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Astrophysics Science Division Colloquium Series
Schedule: April - June 2008

Astrophysics Science Division Colloquium Series
Schedule: April - June 2008


Through the courtesy of the speakers since 2004, most presentations are available on line.

Future schedules:

  • 2008, Third Quarter
  • Past schedules:

  • 2008, First Quarter
  • 2007, Fourth Quarter
  • 2007, Third Quarter
  • 2007, Second Quarter
  • 2007, First Quarter
  • 2006, Fourth Quarter
  • 2006, Third Quarter
  • 2006, Second Quarter
  • 2006, First Quarter

  • Time: 3:45 pm (Meet the Speaker at 3:30 pm) - Location: Bldg 21, Room 183 - unless otherwise noted.
    To view the abstract of a seminar, click on the title.

    April

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    1 Vanessa Lauburg (UMd) - Stellar Mass Black Hole Mergers in Galactic Nuclei
    2
    3
    4
    5
    6
    7
    8 Paolo Coppi (Yale) - The Life and Times of Supermassive Black Holes
    9
    10
    11
    12
    13
    14
    15 Andrey Kravtsov (U Chicago) - Cosmological simulations of clusters of galaxies: status, problems, challenges
    16
    17
    18
    19
    20
    21
    22 Bence Kocsis (Harvard Smithsonian Center for Astrophysics) - Searching for the Electromagnetic Counterparts of Supermassive Black Hole Mergers
    23 Tulun Ergin (U Mass) - Finding TeV Gamma-ray Sources and their Connections to the Unexplored Energy Window through GLAST
    24 Ozlem Celik (UCLA) - VERITAS Observations of the Crab Nebula and Pulsar
    25 Sylvain Guiriec (LPTA) - GLAST-LAT Signatures of UHECRs Production in GRBs
    26
    27
    28
    29 Paul Bryans (Columbia) - A New Approach to Analyzing Solar Coronal Spectra Using Updated Collisional Ionization Equilibrium Calculations
    30 Vlasios Vasileiou (U. Maryland) - Milagro, A Wide Field of View VHE Gamma-Ray Observatory

    May

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    1
    2
    3
    4
    5
    6 Ehud Behar (Technion) - Stellar Coronae and AGN Coronae - Establishing a Physical Analogy
    7
    8
    9
    10
    11
    12
    13 Angela Speck (U Missouri) - Understanding the conditions for crystalline silicate formation: Lessons from an obscured AGB star
    14 Priya Natarajan (Yale) - The accretion history of black holes in galactic nuclei
    15
    16 Daniel Proga (UNLV) Dynamics of Accretion Flows Irradiated by a Quasar
    17
    18
    19
    20 Ioannis Contopoulos (Academy of Athens) - The 3D structure of the pulsar magnetosphere: first results
    21
    22
    23
    24
    25
    26
    27 Adrian Martindale (Leicester) - X-ray Polarimetry with Dichroic Filters Szabolcs Marka (Columbia) - CANCELLED; WILL BE RESCHEDULED -- Gravitational Waves in Multimessenger Astrophysics
    28
    29

    30
    31

    June

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    1
    2
    3 SUMMER HIATUS
    4
    5
    6
    7
    8
    9
    10 SUMMER HIATUS
    11
    12
    13
    14
    15
    16
    17 SUMMER HIATUS
    18
    19
    20
    21
    22
    23
    24 SUMMER HIATUS
    25
    26
    27
    28
    29
    30

    Channels for Stellar-Mass Black Hole Mergers in the Centers of Galaxies

    Vanessa Lauburg

    University of Maryland - College Park

    Tuesday, April 1, 2008

    Abstract

    Gravitational waves will soon take us out of the realm of indirect observation of black holes and allow us to "see" them once and for all. By forcing black holes out of hiding, detectors such as LISA and LIGO will give us insight into the formation and demographics of these objects. Because of the weak nature of gravitational waves and the resultant challenges in data analysis, the success of these detectors depends in large part on a detailed understanding of the radiation sources. Mergers of black holes, on both stellar-mass and supermassive scales, are among the most important such sources. Supermassive black holes lurk at most galactic centers, where they are likely to capture stellar-mass black holes onto close orbits that will lead to inspirals and mergers. These extreme mass ratio inspirals (EMRIs) can be formed by the tidal separation of black hole binaries, which will produce events with near-zero eccentricity in the LISA sensitivity band. The dense environments of galactic nuclei foster close encounters between stellar-mass black hole binaries and stars, often inducing mergers. Dynamically-triggered events such as these might be the dominant formation channel for mergers of black hole binaries, which would indicate that mergers will preferentially involve stellar-mass black holes at the upper end of the mass spectrum. I will present these two new potential formation channels for gravitational radiation sources: tidal separation of binaries by supermassive black holes and induced mergers of stellar-mass black holes in the centers of galaxies.

