Date: Monday 02-Feb-2009
Speaker: Sean O'Neill and Tamara Bogdanovic
Title: Reaction of Accretion Disks to Abrupt Mass Loss During Binary Black Hole Merger
The association of an electromagnetic signal with the merger of a pair of supermassive black holes would have many important implications. For example, it would provide new information about gas and magnetic field interactions in dynamical spacetimes as well as a combination of redshift and luminosity distance that would enable precise cosmological tests. A proposal first made by Bode & Phinney (2007) is that because radiation of gravitational waves during the final inspiral and merger of the holes is abrupt and decreases the mass of the central object by a few percent, there will be waves in the disk that can steepen into shocks and thus increase the disk luminosity in a characteristic way. We evaluate this process analytically and numerically. We find that shocks only occur when the fractional mass loss exceeds the half-thickness (h/r) of the disk, hence significant energy release only occurs for geometrically thin disks which are thus at low Eddington ratios. This strongly limits the effective energy release, and in fact our simulations show that the natural variations in disk luminosity are likely to obscure this effect entirely. However, we demonstrate that the reduction of luminosity caused by the retreat of the inner edge of the disk following mass loss is potentially detectable. This decrease occurs even if the disk is geometrically thick, and lasts for a duration on the order of the viscous time of the modified disk. Observationally, the best prospect for detection would be a sensitive future X-ray instrument with a field of view of on the order of a square degree, or possibly a wide-field radio array such as the Square Kilometer Array, if the disk changes produce or interrupt radio emission from a jet.
Date: Monday 09-Feb-2009
Speaker: Stratos Boutloukos
Title: Near alignment in accretion powered millisecond pulsars
Ten accretion-powered millisecond pulsars are now known. We show that many properties of their X-ray oscillations can be understood if the X-ray emitting regions of most are near their spin axes but wander. This is to be expected if their magnetic poles are close to their spin axes, so that accreting gas is channeled there. As the accretion rate and structure of the inner disk vary, gas will be channeled to different locations on the stellar surface, causing the X-ray emitting regions to move with respect to the magnetic field. This model can explain the small amplitudes and nearly sinusoidal waveforms of most of these pulsars and the large, rapid phase variations of several. It may also explain why accretion-powered millisecond pulsars are difficult to detect, why all found so far are transients, and why the oscillations of a few are intermittent. The model can be tested by comparing with observations the correlated waveform changes that it predicts, including changes with accretion rate.
Date: Monday 16-Feb-2009
Speaker: Teddy Cheung (NASA/GSFC)
Title: Observational Constraints on Gravitational Wave Recoil Kick Velocities in Supermassive Black Hole Binaries from Radio Galaxies
In recent years, general relativity simulations of binary black hole mergers have demonstrated that large `kicks' due to gravitational wave radiation recoil are possible. An observable manifestation of this phenomena would be a displacement of the resultant active nucleus from its parent host galaxy. We test this interesting possibility with measurements of a large sample of radio galaxies, focusing in particular on the class with double-double morphologies. The unusual morphologies of the double-doubles suggest a relatively recent merger, and such angular offset measurements place useful observational constraints on possible kick velocities resulting from mergers of super-massive black hole binaries.
Date: Monday 23-Feb-2009
Speaker: Cole Miller
Title: Dynamics and Fundamental Physics in Dense Stellar Systems
Date: Monday 02-Mar-2009
Speaker: Kwang-Ho Park
Title: Bondi-Hoyle-Lyttleton Accretion of Black Holes Regulated by Radiative Feedback (canceled due to the weather)
Date: Monday 09-Mar-2009
Speaker: Arti Garg (LLNL, Institute of Geophysics and Planetary Physics)
Title: Astrophysics in the Time-Domain: Results and lessons-learned from The SuperMACHO Project
The SuperMACHO Project is a five-year survey toward the Large Magellanic Cloud (LMC) aimed at understanding the nature of the populations of lenses responsible for the excess microlensing rates observed by MACHO (Alcock et al. 2000). Survey observations were completed in 2006. A rich side-product of this survey is a catalog of variable sources down to a depth of VR~23. I will discuss some preliminary findings from the survey and describe simulation techniques used to determine the project's detection efficiency. I will also present some of the auxiliary science from the project including: light echoes of ancient supernovae, type Ia supernova rise times, unusual transients, and periodic variable stars. The purpose of this talk is to provide an overview of the many exciting and diverse areas of research that can be explored in the time domain. I also hope to highlight some of the challenges associated with such surveys and present recent progress toward overcoming them.
