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Astrophysics Science Division Colloquium Series
Schedule: October - December 2007

Astrophysics Science Division Colloquium Series
Schedule: October - December 2007


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

Future schedules:

Recent schedules:

  • 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.

    October

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
      1
    2 Robert Naeye (NASA/GSFC) - My Experience in the Magazine Business and the Journalist/Scientist Relationship
    3
    4 Yoshiyuki Inoue (Kyoto University) - The Cosmic MeV Gamma-ray Background and Hard X-ray Spectra of Active Galactic Nuclei: Implications for the Origin of Hot AGN Coronae
    5
    6
    7
    8
    9 Dong Lai (Cornell) Neutron Star Formation/Evolution
    10 Peter Becker (GMU) A New Analytical Model for Bulk and Thermal Comptonization in Accretion-Powered X-Ray Pulsars
    11
    12
    13
    14
    15
    16 Suvi Gezari (CalTech) UV/Optical Detections of the Tidal Disruption of Stars by Supermassive Black Holes
    17
    18
    19
    20
    21
    22
    23 Tony Moffat (U. Montreal) - The Most Massive Stars
    24
    25
    26
    27
    28 29
    30 Floyd Stecker (NASA/GSFC) Stellar Photon and Blazar Archaeology with Gamma Rays
    31

    November

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
         1
    2
    3
    4
    5
    6 Savvas Koushiappas (LANL) Dark matter physics with GLAST
    7 Marat Gilfanov (MPA/Garching) X-ray Populations in Galaxies
    8
    9
    10
    11
    12
    13 Tamara Bogdanovic (UMCP) Alignment of the Spins of Supermassive Black Holes Prior to Merger
    14
    15
    16
    17
    18
    19
    20 Peter Shawhan (UMCP) LIGO Listens for Gravitational Waves
    21
    22
    23
    24
    25
    26
    27 No Talk (Thanksgiving)
    28 29
    30

    December

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
           1
    2
    3
    4 Mark Trodden (Syracuse) Gravitational >Approaches to Cosmic Acceleration
    5
    6
    7
    8
    9
    10 Paul McNamara (ESA) The LISA Pathfinder Mission
    11 Sean O'Neill (UMCP) Simulating Interactions Between Active Galactic Nuclei and Their Environments
    12
    13
    14
    15
    16
    17
    18 David Leisawitz (GSFC) The Space Infrared Interferometric Telescope (SPIRIT): A Far-IR Observatory for High-resolution Imaging and Spectroscopy
    19
    20
    21
    22
    23
    24
    25 No Talk
    26
    27
    28
    29
    30
    31


    My Experience in the Magazine Business and the Journalist/Scientist Relationship

    Robert Naeye

    NASA/GSFC

    Tuesday, October 2, 2007

    Abstract

    Robert Naeye, Senior Science Writer for Goddard's Astrophysics Science Division, is the only person who has ever worked as a full-time editor on the staffs of both Astronomy and Sky & Telescope magazines. He has also worked on the editorial staff of Discover magazine, and as editor-in-chief of Mercury magazine (published by the Astronomical Society of the Pacific). Robert will talk about his 15 years of experience in the magazine business, including a comparison of Astronomy and S&T. He will also discuss how magazines are produced, the relationship between scientists and journalists, tips on writing popular science articles, and the considerable challenges faced by science magazines in the Internet Age.

    The Cosmic MeV Gamma-ray Background and Hard X-ray Spectra of Active Galactic Nuclei: Implications for the Origin of Hot AGN Coronae

    Yoshiyuki Inoue

    Kyoto University

    Thursday, October 4, 2007

    Abstract

    The origin of the extragalactic gamma-ray background at 1-10 MeV has for a long time been a mystery. Though the cosmic X-ray background (CXB) up to a few hundreds keV can be accounted for by superposition of Active Galactic Nuclei (AGNs), models of AGN spectra are not able to explain the background spectrum beyond about 1 MeV, because of the thermal exponential cutoff of electron spectra assumed in the models. Here we construct a new spectral model by calculating the Comptonizatino process including nonthermal electrons, which are naturally expected to exist in an AGN hot corona if it is heated by magnetic reconnections. We show that the MeV background spectrum can be naturally explained by our model, and discuss some implications for the heating mechanism of AGN coronae, comparing with the observations of nonthermal electrons observed in reconnections in solar flares and the Earth magnetosphere.

