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Astrophysics Science Division Colloquium Series
Schedule: April - June 2007

Astrophysics Science Division Colloquium Series
Schedule: April - June 2007


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

Future schedules:

  • 2007, Third Quarter
  • Past schedules:

  • 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 2 3 Ted Jacobson (UMCP/Physics Dept.) - Astrophysical constraints on Lorentz violation 4 5 6 7
    8 9 10 11 12 Wayne Baumgartner (CIT) - Hard X-ray Astronomy with CZT detectors and the High Energy Focusing Telescope 13 14
    15 16 17 Yosi Gelfand (CfA) - The Evolution of Pulsar Wind Nebulae in Supernova Remnants 18 19 Dave Henley (Univ. Georgia) - XMM-Newton and Suzaku Observations of the Soft X-ray Background 20 21
    22 23 24 25 26 Marta Volonteri (Univ. Michigan/IoA Cambridge) - Building up Massive Black Holes 27 Anna Watts (MPA Garching) - Neutron star seismology - towards a relativistic Richter scale 28
    29 30

    May

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    1 Antonio Codino (University of Perugia) - On the Origin of the Cosmic Ray Knee and Ankle 2 3 4 5
    6 7 8 No Colloquium 9 10 11 12
    13 14 15 Ann Hornschemeier (GSFC) - The Beyond Einstein Missions: Constellation-X 16 17 18 19
    20 21 22 Maurice Leutenegger (Columbia U.) Resolved X-ray Doppler Profiles of O Stars 23 24 25 26
    27 28 29 Scott Ransom (NRAO) - Detecting nHz Gravitational Waves with Millisecond Pulsars 30 31

    June

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    1 2
    3 4 5 Andrew Kerman (MIT Lincoln Lab.) - Superconducting NbN-Nanowire Single Photon Detectors 6 7 8 9
    10 11 12 Savvas Koushiappas (LANL) - GLAST and Dark Energy - POSTPONED 13 14 15 16
    17 18 19 No Colloquium 20 21 22 23
    24 25 26 Parviz Ghavamian (JHU) - Using Young Supernova Remnants as Laboratories for the Study of Collisionless Shocks 27 28 29 30


    Astrophysical constraints on Lorentz violation

    Ted Jacobson

    Dept. of Physics, U. Maryland (College Park)

    Tuesday, April 3, 2007

    Abstract

    Suspicions that Lorentz symmetry may be violated in quantum gravity have motivated recent efforts to improve observational bounds on such effects. I will review the field theory framework for considering Lorentz violation, describe some characteristic phenomena, and discuss current and possible future constraints making use of astrophysical observations.

    Hard X-ray Astronomy with CZT detectors and the High Energy Focusing Telescope

    Wayne Baumgartner

    California Institute of Technology

    Thursday, April 12, 2007

    Time: 2pm, Location: Bldg 2, Rm 8

    Abstract

    New advances in multilayer mirrors and room temperature CZT X-ray detectors have begun to open the hard X-ray band (10 keV - 100 keV) to astronomers. We are just starting to build focusing telescopes that will enable us to view highly energetic non-thermal processes in galactic black holes and neutron stars, supernova remnants, active galactic nuclei and galaxy clusters that are not readily observable at lower energies. I will focus on the balloon-borne High Energy Focusing Telescope (HEFT) and the results from observations of the Crab and Cyg X-1 during its first flight in May 2005. The CZT detectors we have developed for HEFT are at the heart of the instrument, and I will present flight and laboratory performance results that illustrate the excellent spectral resolution (700 eV at 60 keV) and imaging capability we have achieved with these detectors.

    The Evolution of Pulsar Wind Nebulae in Supernova Remnants

    Yosi Gelfand

    Harvard-Smithsonian CfA

    Tuesday, April 17, 2007

    Abstract

    Pulsar wind nebulae (PWNe) are the structures created by the particle winds generated by neutron stars as they lose rotational energy. By studying the emission from these objects, it is possible to determine the acceleration mechanism and particle content of the wind, the initial period of the central neutron star, and the properties of the surrounding medium. In this talk, I will present a simple, semi-analytic model for the evolution of a PWN inside a supernova remnant, apply it to existing observations of pulsar wind nebulae, and describe future applications of this model that will be made possible through forthcoming GLAST observations of PWNe.