    The Life and Times of Supermassive Black Holes

    Paolo Coppi

    Yale University

    Tuesday, April 8, 2008

    Abstract

    Once considered rare and exotic objects, supermassive black holes now seem to be found in the center of every massive galaxy and may have released enough energy into their surroundings to profoundly influence the structure we see around us today. I will review recent developments in our understanding of the life history of these objects, e.g., how these objects form and grow (by accretion and/or merging) to their present size. I will highlight recent work at Yale on the importance of gas in understanding whether the central black holes of two merging galaxies will also merge. I will speculate on possible solutions for some of the many issues that remain outstanding and discuss how they would impact the event rates expected from gravitational wave experiments like LISA.

    Cosmological simulations of clusters of galaxies: status, problems, challenges

    Andrey Kravtsov

    Yale University

    Tuesday, April 15, 2008

    Abstract

    I will describe high-resolution self-consistent cosmological simulations of clusters forming in the concordance Cold Dark Matter model with vacuum energy. The resolution of the simulations is sufficiently high to resolve formation and evolution of cluster galaxies and their impact on cluster gas. We use these simulations to study the effects of galaxy formation on the global properties of clusters, such as the shape of cluster dark matter halo and its density profile, the baryon fractions, gas density and temperature profiles. I will present comparisons of simulations results with the recent X-ray Chandra, Sunyaev-Zeldovich, and optical observations of clusters with highlights of both successes and problems of the models. I will show that despite complexities of their formation and uncertainties in their modeling, clusters of galaxies both in observations and numerical simulations are remarkably regular and consistent outside of their core region (~5% of the virial radius), which holds great promise for their use as cosmological probes. I will briefly describe the current status of cosmological constraints with clusters.

    Searching for the Electromagnetic Counterparts of Supermassive Black Hole Mergers

    Bence Kocsis

    Harvard-Smithsonian Center for Astrophysics

    Tuesday, April 22, 2008

    Abstract

    The anticipated detection of the gravitational waves (GWs) by the future Laser Interferometric Space Antenna (LISA) will constitute a milestone for fundamental physics and astrophysics. In this talk, I will discuss LISA's capability of providing an advance warning of supermassive black hole mergers (SMBH) and show how the size and geometry of the localization volume evolves during the observation. While the GW signatures themselves will provide a treasure trove of information, if the source can be securely identified in electromagnetic (EM) bands, this would open up entirely new scientific opportunities, to probe fundamental physics, astrophysics, and cosmology. I will describe several mechanisms that might produce EM variability during a SMBH merger. In particular, the binary may produce a roughly periodic variable electromagnetic flux, due to the orbital motion prior to coalescence, a transient signal caused by shocks in the circumbinary disk when the SMBH binary recoils and ``shakes'' the disk, or a prompt EM flare caused by the viscous dissipation of GWs in the ambient gas. I will discuss whether these time-variable EM signatures may be detectable. A LISA-triggered EM counterpart search campaign will require monitoring a several-square degree area. It could aim for variability at the 24-27 mag level in optical bands, for example, which corresponds to 1-10% of the Eddington luminosity of the prime LISA sources of ~10^6 - 10^7 Msun BHs at z=1-2, on time-scales of minutes to hours, the orbital time-scale of the binary in the last 2-4 weeks of coalescence.