    Neutron Star Formation/Evolution

    Dong Lai

    Cornell University

    Tuesday, October 9, 2007

    Abstract

    The origin of the extragalactic gamma-ray background at 1-10 MeV has for a long time been a mystery. Though the cosmic X-ray background (CXB) up to a few hundreds keV can be accounted for by superposition of Active Galactic Nuclei (AGNs), models of AGN spectra are not able to explain the background spectrum beyond about 1 MeV, because of the thermal exponential cutoff of electron spectra assumed in the models. Here we construct a new spectral model by calculating the Comptonizatino process including nonthermal electrons, which are naturally expected to exist in an AGN hot corona if it is heated by magnetic reconnections. We show that the MeV background spectrum can be naturally explained by our model, and discuss some implications for the heating mechanism of AGN coronae, comparing with the observations of nonthermal electrons observed in reconnections in solar flares and the Earth magnetosphere.

    A New Analytical Model for Bulk and Thermal Comptonization in Accretion-Powered X-Ray Pulsars

    Peter Becker

    George Mason University

    Wednesday, October 10, 2007

    Abstract

    Accretion-powered X-ray pulsars are among the most luminous X-ray sources in the galaxy. However, until recently no satisfactory model for the formation of the observed X-ray spectra based on fundamental physics was available. In this talk, I discuss the first self-consistent model for the formation of the observed X-ray pulsar spectra based on the radiative transfer and the dynamics occurring in the accreting gas as it decelerates through a radiation-dominated standing shock onto one of the magnetic poles of a rotating neutron star. The spectral formation process is heavily influenced by the bulk and thermal Comptonization occurring in the accreting gas. Using an eigenfunction expansion method, we obtain a closed-form solution for the Green's function describing the scattering of monochromatic seed radiation injected into the column. The Green's function is convolved with bremsstrahlung, cyclotron, and blackbody source terms to calculate the photon spectrum that emerges from the accretion column. The energization of the photons in the shock, combined with cyclotron absorption, naturally produces an X-ray spectrum with a relatively flat continuum, leading up to a high-energy exponential cutoff. The results are in good agreement with the observed X-ray pulsar spectra over a wide range in luminosity, including bright sources such as LMC X-4, Her X-1, and Cen X-3, and low-luminosity sources such as X Per. It is shown that in the luminous sources, the emergent spectrum is dominated by Comptonized bremsstrahlung emission, and in the low-luminosity sources the spectrum is dominated by Comptonized blackbody emission. We speculate on possible extensions of the model to treat millisecond X-ray pulsars such as SAX J1808.4.

    UV/Optical Detections of the Tidal Disruption of Stars by Supermassive Black Holes

    Suvi Gezari

    CalTech

    Tuesday, October 16, 2007

    Abstract

    A supermassive black hole lurking in the nucleus of a normal galaxy will be revealed when a star approaches close enough to be torn apart by tidal forces, and a flare of radiation is emitted as the stream of stellar debris plunges into the black hole. The luminosity, temperature, and decay of a tidal disruption flare are dependent on the mass and spin of the central black hole, and can be used to directly probe dormant black holes in distant galaxies for which the sphere of influence of the black hole is unresolved, and a dynamical measurement of the black hole mass is not possible. I will present simultaneous Ultraviolet and Optical detections of tidal disruption flares by GALEX and CFHTLS, and compare the observed properties and detection rates of the flares to the theoretical predictions.

    The Most Massive Stars

    Tony Moffat

    University of Montreal

    Tuesday, October 23, 2007

    Abstract

    Locating the most massive stars and measuring their masses is key to understanding the formation and nature of (massive) stars in general. This is best done by using the least model-dependent technique, i.e. Keplers laws (combined with techniques to extract the orbital inclination) for binary systems that contain very luminous stars on or near the main sequence, where interaction effects are not (yet) important. In the local Universe, it appears that the potentially most massive, main-sequence stars are the most luminous hydrogen-rich Wolf-Rayet stars of type WN5-7ha, which are even more luminous than, and probably at least as hot as, the recently recognized hottest main-sequence stars of type O2. WN5-7ha stars exhibit strong, broad WR-like emission lines because of their high luminosity, not because of their compactness and high ratio of L/M as classical He-burning, WR stars. It is probably no coincidence that the currently most massive stars measured this way are in WR20a, an eclipsing binary system containing two identical WN6ha stars of 83 and 82 solar masses. Other WN5-7ha stars, some even more luminous than WR20a, are being measured currently and I will report on the overall results, with implications for stellar evolution.