    XMM-Newton and Suzaku Observations of the Soft X-ray Background

    Dave Henley

    University of Georgia

    Thursday, April 19, 2007

    Time: 11am, Location: Bldg 2, Rm 8

    Abstract

    The soft X-ray background (SXRB) is composed of emission from several different sources: solar wind charge exchange, the Local Bubble, the Galactic halo, and the extragalactic background due to unresolved AGN. X-ray spectroscopy of the SXRB enables us to separate out the different emission components, and also to determine the thermal and ionization state of the emitting plasmas, which helps constrain models for their origin.

    I will discuss the analysis of two XMM-Newton and two Suzaku observations of the SXRB. For each satellite, one observation was toward a nearby (d ~ 230 pc) absorbing filament at high southern Galactic latitude (b ~ -45 deg), and the other was toward a neighboring, unobscured region. The different absorbing columns in the two directions is used to disentangle the contributions of the unabsorbed foreground emission and the absorbed background emission. We find that the XMM-Newton spectra contain an extra emission component, over the best-fitting Suzaku model, which we attribute to solar wind charge exchange. I will discuss some of the implications of our results for models of the various components of the SXRB.

    Building up Massive Black Holes

    Marta Volonteri

    Univ. Michigan, IoA Cambridge

    Thursday, April 26, 2007

    Time: 3:30, Location: Bldg 21, Rm 183

    Abstract

    I'll discuss how massive black hole "seeds" may form in proto-galaxies, within a hierarchical cosmological framework. The growth from "seeds" to supermassive black holes, via accretion, mergers and dynamical interactions, as well as their implications, will be critically addressed.

    Neutron star seismology - towards a relativistic Richter scale

    Anna Watts

    MPA Garching

    Friday, April 27, 2007

    Time: 11am, Location: Bldg 2, Rm 8

    Abstract

    The detection of seismic vibrations in the aftermath of giant flares from two magnetars has opened up the prospect of using seismological techniques to study neutron star properties. Initial results have included the first direct estimate of the thickness of the neutron star crust, a strong constraint on the nuclear equation of state. I will discuss current efforts to improve our models of the starquake process, focusing in particular on the effects of the strong field on the oscillations, their excitation, damping and detectability. I will also cover prospects for future observations using both electromagnetic and gravitational wave telescopes. The detection of seismic vibrations in the aftermath of giant flares from two magnetars has opened up the prospect of using seismological techniques to study neutron star properties. Initial results have included the first direct estimate of the thickness of the neutron star crust, a strong constraint on the nuclear equation of state. I will discuss current efforts to improve our models of the starquake process, focusing in particular on the effects of the strong field on the oscillations, their excitation, damping and detectability. I will also cover prospects for future observations using both electromagnetic and gravitational wave telescopes.

    The Origin of the Knee and the Ankle in the Cosmic Ray Spectrum

    Antonio Codino

    INFN and Dipartimento di Fisica, Universita degli Studi di Perugia, Italy

    Tuesday, May 1, 2007

    Abstract

    The differential energy spectra of the cosmic radiation presents two deviations (the knee and the ankle) with respect to a constant spectral index at energies above 1015 eV (the knee) and above 5 x 1018 (the ankle). A new mechanism accounting for both the deviations is presented. A comparison between the computed and the measured spectra is given and discussed. The mechanism accounting for the knee and the ankle is based on the observational facts regarding the form and strength of the magnetic field in the Galaxy, the trend of the nuclear cross sections versus energy, the position of the Solar cavity in the disc and some others of minor importance.

    The Beyond Einstein Missions: Constellation-X

    Ann Hornschemeier

    NASA/GSFC

    Tuesday, May 15, 2007

    Abstract

    The Constellation-X Observatory will provide a 100-fold increase in collecting area over previous X-ray astrophysics missions enabling a great breadth of science currently inaccessible to Chandra and XMM-Newton. The primary science objectives for Con-X are (1) using black holes to test General Relativity (GR) and measuring black hole spin, (2) improving the constraints on the key Dark Energy parameters by a factor of ten, (3) unambiguous detection of the hot phase of the Warm-Hot Intergalactic Medium (WHIM) at z>0 and (4) measuring the mass-radius relation of neutron stars to determine the Equation of State (EOS) of ultra-dense matter. This talk will cover these science areas and also describe the Atlas V launch vehicle configuration of the mission. This configuration allows all three detector systems to be flown on a single launch vehicle in a single spacecraft and presents several advantages over the 4-spacecraft configuration of >2 years ago.