    Finding TeV Gamma-ray Sources and their Connections to the Unexplored Energy Window through GLAST

    Tulun Ergin

    University of Massachusetts

    Wednesday, April 23, 2008

    NOTE TIME/LOCATION: Bldg 2, Room 8, 10 AM

    Abstract

    Recent observations of imaging atmospheric Cherenkov telescope systems like H.E.S.S., VERITAS and MAGIC resulted in detections of very high energy gamma rays from various galactic and extra-galactic sources. Some of the detected galactic sources showed extended TeV emission and some of them were within or close to the large error boxes of the EGRET observations. Due to the better angular resolution of GLAST, it will be easier to resolve and identify all the sources within the EGRET error boxes so that they can be better spatially correlated with the detected TeV sources. For these overlapping sources GLAST will measure spectra in the so far unexplored energy range that spans from 30 GeV to 300 GeV. These observations will help us to study the energy dependence of the source extensions and the spectra of the sources. The GLAST spectral measurements of the TeV detected supernova remnants will reveal if the gamma-rays are products of proton or electron interactions. Good spectral measurements are mostly coupled with timing analysis of the pulsars where pulsed and not-pulsed emissions are analyzed separately to find the cut-off energies in pulsed spectra and set limits to the pulsed emission models. Moreover, other time critical measurements will be the observations of the binaries which show periodic changes in the gamma-ray flux and the multi-wavelength campaigns to detect the variable emission in the AGN. GLAST is also expected to detect many new sources that will bring new challenges to develop new methods to identify and classify the sources. Data taken in other wavelengths including in the TeV energies will be useful in understanding these new sources. In this talk I want to give a summary on my research done in the the H.E.S.S. and VERITAS experiments by focusing on TeV observations of galactic sources, especially supernova remnants, pulsars, and binaries. Then I will discuss the motivations for future observations of these and other sources with GLAST and the possible fields of contribution to the research going on in GSFC.

    VERITAS Observations of the Crab Nebula and Pulsar

    Ozlem Celik

    UCLA

    Thursday, April 24, 2008

    NOTE TIME/LOCATION: Bldg 2, Room 8, 1:00 PM

    Abstract

    VERITAS Observations of the Crab Nebula and Pulsar Observations of the Crab Nebula, the standard candle in TeV astronomy, has proven to be the best tool to test and characterize the performance of a new gamma-ray instrument. Demonstration of the consistency of the results from these observations with the well established properties of Crab builds confidence that the results from other observations with the instrument are correct. Scientifically, it is interesting to measure the energy spectrum of the Crab Nebula close to the inverse-Compton peak where a deviation from the power law seen at energies above 300 GeV is expected. Additionally, it is important to search for pulsed emission from the Crab Pulsar at energies beyond 10 GeV, the highest energy at which EGRET detected pulsed emission from the Crab. With these motivations, the Crab has been observed extensively during the 2-, 3-, and 4-telescope commissioning phases of VERITAS. The energy spectrum of the Crab Nebula between 200 GeV and 7 TeV is constructed from these data sets. A search for pulsed emission from the Crab Pulsar at gamma-ray energies above 100 GeV is also performed. The Crab data set from these observations does not show any significant pulsed excess, so an upper limit on the pulsed emission is obtained. The results of these studies will be presented on the talk. The advantages and the capabilities of GLAST in detection of pulsed emission from many pulsars in the high-energy gamma-ray band and answering most of the current open questions in pulsar physics will be discussed in the conclusion.

    GLAST-LAT Signatures of UHECRs Production in GRBs.

    Sylvain Guiriec

    Laboratoire de Physique Theorique et Astroparticles (LPTA)

    Friday, April 25, 2008

    NOTE TIME/LOCATION: Bldg 2, Room 8, 1:00 PM

    Abstract

    After a decade of preparation, NASA's "Gamma-ray Large Area Space Telescope" (GLAST) will observe, using the "Large Area Telescope" (LAT), the gamma-ray sky from 20 MeV up to >300 GeV. With an unprecedented sensitivity, the LAT will replace the "Energetic Gamma-ray Experiment Telescope" (EGRET) on-board the well known "Compton Gamma-Ray Observatory" (CGRO). With an energy range from 8 keV up to 30 MeV and a sky coverage over 9.5 sr, the "GLAST Burst Monitor" (GBM) on-board GLAST will replace CGRO-BATSE ("Burst And Transient Source Experiment") to detect and study transient sources and "Gamma-Ray Bursts" (GRBs). In this presentation, I will briefly describe both GLAST instruments as well as the current status of the mission whose launch is scheduled for May 16th 2008. The mission is expected to last from 5 to 10 years. I will emphasize on the interest of these two instruments for the study of GRBs at high energy. GRBs are very intense and short gamma-ray emissions which could be produced by stellar mass black holes or magnetars. The central engine responsible of the gamma-ray burst could result from the collapse of supermassive stars or from the coalescence of compact objects. I will also present some high energy emission models that GLAST will constrain. In the last part, I will propose a semi leptonic and hadronic model for GRBs based on the "fireball model" which currently best reproduces the prompt and afterglow emission. I will show you how past and future observations of the prompt emission (few keV up to some MeV) interpreted as the synchrotron emission from accelerated electrons during the internal shocks, can constrain the parameters of the GRB jet magnetic field. According to these parameters, I will Energy Cosmic Rays (UHECRs) at the beginning of the internal shocks. For this purpose, I will present an original "second order Fermi" like process. Finally, I will test the possibility of detecting synchrotron emission from UHECRs using LAT.

