Astrophysics Science Division Colloquium Series
Schedule: April - June 2007
Astrophysics Science Division Colloquium Series
Schedule: April - June 2007
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.
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Randall Smith
