Astrophysics Science Division Colloquium Series
Schedule: January - March 2007
Astrophysics Science Division Colloquium Series
Schedule: January - March 2007
Through the courtesy of the speakers since 2004,
most presentations are available on line.
Future schedules:
2007, Second Quarter
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Past schedules:
2006, Fourth Quarter
2006, Third Quarter
2006, Second Quarter
2006, First Quarter
2005, Fourth Quarter
2005, Third Quarter
2005, Second Quarter
2005, First Quarter
2004, Fourth Quarter
2004, Third Quarter
2004, Second Quarter
2004, First Quarter
2003, Fourth Quarter
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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.
The Role of Reconnection in the Pulsar Magnetosphere
Ioannis Contopoulos
Academy of Athens
Tuesday, January 16, 2007
Abstract
We will present an overview of our current understanding
of the structure of the pulsar magnetosphere. We will show
that ideal MHD is a valid description of the largest part of
the magnetosphere except for a thin equatorial region where
the main magnetospheric electric current flows. We will
show that equatorial reconnection is related to the global
magnetic field topology, and therefore it cannot be studied
independently. We will argue that reconnection efficiency
may be measurable through the pulsar braking index.
Finally, we will argue that equatorial reconnection is able to
accelerate particles that reach the pulsar wind termination
shock with energies of the order of 10^16 eV.
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Cosmology and the Atacama Cosmology Telescope (ACT)
Suzanne Staggs
Princeton
Tuesday, January 30, 2007
Abstract
The Atacama Cosmology Telescope (ACT) is a new 6m millimeter-wave
telescope in the high dry desert of Chile. The ACT team is
outfitting the telescope with three kilopixel arrays of TES bolometer
detectors made at NASA/GSFC. The ACT cameras will image the sky at
three different frequencies with arcminute resolution and microKelvin
sensitivity. These maps will allow characterization of the cosmic
microwave background (CMB) angular power spectrum out to very small
angular scales (never before measured). Since the WMAP data
constrain most of the other relevant cosmological parameters with
their measurements of the larger-angular-scale part of the spectrum,
these new ACT measurements can reveal something about the initial
conditions of the primordial power spectrum of density
fluctuations. The ACT maps will also encode information about the
intervening universe between the primordial plasma at redshift 1000
and the present epoch, in the form of spatial correlations imparted
to the microwave fluctuations by gravitational lensing of the matter
in the lines of sight. Finally, the three frequencies of the ACT
detector arrays have been chosen for ready identification of the
spectral distortions the CMB spectrum develops upon scattering from
the hot electrons in clusters of galaxies. Thus, ACT will catalog
(nearly) all the clusters in the fields it observes. The ACT project
includes follow-up to determine cluster redshifts. We will
elaborate on these science goals, and also describe the experiment in
some detail.
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Binary Black Holes, Gravitational Waves, and Numerical Relativity
Joan Centrella
NASA/Goddard Space Flight Center
Tuesday, March 27, 2007
Abstract
The final merger of two black holes releases a tremendous
amount of energy and is one of the brightest sources in
the gravitational wave sky. Observing these sources with
gravitational wave detectors requires that we know the
radiation waveforms they emit. Since these mergers take
place in regions of extreme gravity, we need to solve
Einstein's equations of general relativity on a computer
in order to calculate these waveforms.
For more than 30 years, scientists have tried to compute
these waveforms using the methods of numerical relativity.
The resulting computer codes have been plagued by instabilities,
causing them to crash well before the black holes in the binary
could complete even a single orbit. REcently this situation has changed
dramatically, with a series of
amazing breakthroughs. This talk will take you on this
quest for the holy grail of numerical relativity, showing
how a spacetime is constructed on a computer to build a
simulation laboratory for binary black hole mergers.
We will focus on the recent advances that are revealing
these waveforms, and the dramatic new potential for discoveries
that arises when these sources will be observed by LISA
and LIGO.