    A New Approach to Analyzing Solar Coronal Spectra Using Updated Collisional Ionization Equilibrium Calculations

    Paul Bryans

    Columbia University

    Tuesday, April 29, 2008

    Abstract

    Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. Using recently published state-of-the-art electron-ion recombination data, we have calculated new fractional ionic abundances for collisional ionization equilibrium (CIE) for all elements from H through Zn. Utilizing these new results, we have reanalyzed a SUMER observation of the solar corona using our newly developed systematic method for determining the emission measure and electron temperature of the emitting plasma. Using this technique I will discuss the impact of the results on the first ionization potential (FIP) effect and the long-standing Li- and Na-like ion abundance discrepancy in the solar corona.

    Milagro, A Wide Field of View VHE Gamma-Ray Observatory

    Vlasios Vasileiou

    University of Maryland (College Park)

    Wednesday, April 30, 2008

    NOTE TIME/LOCATION: Bldg 2, Room 8: 2:00 PM

    Abstract

    Milagro is a wide field of view gamma-ray observatory. It is using the water-Cherenkov technique to detect cosmic gamma-rays of energies ranging from 100 GeV to 500 TeV. I will talk about my graduate work with Milagro; namely, the blind whole-sky search for VHE emission from GRBs, the Monte Carlo simulation of the detector with Geant4 and the photocathode-uniformity tests on Milagro's Photomultiplier Tubes

    Stellar Coronae and AGN Coronae - Establishing a Physical Analogy

    Ehud Behar

    Technion (on sabbatical at NASA/GSFC)

    Tuesday, May 6, 2008

    Abstract

    The talk will discuss new evidence for a high-energy physical analogy between the extensively studied coronae of stars and the putative, poorly understood coronae of accretion disks in active galactic nuclei (AGNs). The current picture of magnetic energy powering the X-ray and radio emission of stellar coronae will be briefly reviewed with an emphasis on recent results for large X-ray flares. Subsequently, empirical evidence will be presented suggesting that the X-ray and radio emission from radio-quiet AGNs originates in a corona akin to stellar. It will be demonstrated how the AGN coronal plasma parameters can be estimated using basic Compton scattering and synchrotron self-absorption theory. The physical similarities between stellar and AGN coronal sources will be pointed out along with some of the obvious differences. Finally, future X-ray and radio observations will be proposed to further explore the AGN coronal hypothesis.

    Understanding the conditions for crystalline silicate formation: Lessons from an obscured AGB star

    Angela Speck

    University of Missouri

    Tuesday, May 13, 2008

    Abstract

    IRAS 17495-2534 is a highly obscured oxygen-rich AGB star that does not exhibit OH/IR maser emission. However, its IRAS LRS spectrum exhibits the strongest mid-infrared crystalline silicate absorption features seen to date. Consequently this source provides an unprecedented opportunity to test competing hypotheses for dust formation and to understand the formation of crystalline silicates. This source is located in the Galactic plane at a distance of ~4kpc, and thus may have higher than solar metallicity. However, comparing the spectrum to those of OH/IR stars in the Galactic Bulge shows that this source has significantly stronger crystalline silicate features, indicating that metallicity is not the only factor influencing crystal production. Radiative transfer modeling of IRAS 17495-2534 suggests that both the crystalline and amorphous components of silicate dust are dominated by forsterite (Mg2SiO4) rather than enstatite (MgSiO3) or other more silica-rich compositions. Calculations predict that, in stars with relatively high mass-loss rates, the high pressure (leading to high dust formation temperatures) and slow outflow velocities (leading to slow cooling) should result in either direct formation of crystalline grains or rapid annealing and crystallization of initially amorphous silicates. Competing dust formation mechanisms will be compared to determine whether they can explain the spectrum of this particular star. In addition, since the spectra of most high mass-loss stars are interpreted to contain little crystalline material, we investigate the effects of C/O, metallicity, mass-loss rate and initial stellar mass on crystalline dust formation and discuss why there is apparently so little crystalline dust around most O-rich stars.

