxy clusters from z=20 to z=0. We present a census of black holes as function of redshift and will determine their mass assembly history under a variety of assumptions regarding the importance of gas accretion in black hole growth, from early supercritical Eddington accretion to gas-poor hierarchical assembly. Following a galaxy merger, black holes are expected to form, inspiral and merge after strongly radiating energy via gravitational waves. For each binary black hole inspiral and merger, we determine the expected gravitational wave signal for the Laser Interferometer Space Antenna (LISA), and calculate the LISA event rate as a function of time. We will calculate the black hole mass assembly history for several black hole growth scenerios, so that we can explore tests to characterize each model observationally. In particular, we will study how well LISA observations will be able to distinguish between these very different assembly scenarios.Comment: 5 pages, 11 figures, proceedings, Sixth International LISA Symposium, June 19-23, 2006 Goddard Space Flight Center Greenbelt, Marylan

Interpretation of the 115 Day Periodic Modulation in the X-ray Flux of NGC 5408 X-1

We comment on the recent observation of a 115-day modulation in the X-ray flux of the ultraluminous X-ray source (ULX) NGC 5408 X-1, and in particular, the interpretation of this modulation as the orbital period. We suggest that this modulation may instead be due to a precessing jet, and is thus superorbital in nature. Comparing the properties of this ULX with those of the prototypical microquasar SS 433, we argue that NGC 5408 X-1 is very similar to SS 433: a hyper-accreting stellar mass black hole in a shorter-period binary. If the analogy holds, the 115-day modulation is best explained by the still poorly-understood physics of inner-disc/jet precession and a longer observing baseline would be able to reveal an intrinsic phase jitter that is associated with such a precession.Comment: 5 pages, 1 figure, Accepted for publication in Ap

Event Rate for Extreme Mass Ratio Burst Signals in the LISA Band

Stellar mass compact objects in short period orbits about a $10^{4.5}$--$10^{7.5}$ solar mass massive black hole (MBH) are thought to be a significant continuous-wave source of gravitational radiation for the ESA/NASA Laser Interferometer Space Antenna (LISA) gravitational wave detector. However, these extreme mass-ratio inspiral sources began in long-period, nearly parabolic orbits that have multiple close encounters with the MBH. The gravitational radiation emitted during these close encounters may be detectable by LISA as a gravitational wave burst if the characteristic passage timescale is less than $10^5$ seconds. Scaling a static, spherical model to the size and mass of the Milky Way bulge we estimate an event rate of ~ 15 per year for such burst signals, detectable by LISA with signal-to-noise greater than five, originating in our galaxy. When extended to include Virgo cluster galaxies our estimate increases to a gravitational wave burst rate of ~ 18. We conclude that these extreme mass-ratio burst sources may be a steady and significant source of gravitational radiation in the LISA data streams.Comment: 4 pages, minor revisions. Accepted for ApJ Letter