    Stellar Photon and Blazar Archaeology with Gamma Rays

    Floyd Stecker

    NASA/GSFC

    Tuesday, October 30, 2007

    Abstract

    Ongoing deep surveys of galaxy luminosity distribution functions, spectral energy distributions and backwards evolution models of star formation rates can be used to calculate the past history of intergalactic photon densities and, from them, the present and past optical depth of the universe to gamma-rays from pair production interactions with these photons. This procedure can also be reversed by looking for sharp cutoffs in the spectra of extragalactic gamma-ray sources at high redshifts in the multi-GeV energy range with GLAST (the Gamma-ray Large Area Space Telescope). Determining the cutoff energies of sources with known redshifts and little intrinsic absorption may enable a more precise determination of the IBL photon densities in the past, i.e., the "archaeo-IBL," and therefore allow a better measure of the past history of the total star formation rate, including that from galaxies too faint to be observed. Conversely, observations of sharp high energy cutoffs in the gamma-ray spectra of blazars at unknown redshifts can be used instead of spectral lines to determine their redshifts. Given a knowledge of the archaeo-IBL, one can derive the intrinsic gamma-ray spectra and luminosities of blazars over a range of redshifts to look for possible trends in blazar evolution. There is now some evidence hinting that TeV blazars with flatter spectra have higher intrinsic TeV gamma-ray luminosities and indicating that there may be a correlation of flatness and luminosity with redshift. GLAST will observe and investigate many blazars in the GeV energy range and will therefore provide much new information regarding this possibility.

    Dark matter physics with GLAST

    Savvas Koushiappas

    Los Alamos National Lab

    Tuesday, November 6, 2007

    Abstract

    GLAST is a unique tool in the search for dark matter. As a space mission which will survey the whole sky, it can provide information on the nature of dark matter in a way that no other existing or proposed experiment can do. One such area is the search for gamma-rays from the annihilation of dark matter particles in the Milky Way substructure. In general, a detection of gamma-rays from dark matter annihilation (or the lack of a detection) places constrains on the nature of the dark matter particle. It is thus crucial to understand the uncertainties and assumptions that enter in any prediction. I will present an overview of different substructure candidates, and specifically show that dwarf spheroidals of the local group are the most promising targets for detecting dark matter annihilations.

    X-ray Populations in Galaxies

    Marat Gilfanov

    Max Planck Institute / Garching

    Wednesday, November 7, 2007

    Abstract

    I will discuss various aspects of populations of compact X-ray sources in nearby galaxies. Based on Chandra and XMM-Newton observations I will demonstrate that the populations of high- and low-mass X-ray binaries are closely related to the star-formation history and present stellar mass of the host galaxy and can be used as independent estimators to measure these quantities in more distant galaxies, included the ones located at cosmologically interesting redshifts. The X-ray luminosity distributions of these two types of accreting X-ray sources differ dramatically, reflecting the difference in their accretion regimes. The HMXB XLF is intimately related to the massive star IMF. The numbers of HMXBs in the nearby galaxies indicate that a significant (up to a half) fraction of relativistic objects of stellar mass once in their life-time experience an accretion episode and become less or more luminous sources of X-ray emission. Overall, they contribute about ~7-10% to the cosmic X-ray background below ~10 keV and can explain about a ~half of its unresolved part. Finally, I will discuss dynamical formation of X-ray binaries in the dense stellar environment of galactic bulges using the bulge of Andromeda galaxy as an example and compare it with a similar process taking place in globular clusters.

    Alignment of the Spins of Supermassive Black Holes Prior to Merger

    Tamara Bogdanovic

    University of Maryland - College Park

    Tuesday, November 13, 2007

    Abstract

    Fully relativistic simulations of merging spinning black holes suggest that the emission of gravitational waves can impart a kick of several thousand kilometers per second on the merger remnant if the spin axes of pre-merger black holes lie close to the plane of the orbit. This exceeds galactic escape speeds and would suggest that some fraction of major galaxy mergers would lead to complete ejection of the final black hole. We suggest that, in most gas-rich galactic mergers, accretion torques will effectively align the black hole spins with the orbital angular momentum of the binary thereby allowing these large kick velocities to be avoided. We predict, however, that alignment will not occur in gas-free mergers and that the merger remnant will indeed be ejected in some small fraction of major gas-free mergers. I will also discuss other predictions of our scenario including implications for X-shaped radio galaxies.

