Astrophysics Science Division Colloquium Series
Schedule: October - December 2007
Astrophysics Science Division Colloquium Series
Schedule: October - December 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.
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Randall Smith