Date: Monday 16-Mar-2009
Speaker: Mia Bovill
Title: The distribution of ultra-faint dwarfs in the voids
We build upon the argument that at least some of the 21 new ultra-faint Local Group dSphs are fossils of the first galaxies which formed before reionization to investigate the evolution of this population at late times. We have developed a new N-body technique which allows us to follow these fossils to the modern epoch and explore their z=0 distribution and properties. While we see the most luminous fossils concentrated in high density regions, their lower luminosity counterparts are scattered throughout the filaments and the voids. For the second part of this work we examine our assumption that fossils do not accrete gas after reionization by applying the theory of secondary infall to halos with masses below the Jeans mass of the IGM. We use high resolution N-body simulations of a void combined with 1D hydrodynamical and SPH runs to explore the evolution of fossils at late times and look for a population of hereto undiscovered dark galaxies in the voids.
Date: Monday 23-Mar-2009
Speaker: Stacy McGaugh
Title: The Mass Profile of the Milky Way
I will discuss some atronomical constraints on the mass distribution of the Milky Way, and make connections to current efforts at direct and indirect dark matter detection.
Date: Monday 30-Mar-2009
Speaker: Susmita Chakravorty (Pune, India)
Title: Stability of warm absorbers in AGN
Warm absorbers (WA) are found in soft X-ray spectra of about half of all Seyfert1 galaxies and in some quasars and blazars. We use the thermal equilibrium curve generated by the photoionization code CLOUDY to study the influence, of the shape of the ionizing continuum, density and the chemical composition of the absorbing gas, on the existence and nature of the WA. We have shown that use of, recently derived, more reliable, dielectronic recombination rates gives different results, necessitating revision of these analyses. Systematic stability curve analysis shows that i) the value of the spectral index of the X-ray power-law ionizing continuum need to be more than 0.2 for the WA to exist and should be about 0.8 for its multiphase nature ii) thermal and ionization states of highly dense WA are sensitive to their density if the ionizing continuum is sufficiently soft, i.e. dominated by the ultraviolet a significant new result opening new avenues for density estimation; iii) absorbing gas with super Solar metallicity and / or rich in iron and associated elements is more likely to have a multiphase nature iv) the soft excess component in the ionizing continuum in the form of a blackbody with temperature in the range 100 to 200 eV increases the stability of 10^5 K gas. The final test is to include magnetic fields, of appropriate structure, in this analysis which is likely to influence the dynamics of the gas and stabilizes the WA - providing a robust description of the system.
Date: Monday 06-Apr-2009
Speaker: Sean O'Neill, Cole Miller, Tamara Bogdanovic
Title: Highlights from the conference on Observational Signatures of Black Hole Mergers
Conference on "Observational Signatures of Black Hole Mergers" was held at the Space Telescope Science Institute, in Baltimore, MD from 30 March until 1 April 2009. We will give a summary of topics and discuss the highlights, advances, and current challenges in this field of research.