    LIGO Listens for Gravitational Waves

    Peter Shawhan

    University of Maryland - College Park

    Tuesday, November 20, 2007

    Abstract

    The general theory of relativity predicts that massive objects in motion create perturbations in the geometry of spacetime which propagate away as gravitational waves. However, these waves have not yet been detected directly. The Laser Interferometer Gravitational-Wave Observatory (LIGO) leads the ground-based effort to detect gravitational waves at frequencies above 10 Hz, where plausible sources include binary systems of neutron stars and/or black holes, as well as the collapsing cores of massive stars. The LIGO detectors in Washington State and Louisiana have recently completed a two-year "science run" during which they operated at their design sensitivity. The GEO600 and Virgo detectors in Europe collected data for part of the same time, forming a coherent network of gravitational wave detectors. I will review the design and operation of LIGO and present results from some recent searches for gravitational-wave signals in the data. I will also describe the upgrades in progress to improve the sensitivity of the detectors and ultimately enter an era of gravitational-wave astronomy.

    Gravitational Approaches to Cosmic Acceleration

    Mark Trodden

    Syracuse University

    Tuesday, December 4, 2007

    Abstract

    Among the possible explanations for the observed acceleration of the universe, perhaps the boldest is the idea that new gravitational physics might be the culprit. In this colloquium I will discuss some of the challenges of constructing a sensible phenomenological extension of General Relativity, give examples of some candidate models of modified gravity and survey existing observational constraints on this approach. I will conclude by discussing how we might hope to distinguish between modifications of General Relativity and dark energy as competing hypotheses to explain cosmic acceleration.

    Simulating Interactions Between Active Galactic Nuclei and Their Environments

    Sean O'Neill

    University of Maryland - College Park

    Tuesday, December 11, 2007

    Abstract

    X-ray observations of galaxy clusters illustrate the immense energies associated with interactions between active galactic nuclei (AGN) and their environments. While many AGN deliver sufficient amounts of energy to stifle cooling flows and reheat the intra-cluster medium, it is unclear how this energy can be adequately transferred to and distributed within the surrounding medium. Here, I will present the results of an ensemble of high-resolution, three-dimensional, magnetohydrodynamic simulations of AGN jets and jet-produced structures in realistic galaxy cluster environments. While the focus of the discussion will be on energy flow and ambient heating, I will also briefly address the morphologies of our simulated jets, lobes, and bubbles, and some general features of magnetic field evolution in these systems.

    The LISA Pathfinder Mission

    Paul McNamara

    ESA

    Building 21, Room 183A, 3:45pm

    Monday, December 10, 2007

    Abstract

    LISA Pathfinder (formerly known as SMART-2) is a European Space Agency (ESA) mission designed to pave the way for the joint ESA/NASA LISA mission by testing in flight, to an unprecedented accuracy, one of the core assumptions of General Relativity: that free-particles follow geodesics. LISA Pathfinder achieves this goal by putting two test masses in a near-perfect gravitational free-fall, and controlling and measuring their motion with unprecedented accuracy. This is achieved through technology comprising inertial sensors, high precision laser metrology, drag-free control and an ultra-precise micro-Newton propulsion system. LISA Pathfinder is due to be launched in mid-2010, with first results on the performance of the system being available 6 months later. Here I will give an introduction to, and current status of, the mission, followed by a more detailed discussion of the technologies to be tested.

    The Space Infrared Interferometric Telescope (SPIRIT): A Far-IR Observatory for High-resolution Imaging and Spectroscopy

    David Leisawitz

    NASA/GSFC

    Tuesday, December 18, 2007

    Abstract

    By providing sensitive sub-arcsecond resolution images and integral field spectroscopy in the 25 to 400 micron wavelength range, the Space Infrared Interferometric Telescope (SPIRIT) will: (1) revolutionize our understanding of the formation of planetary systems and enable us to "follow the water" as these systems develop; (2) reveal otherwise-undetectable exoplanets; (3) probe the atmospheres of extrasolar giant planets; and (4) make profound contributions to our understanding of the formation, merger history, and star formation history of galaxies. SPIRIT was originally studied as a candidate Origins Probe mission and recently proposed for study as an Astrophysics Strategic Mission. I will describe the mission's main science goals and measurement capabilities, the Origins Probe mission concept, and our plan to study SPIRIT as a facility-class mission in preparation for the upcoming Decadal Survey.


    Randall Smith
    NASA Logo, National Aeronautics and Space Administration