    The accretion history of black holes in galactic nuclei

    Priya Natarajan

    Yale University

    Wednesdayday, May 14, 2008

    Abstract

    In this talk I will present some key issues in the formation, assembly and evolution of black holes in galactic nuclei. I will summarize our current theoretical understanding of black hole assembly consistent with existing observations. I will then discuss in detail two key aspects of accretion disk physics that are crucial to our understanding of the growth of black holes (i) the formation of the first seed black holes and (ii) the role of gas accretion in the merger of binary black holes.

    Dynamics of Accretion Flows Irradiated by a Quasar

    Daniel Proga

    University of Nevada (Las Vegas)

    Friday, May 16, 2008

    NOTE: To be held in Bldg 2, Room 8

    Abstract

    I will present results from recent time-dependent radiation-hydrodynamical calculations of flows that are under the influence of supermassive black hole gravity and radiation from an accretion disk surrounding the black hole. The calculations take into account radiation heating/cooling and the radiation force due to electron scattering and spectral lines. I show how the mass accretion and outflow rates, and the flow dynamics respond to thermal effects and effects of radiation pressure and gas rotation. The main focus of this work is to gain insights into the problem of AGN feedback and the formation and evolution of cold clouds in AGN.

    The 3D structure of the pulsar magnetosphere: first results

    Ioannis Contopoulos

    Academy of Athens

    Tuesday, May 20, 2008

    Abstract

    We will present our preliminary results on the 3D structure of the pulsar magnetosphere obtained through time dependent numerical simulations of a rotating magnetic dipole. We discovered that after about ten neutron star rotations, the magnetosphere settles to a dynamic steady-state consisting of a sequence of openings and closings of magnetic field lines at the period of the pulsar rotation. We will discuss the prospects of our code for future high resolution investigations of dissipation, particle accelerationa, and high-energy emission in the pulsar magnetosphere.

    CANCELLED; WILL BE RESCHEDULED -- Gravitational Waves in Multimessenger Astrophysics

    Szabolcs Marka

    Columbia University

    Tuesday, May 27, 2008

    THIS TALK WILL BE RESCHEDULED FOR THE FALL

    Abstract

    Gamma-ray, X-ray, optical and neutrino observations of cataclysmic cosmic events with plausible gravitational wave emission can be used in combination with searches for gravitational waves. Information on the progenitor, such as trigger time, direction and expected frequency range, shall enhance our ability to identify gravitational wave signatures with amplitudes close to the noise floor of the detector. Even in the absence of detection, the association of the astrophysical trigger with a particular source distance allows to interpret upper limits on the observed flux of gravitational waves in terms of the energy emitted in the form of gravitational waves. After a summary of past multimessenger based gravitational wave searches, I will discuss the implications of these results. I will close by giving an outlook on the future.

    X-ray Polarimetry with Dichroic Filters

    Adrian Martindale

    University of Leicester

    Tuesday, May 27, 2008

    Abstract

    X-ray polarimetry remains the most underdeveloped method of studying celestial X-ray sources. Indeed, no progress has been made in this field for over 30 years, despite intense scientific interest in measuring X-ray polarisation. New X-ray dichroic filters offer a simple and effective science enhancement opportunity for the cryogenic spectrometers proposed for XEUS and Constellation-X by providing them with polarisation sensitivity. I will describe experimental and theoretical work undertaken to demonstrate the Dichroic filter concept. Modelling based on experimental data and the proposed performance of XEUS predicts a minimum detectable polarisation (MDP) of about 1-2% at 8.9 keV and about 4-5% at 13.4 keV in a 100 ksec observation of a number of sources. Demonstration of X-ray linear dichroism at lower energy edges would offer significantly improved MDP for a much broader range of celestial objects and is a major goal of our current research.


    Randall Smith
    NASA Logo, National Aeronautics and Space Administration