    Resolved X-ray Doppler Profiles of O stars

    Maurice Leutenegger

    Columbia University

    Tuesday, May 22, 2007

    Abstract

    High-resolution X-ray spectroscopy of O stars with the XMM-Newton RGS and the Chandra HETGS has shown that the wind-shock paradigm can explain the X-ray emission of most O stars. The wind-broadened Doppler profiles contain information about the distribution of X-ray emitting plasma in the wind. However, the profiles are far more symmetric than would be predicted from the published mass-loss rates. In addition to the obvious hypothesis that the published mass-loss rates of O stars might be systematically too high, two other mechanisms are available which can symmetrize Doppler profiles: porosity and resonance scattering. I will discuss current modeling efforts aimed at disentangling these three possibilities.

    Resolved X-ray Doppler Profiles of O stars

    Maurice Leutenegger

    Columbia University

    Tuesday, May 22, 2007

    Abstract

    High-resolution X-ray spectroscopy of O stars with the XMM-Newton RGS and the Chandra HETGS has shown that the wind-shock paradigm can explain the X-ray emission of most O stars. The wind-broadened Doppler profiles contain information about the distribution of X-ray emitting plasma in the wind. However, the profiles are far more symmetric than would be predicted from the published mass-loss rates. In addition to the obvious hypothesis that the published mass-loss rates of O stars might be systematically too high, two other mechanisms are available which can symmetrize Doppler profiles: porosity and resonance scattering. I will discuss current modeling efforts aimed at disentangling these three possibilities.

    Detecting nHz Gravitational Waves with Millisecond Pulsars

    Scott Ransom

    NRAO

    Tuesday, May 29, 2007

    Abstract

    Millisecond pulsars are extremely stable rotators whose radio pulses can be measured with extraordinary precision (hundreds of nanoseconds). An ensemble of pulsars spread across the sky can function as a gigantic detector for nHz-scale gravitational waves, in a regime which is complementary to LIGO and LISA. I descibe an ongoing international effort, using the largest radio telescopes on Earth, to detect a stochastic gravitational wave background from relic cosmological gravitational waves, cosmic strings, or (perhaps most likely) coalescing super-massive black-holes.

    Superconducting NbN-Nanowire Single Photon Detectors

    Andrew J. Kerman

    MIT Lincoln Laboratory

    Tuesday, June 5, 2007

    Abstract

    I will discuss our ongoing work at MIT on single-photon detectors based on superconducting NbN nanowires. These nanometer-scale devices exploit the ultrafast nonequilibrium electronic response in ultrathin films of the highly disordered superconductor NbN to produce a photon counter of unprecedented speed and sensitivity. With better than 30 ps timing resolution, ~few ns reset time after a detection, and high detection efficiency (70% demonstrated at 1550 nm), these devices show promise as an enabling technology in a number of areas, such as high data rate optical communications, spectroscopy of ultrafast quantum phenomena in biological and solid-state physics, quantum key distribution and quantum computation, astrophysics, laser radar, and high-speed noninvasive digital circuit testing. After providing an overview of the basic operating principles of these devices, I will present some of our recent results, including investigations into their detection mechanism, detection efficiency, and reset time; improvements to their optical design; and progress towards detector arrays. I will also show results from a demonstration experiment in which we used one of these detectors as a high-data-rate photon-counting optical receiver.

    This work is sponsored by the United States Air Force under Air Force Contract #FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by the United States Government.


    Using Young Supernova Remnants as Laboratories for the Study of Collisionless Shocks

    Parviz Ghavamian

    Dept. of Physics and Astronomy, Johns Hopkins University

    Tuesday, June 26, 2007

    Abstract

    Our understanding of astrophysical collisionless shocks is poor because they occur in conditions too extreme to model in terrestrial laboratories. When a non-relativistic collisionless shock propagates through a partially ionized medium, the excitation of cold Hydrogen and charge exchange with hot ions produces both narrow and broad Balmer emission lines. I will describe spectroscopic observations of these 'Balmer-dominated' shocks in Galactic and LMC supernova remnants. Numerical models of these structures, combined with the observational data, suggest that the electron to proton temperature ratio at the shock front declines as the inverse square of the shock speed. This provides important clues to the nature of the collisionless heating at the shock front, and I will outline a physical model which can account for the observations. Aside from providing a powerful tool for interpreting X-ray observations of young SNRs, results from our studies may also be applicable in predicting the temperature and ionization state of the warm-hot intergalactic medium (WHIM), where accretion shocks produce UV and X-ray emitting plasma during large scale structure formation.


    Randall Smith
    NASA Logo, National Aeronautics and Space Administration