Date: Monday 13-Apr-2009
Speaker: Dr. Massimo Dotti (University of Michigan)
Title: Dual black holes in merger remnants: linking accretion to dynamics
We study the orbital evolution and accretion history of massive black hole (MBH) pairs in rotationally supported circumnuclear discs up to the point where MBHs form binary systems. Our simulations have unprecedented resolution in mass and space which, for the first time, makes it feasible to follow the orbital decay of a MBH either counter- or co-rotating with respect to the circumnuclear disc. We show that a moving MBH on an initially counter-rotating orbit experiences an "orbital angular momentum flip" due to the gas-dynamical friction, i.e., it starts to corotate with the disc before a MBH binary forms. We stress that this effect can only be captured in very high resolution simulations. Given the extremely large number of gas particles used, the dynamical range is sufficiently large to resolve the Bondi-Hoyle-Lyttleton radii of individual MBHs. As a consequence, we are able to link the accretion processes to the orbital evolution of the MBH pairs. We predict that the accretion rate is significantly suppressed and extremely variable when the MBH is moving on a retrograde orbit. It is only after the orbital angular momentum flip has taken place that the secondary rapidly "lights up" at which point both MBHs can accrete near the Eddington rate for a few Myr. The separation of the double nucleus is expected to be around ~10 pc at this stage. We show that the accretion rate can be highly variable also when the MBH is co-rotating with the disc (albeit to a lesser extent) provided that its orbit is eccentric. Our results have significant consequences for the expected number of observable double AGNs at separations of ~100 pc.
Date: Monday 20-Apr-2009
Speaker: Mike Koss
Title: A Multiwavelength Study of the BAT Hard X-ray Selected AGN and their Host Galaxies (PhD proposal talk practice)
Surveys of AGN taken in the optical, UV, and soft X-rays miss an important population of obscured AGN only visible in the hard X-rays. The SWIFT all sky hard X-ray survey has provided a uniquely unbiased sample because of its ability to find these hidden AGN. Multiwavelength followup of this unbiased sample of AGN in the optical, NIR, and soft X-rays will attempt to answer the critical question of how AGN fit into galaxy evolution. Does the supermassive black hole quench star formation? What is the environment of the host galaxy that triggers the AGN? Do galaxy mergers trigger AGN? How is the mass of the black hole related to host galaxy properties? Finally, how are hard X-ray selected AGN different from methods of selection at other wavelengths and do the newly found heavily obscured AGN challenge the Unified Model of AGN? This talk is the the thesis proposal of Mike Koss.
Date: Monday 27-Apr-2009
Speaker: Dimitri Veras (UFL)
Title: Secular Orbital Dynamics of Hierarchical Two Planet Systems
The preponderance of multi-planet extrasolar systems has kindled interest in using their resonant and secular dynamical evolution as a probe of planet formation. We combine published radial velocity data with Markov Chain Monte Carlo analyses in order to obtain an *ensemble* of masses, semimajor axes, eccentricities and orbital angles for each of 5 multi-planet systems: HD 11964, HD 38529, HD 108874, HD 168443, and HD 190360. We dynamically evolve these systems with 32,500 N-body integrations that consider the full range of possible initial line-of-sight and relative inclinations which span their entire ranges, and report on the planets' resulting stability, secular evolution and resonant influences. The simulations help pinpoint the likelihood of different dynamical regimes for each system, and highlight the dangers of restricting simulation phase space to a single set of initial conditions.
Date: Friday 08-May-2009
Speaker: Dr. Sylvio Ferraz-Mello (Univ. of Sao Paulo)
Title: Tidal evolution of close-in satellites and exoplanets
Our knowledge of tidal friction is even today directly founded on Darwin's theory. Many progresses from studies done in the past century deserve mention. To quote just a few, we may mention Love?s theory on the elastic response of one body submitted to an external potential and the understanding of the role played by tides in generating heat in synchronous planetary satellites. We may also mention the many applications that leaded to the understanding of the evolution of systems with close-in satellites, the Earth-Moon system in the first place. However, notwithstanding the existence of some high-order formal theories, the essential of our knowledge is yet nowadays the one established by Darwin and crucial questions on the action of viscosity, for instance, remains unanswered. I intend to critically review our current understanding of Darwin?s theory and several problems concerning the tidal evolution of Titan, which may be considered as deserving attention.
Date: Monday 11-May-2009
Speaker: TBA
Title:
