Astrophysics Science Division Colloquium Series
Schedule: April - June 2008
Astrophysics Science Division Colloquium Series
Schedule: April - June 2008
Through the courtesy of the speakers since 2004,
most presentations are available on line.
Future schedules:
2008, Third Quarter
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Past schedules:
2008, First Quarter
2007, Fourth Quarter
2007, Third Quarter
2007, Second Quarter
2007, First Quarter
2006, Fourth Quarter
2006, Third Quarter
2006, Second Quarter
2006, First 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.
Channels for Stellar-Mass Black Hole Mergers in the Centers of Galaxies
Vanessa Lauburg
University of Maryland - College Park
Tuesday, April 1, 2008
Abstract
Gravitational waves will soon take us out of the realm of indirect
observation of black holes and allow us to "see" them once and for all.
By forcing black holes out of hiding, detectors such as LISA and LIGO
will give us insight into the formation and demographics of these
objects. Because of the weak nature of gravitational waves and the
resultant challenges in data analysis, the success of these detectors
depends in large part on a detailed understanding of the radiation
sources. Mergers of black holes, on both stellar-mass and supermassive
scales, are among the most important such sources. Supermassive black
holes lurk at most galactic centers, where they are likely to capture
stellar-mass black holes onto close orbits that will lead to inspirals
and mergers. These extreme mass ratio inspirals (EMRIs) can be formed
by the tidal separation of black hole binaries, which will produce
events with near-zero eccentricity in the LISA sensitivity band. The
dense environments of galactic nuclei foster close encounters between
stellar-mass black hole binaries and stars, often inducing mergers.
Dynamically-triggered events such as these might be the dominant
formation channel for mergers of black hole binaries, which would
indicate that mergers will preferentially involve stellar-mass black
holes at the upper end of the mass spectrum. I will present these two
new potential formation channels for gravitational radiation sources:
tidal separation of binaries by supermassive black holes and induced
mergers of stellar-mass black holes in the centers of galaxies.
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The Life and Times of Supermassive Black Holes
Paolo Coppi
Yale University
Tuesday, April 8, 2008
Abstract
Once considered rare and exotic objects, supermassive black holes now
seem to be found in the center of every massive galaxy and may have
released enough energy into their surroundings to profoundly influence
the structure we see around us today. I will review recent developments
in our understanding of the life history of these objects, e.g., how
these objects form and grow (by accretion and/or merging) to their
present size. I will highlight recent work at Yale on the importance of
gas in understanding whether the central black holes of two merging
galaxies will also merge. I will speculate on possible solutions for
some of the many issues that remain outstanding and discuss how they
would impact the event rates expected from gravitational wave
experiments like LISA.
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Cosmological simulations of clusters of galaxies: status, problems, challenges
Andrey Kravtsov
Yale University
Tuesday, April 15, 2008
Abstract
I will describe high-resolution self-consistent cosmological
simulations of clusters forming in the concordance Cold Dark Matter
model with vacuum energy. The resolution of the simulations is
sufficiently high to resolve formation and evolution of cluster
galaxies and their impact on cluster gas. We use these simulations to
study the effects of galaxy formation on the global properties of
clusters, such as the shape of cluster dark matter halo and its
density profile, the baryon fractions, gas density and temperature
profiles. I will present comparisons of simulations results with the
recent X-ray Chandra, Sunyaev-Zeldovich, and optical observations of
clusters with highlights of both successes and problems of the models.
I will show that despite complexities of their formation and
uncertainties in their modeling, clusters of galaxies both in
observations and numerical simulations are remarkably regular and
consistent outside of their core region (~5% of the virial radius),
which holds great promise for their use as cosmological probes. I will
briefly describe the current status of cosmological constraints with
clusters.
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Searching
for the Electromagnetic Counterparts of Supermassive Black
Hole Mergers
Bence Kocsis
Harvard-Smithsonian Center for Astrophysics
Tuesday, April 22,
2008
Abstract
The anticipated detection of the gravitational waves (GWs) by the future
Laser Interferometric Space Antenna (LISA) will constitute a milestone
for fundamental physics and astrophysics. In this talk, I will discuss
LISA's capability of providing an advance warning of supermassive black
hole mergers (SMBH) and show how the size and geometry of the
localization volume evolves during the observation. While the GW
signatures themselves will provide a treasure trove of information, if
the source can be securely identified in electromagnetic (EM) bands,
this would open up entirely new scientific opportunities, to probe
fundamental physics, astrophysics, and cosmology. I will describe
several mechanisms that might produce EM variability during a SMBH
merger. In particular, the binary may produce a roughly periodic
variable electromagnetic flux, due to the orbital motion prior to
coalescence, a transient signal caused by shocks in the circumbinary
disk when the SMBH binary recoils and ``shakes'' the disk, or a prompt
EM flare caused by the viscous dissipation of GWs in the ambient gas. I
will discuss whether these time-variable EM signatures may be
detectable. A LISA-triggered EM counterpart search campaign will require
monitoring a several-square degree area. It could aim for variability at
the 24-27 mag level in optical bands, for example, which corresponds to
1-10% of the Eddington luminosity of the prime LISA sources of ~10^6 -
10^7 Msun BHs at z=1-2, on time-scales of minutes to hours, the orbital
time-scale of the binary in the last 2-4 weeks of coalescence.
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Finding TeV Gamma-ray Sources and their Connections to the Unexplored Energy
Window through GLAST
Tulun Ergin
University of Massachusetts
Wednesday, April 23, 2008
NOTE TIME/LOCATION: Bldg 2, Room 8, 10 AM
Abstract
Recent observations of imaging atmospheric Cherenkov telescope systems
like H.E.S.S., VERITAS and MAGIC resulted in detections of very high energy
gamma rays from various galactic and extra-galactic sources. Some of the
detected galactic sources showed extended TeV emission and some of them
were within or close to the large error boxes of the EGRET observations. Due
to the better angular resolution of GLAST, it will be easier to resolve and
identify all the sources within the EGRET error boxes so that they can be
better spatially correlated with the detected TeV sources. For these
overlapping sources GLAST will measure spectra in the so far unexplored
energy range that spans from 30 GeV to 300 GeV. These observations
will help us to study the energy dependence of the source extensions and
the spectra of the sources. The GLAST spectral measurements of the TeV
detected supernova remnants will reveal if the gamma-rays are products
of proton or electron interactions. Good spectral measurements are mostly
coupled with timing analysis of the pulsars where pulsed and not-pulsed
emissions are analyzed separately to find the cut-off energies in pulsed
spectra and set limits to the pulsed emission models. Moreover, other time
critical measurements will be the observations of the binaries which show
periodic changes in the gamma-ray flux and the multi-wavelength campaigns
to detect the variable emission in the AGN. GLAST is also expected to detect
many new sources that will bring new challenges to develop new methods to
identify and classify the sources. Data taken in other wavelengths
including in the TeV energies will be useful in understanding these new
sources.
In this talk I want to give a summary on my research done in the the H.E.S.S.
and VERITAS experiments by focusing on TeV observations of galactic sources,
especially supernova remnants, pulsars, and binaries. Then I will discuss the
motivations for future observations of these and other sources with GLAST
and the possible fields of contribution to the research going on in GSFC.
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VERITAS Observations of the Crab Nebula and Pulsar
Ozlem Celik
UCLA
Thursday, April 24, 2008
NOTE TIME/LOCATION: Bldg 2, Room 8, 1:00 PM
Abstract
VERITAS Observations of the Crab Nebula and Pulsar Observations of the
Crab Nebula, the standard candle in TeV astronomy, has proven to be the
best tool to test and characterize the performance of a new gamma-ray
instrument. Demonstration of the consistency of the results from these
observations with the well established properties of Crab builds
confidence that the results from other observations with the instrument
are correct. Scientifically, it is interesting to measure the energy
spectrum of the Crab Nebula close to the inverse-Compton peak where a
deviation from the power law seen at energies above 300 GeV is expected.
Additionally, it is important to search for pulsed emission from the
Crab Pulsar at energies beyond 10 GeV, the highest energy at which EGRET
detected pulsed emission from the Crab.
With these motivations, the Crab has been observed extensively during
the 2-, 3-, and 4-telescope commissioning phases of VERITAS. The energy
spectrum of the Crab Nebula between 200 GeV and 7 TeV is constructed
from these data sets. A search for pulsed emission from the Crab Pulsar
at gamma-ray energies above 100 GeV is also performed. The Crab data set
from these observations does not show any significant pulsed excess, so
an upper limit on the pulsed emission is obtained.
The results of these studies will be presented on the talk. The
advantages and the capabilities of GLAST in detection of pulsed emission
from many pulsars in the high-energy gamma-ray band and answering most
of the current open questions in pulsar physics will be discussed in the
conclusion.
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GLAST-LAT
Signatures of UHECRs Production in GRBs.
Sylvain Guiriec
Laboratoire de Physique Theorique et Astroparticles
(LPTA)
Friday, April 25, 2008
NOTE TIME/LOCATION: Bldg 2, Room 8, 1:00 PM
Abstract
After a decade of preparation, NASA's "Gamma-ray Large Area
Space Telescope" (GLAST) will observe, using the "Large Area
Telescope"
(LAT), the gamma-ray sky from 20 MeV up to >300 GeV. With an
unprecedented sensitivity, the LAT will replace the "Energetic
Gamma-ray Experiment Telescope" (EGRET) on-board the well known
"Compton Gamma-Ray Observatory" (CGRO). With an energy range from 8
keV up to 30 MeV and a sky coverage over 9.5 sr, the "GLAST Burst
Monitor" (GBM) on-board GLAST will replace CGRO-BATSE ("Burst And
Transient Source Experiment") to detect and study transient sources
and "Gamma-Ray Bursts" (GRBs).
In this presentation, I will briefly describe both GLAST
instruments as well as the current status of the mission whose launch
is scheduled for May 16th 2008. The mission is expected to last from 5
to 10 years. I will emphasize on the interest of these two instruments
for the study of GRBs at high energy. GRBs are very intense and short
gamma-ray emissions which could be produced by stellar mass black
holes or magnetars. The central engine responsible of the gamma-ray
burst could result from the collapse of supermassive stars or from the
coalescence of compact objects. I will also present some high energy
emission models that GLAST will constrain.
In the last part, I will propose a semi leptonic and hadronic
model for GRBs based on the "fireball model" which currently best
reproduces the prompt and afterglow emission. I will show you how past
and future observations of the prompt emission (few keV up to some
MeV) interpreted as the synchrotron emission from accelerated
electrons during the internal shocks, can constrain the parameters of
the GRB jet magnetic field. According to these parameters, I will
Energy Cosmic Rays (UHECRs) at the beginning of the internal shocks.
For this purpose, I will present an original "second order Fermi" like
process. Finally, I will test the possibility of detecting
synchrotron emission from UHECRs using LAT.
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A New Approach to Analyzing Solar Coronal Spectra Using Updated
Collisional
Ionization Equilibrium Calculations
Paul Bryans
Columbia University
Tuesday, April 29,
2008
Abstract
Reliably interpreting spectra from electron-ionized cosmic plasmas requires
accurate ionization balance calculations for the plasma in question. Using
recently published state-of-the-art electron-ion recombination data, we have
calculated new fractional ionic abundances for collisional ionization
equilibrium (CIE) for all elements from H through Zn. Utilizing these new
results, we have reanalyzed a SUMER observation of the solar corona using our
newly developed systematic method for determining the emission measure and
electron temperature of the emitting plasma. Using this technique I will
discuss the impact of the results on the first ionization potential (FIP) effect
and the long-standing Li- and Na-like ion abundance discrepancy in the solar
corona.
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Milagro, A Wide Field of View VHE Gamma-Ray Observatory
Vlasios Vasileiou
University of Maryland (College Park)
Wednesday, April 30, 2008
NOTE TIME/LOCATION: Bldg 2, Room 8: 2:00 PM
Abstract
Milagro is a wide field of view gamma-ray observatory. It
is using the water-Cherenkov technique to detect cosmic gamma-rays of
energies ranging from 100 GeV to 500 TeV.
I will talk about my graduate work with Milagro; namely, the blind
whole-sky search for VHE emission from GRBs, the Monte Carlo
simulation of the detector with Geant4 and the photocathode-uniformity
tests on Milagro's Photomultiplier Tubes
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Stellar Coronae and AGN Coronae - Establishing a Physical Analogy
Ehud Behar
Technion (on sabbatical at NASA/GSFC)
Tuesday, May 6, 2008
Abstract
The talk will discuss new evidence for a high-energy physical analogy
between the extensively studied coronae of stars and the putative,
poorly understood coronae of accretion disks in active galactic nuclei
(AGNs). The current picture of magnetic energy powering the X-ray and
radio emission of stellar coronae will be briefly reviewed with an
emphasis on recent results for large X-ray flares. Subsequently,
empirical evidence will be presented suggesting that the X-ray and
radio emission from radio-quiet AGNs originates in a corona akin to
stellar. It will be demonstrated how the AGN coronal plasma
parameters can be estimated using basic Compton scattering and
synchrotron self-absorption theory. The physical similarities between
stellar and AGN coronal sources will be pointed out along with some of
the obvious differences. Finally, future X-ray and radio observations
will be proposed to further explore the AGN coronal hypothesis.
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Understanding the conditions for crystalline silicate formation:
Lessons from an obscured AGB star
Angela Speck
University of Missouri
Tuesday, May 13, 2008
Abstract
IRAS 17495-2534 is a highly obscured oxygen-rich AGB star that does
not exhibit OH/IR maser emission. However, its IRAS LRS spectrum
exhibits the strongest mid-infrared crystalline silicate absorption
features seen to date. Consequently this source provides an
unprecedented opportunity to test competing hypotheses for dust
formation and to understand the formation of crystalline
silicates. This source is located in the Galactic plane at a distance
of ~4kpc, and thus may have higher than solar metallicity. However,
comparing the spectrum to those of OH/IR stars in the Galactic Bulge
shows that this source has significantly stronger crystalline silicate
features, indicating that metallicity is not the only factor
influencing crystal production. Radiative transfer modeling of IRAS
17495-2534 suggests that both the crystalline and amorphous components
of silicate dust are dominated by forsterite (Mg2SiO4) rather than
enstatite (MgSiO3) or other more silica-rich
compositions. Calculations predict that, in stars with relatively high
mass-loss rates, the high pressure (leading to high dust formation
temperatures) and slow outflow velocities (leading to slow cooling)
should result in either direct formation of crystalline grains or
rapid annealing and crystallization of initially amorphous silicates.