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The Unidentified, Extended TeV Gamma-ray Source HESS J1303-631
and Fast Variability of Radio Galaxy M87 Observed with H.E.S.S.
Matthias Beilicke
University Hamburg
Friday, February 16, 2007
Time: 11am / Location: Bldg 2, Rm 8
Abstract
The TeV gamma-ray source HESS J1303-631 was serendipitously discovered
in the field of view of the binary system PSR B1259-63 by the H.E.S.S.
Cherenkov telescopes in 2004. A counterpart at other wavelengths was
not found so far, making HESS J1303-631 an unidentified TeV gamma-ray
source. Results from the TeV gamma-ray observations and implications
are presented. The giant radio galaxy M87 was observed by H.E.S.S. in
2003-2006. The observations confirm M87 as the first extragalactic TeV
gamma-ray source not of the blazar type (first indications of a signal
were reported by the HEGRA collaboration earlier). The TeV gamma-ray
flux was found to be variable on time-scales of days which strongly
constrains the size of the emission region. The results as well as
theoretical interpretations will be presented.
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The Partitioning of Energy among Electrons, Ions, and Cosmic Rays at
Collisionless Shocks in SNRs.
Cara E. Rakowski
NRC Fellow, Naval Research Lab
Tuesday, February 20, 2007
Abstract
The outer blast-waves of supernova remnants (SNRs) are an example of
``collisionless shocks'', i.e. the width of the shock transition is tiny
compared to the Coulomb mean-free-path. In these shocks the particle
heating to postshock temperatures and acceleration to cosmic ray
energies must be mediated by plasma waves arising from instabilities,
and not just from random Coulomb collisions. The 1/1835 mass ratio of
electrons to protons makes the heating and acceleration of electrons
particularly difficult. In this talk I will explain the spectroscopic
techniques for determining the proton, ion and electron temperatures at
a variety of supernova remnant shocks, and present the latest data on
the electron to proton temperature ratio from this survey. The observed
inverse square dependence of the electron to proton temperature ratio
with shock velocity can be explained by a physical model for the
electron heating, whereby lower hybrid waves excited in the shock
cosmic-ray precursor damp by accelerating electrons along the local
magnetic field, echoing recent suggestions in the literature that the
cosmic rays are an integral part of the collisionless shock structure.
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Testing Theories of Gravity with Black Hole Lensing
Charles Keeton
Dept. of Physics, Rutgers
Tuesday, March 13, 2007
Abstract
The gravitational deflection of light provided one of the first
observational confirmations of general relativity. Now I am considering
how gravitational lensing can provide a stronger and more fundamental
test of Einstein's theory, and of intriguing new alternatives. I will
introduce a rigorous and comprehensive analytical framework for black
hole lensing, and use it to make concrete predictions that are testable
with current or near-future technology. Here are two examples: (1) In
post-post-Newtonian theories of gravity, there are universal relations
among lensing observables. Observed violations of these relations would
falsify all PPN models in one fell swoop. (2) In braneworld gravity,
there could be many primordial black holes (even in our Solar System) that
may be detectable in "attolensing" of gamma ray bursts observed by GLAST.
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Supermassive Black Holes: Their Growth and the Link to Galaxy Formation
Lance Miller
Dept. of Physics, Oxford
Tuesday, March 20, 2007
Abstract
We now recognise that every massive galaxy contains a supermassive
black hole at its heart, and that the growth of those black holes is
both observed in the luminous phases of active galactic nuclei (AGN)
and quasars, and is likely responsible for the observed hard X-ray
background. We still don't understand what the relationship is between
black hole growth and galaxy growth, or why accretion onto black holes
turned off as the universe evolved towards the present day. I will argue
that in fact black hole growth, and hence AGN evolution, is intimately
connected to galaxy growth, and that AGN cosmological evolution has a
natural explanation within hierarchical galaxy formation.
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Jerry Bonnell