Competing dust formation mechanisms will be compared to determine
whether they can explain the spectrum of this particular star. In
addition, since the spectra of most high mass-loss stars are
interpreted to contain little crystalline material, we investigate the
effects of C/O, metallicity, mass-loss rate and initial stellar mass
on crystalline dust formation and discuss why there is apparently so
little crystalline dust around most O-rich stars.
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The accretion history of black holes in galactic nuclei
Priya Natarajan
Yale University
Wednesdayday, May 14, 2008
Abstract
In this talk I will present some key issues in the formation, assembly
and evolution of black holes in galactic nuclei. I will summarize our
current theoretical understanding of black hole assembly consistent
with existing observations. I will then discuss in detail two key
aspects of accretion disk physics that are crucial to our
understanding of the growth of black holes (i) the formation of the
first seed black holes and (ii) the role of gas accretion in the
merger of binary black holes.
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Dynamics of Accretion Flows Irradiated by a Quasar
Daniel Proga
University of Nevada (Las Vegas)
Friday, May 16, 2008
NOTE: To be held in Bldg 2, Room 8
Abstract
I will present results from recent time-dependent
radiation-hydrodynamical calculations of flows that are under the
influence of supermassive black hole gravity and radiation from an
accretion disk surrounding the black hole. The calculations take into
account radiation heating/cooling and the radiation force due to
electron scattering and spectral lines. I show how the mass accretion
and outflow rates, and the flow dynamics respond to thermal effects
and effects of radiation pressure and gas rotation. The main focus of
this work is to gain insights into the problem of AGN feedback and the
formation and evolution of cold clouds in AGN.
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The 3D structure of the pulsar magnetosphere: first results
Ioannis Contopoulos
Academy of Athens
Tuesday, May 20, 2008
Abstract
We will present our preliminary results on the 3D structure of the
pulsar magnetosphere obtained through time dependent numerical
simulations of a rotating magnetic dipole. We discovered that after
about ten neutron star rotations, the magnetosphere settles to a
dynamic steady-state consisting of a sequence of openings and closings
of magnetic field lines at the period of the pulsar rotation. We will
discuss the prospects of our code for future high resolution
investigations of dissipation, particle accelerationa, and high-energy
emission in the pulsar magnetosphere.
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CANCELLED; WILL BE RESCHEDULED -- Gravitational Waves in Multimessenger Astrophysics
Szabolcs Marka
Columbia University
Tuesday, May 27, 2008
THIS TALK WILL BE RESCHEDULED FOR THE FALL
Abstract
Gamma-ray, X-ray, optical and neutrino observations of cataclysmic
cosmic events with plausible gravitational wave emission can be used
in combination with searches for gravitational waves. Information on
the progenitor, such as trigger time, direction and expected frequency
range, shall enhance our ability to identify gravitational wave
signatures with amplitudes close to the noise floor of the
detector. Even in the absence of detection, the association of the
astrophysical trigger with a particular source distance allows to
interpret upper limits on the observed flux of gravitational waves in
terms of the energy emitted in the form of gravitational waves. After
a summary of past multimessenger based gravitational wave searches, I
will discuss the implications of these results. I will close by giving
an outlook on the future.
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X-ray Polarimetry with Dichroic Filters
Adrian Martindale
University of Leicester
Tuesday, May 27, 2008
Abstract
X-ray polarimetry remains the most underdeveloped method of studying
celestial X-ray sources. Indeed, no progress has been made in this
field for over 30 years, despite intense scientific interest in
measuring X-ray polarisation. New X-ray dichroic filters offer a
simple and effective science enhancement opportunity for the cryogenic
spectrometers proposed for XEUS and Constellation-X by providing them
with polarisation sensitivity.
I will describe experimental and theoretical work undertaken to
demonstrate the Dichroic filter concept. Modelling based on
experimental data and the proposed performance of XEUS predicts a
minimum detectable polarisation (MDP) of about 1-2% at 8.9 keV and
about 4-5% at 13.4 keV in a 100 ksec observation of a number of sources.
Demonstration of X-ray linear dichroism at lower energy edges would
offer significantly improved MDP for a much broader range of celestial
objects and is a major goal of our current research.
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Randall Smith
