Exploration of the Universe Colloquium Series
Schedule: April - June 2005
Time: 3:45 pm (Meet the Speaker at 3:30 pm) -
Location: Bldg 2 Conference Room (Ground Floor, Rm 8) -
unless otherwise noted.
To view the abstract of a seminar, click on the title.
How To Distinguish an X-ray Neutron Star (NS) From a
Black Hole (BH): Spectral Index and Quasi-Periodic
Oscillation Frequency Correlation in NS and BH sources
Dr. Lev Titarchuk
GSFC
Tuesday, 5 April 2005
Abstract
Recent studies have shown that strong correlations are observed between
the low frequencies (1-10 Hz) of quasiperiodic oscillations (QPOs)
and the spectral power law index of several Black Hole (BH) candidate
sources in low hard state, steep power-law (soft) state and in transition
between these states. All data points of correlating photon index vs
QPO low frequency for GRS 1915+105, XTE 1550-564, 4U 1630-47 and Cyg X-1
will be shown in the talk. In the soft state this index-QPO frequency
correlations show the saturation to the photon index 2.7 at high values
of the low frequency which was identified as a black hole signature.
In this talk I argue that this saturation is not the case at least for
NS source 4U 1728-34 for which the index monotonically increases with
the low frequency to the values of 6 and higher. This effect has been
revealed analyzing almost all available data for 4U 1728-34 in the RXTE
data archive. I show the spectral evolution of the Comptonized blackbody
spectra when the source undergoes the transition from the hard to soft
states. The hard state spectrum is a typical Comptonization spectrum
of the soft photon radiation (from the disk and the NS surface) which
index 1.8 whereas the soft state spectrum consists of two blackbody
components which are only slightly Comptonized. The color disk and
NS surface temperatures slightly decreases from 0.93 keV to 0.83 keV
and from 2.9 keV to 2.2 keV respectively when the source undergoes
the transition from the hard to soft states. Thus one can claim (as
expected) that in the NS source the thermal equilibrium is established
when the sources goes to the soft state whereas in BH sources because of
BH horizon (the drain in system) the equilibrium is never established. The
BH emergent spectrum, even in high/soft state, has a power law component.
A new method for evaluation of the BH mass using this observable
index-frequency correlation will be demonstrated.
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Infrared radiation from Extrasolar Planets
Dr. Drake Deming
GSFC
Tuesday, 12 April 2005
Abstract
A class of extrasolar giant planets - the so-called `hot Jupiters' -
orbit within 0.05 AU of their primary stars. These planets should be hot
and so emit detectable infrared radiation. The planet HD 209458b is an
ideal candidate for the detection and characterization of this infrared
light because it is eclipsed by the star. This planet has an anomalously
large radius (1.35 times that of Jupiter), which may be the result of
ongoing tidal dissipation, but this explanation requires a non-zero
orbital eccentricity (~0.03), maintained by interaction with a
hypothetical second planet. Here we report detection of infrared (24
micron) radiation from HD 209458b, by observing the decrement in flux
during secondary eclipse, when the planet passes behind the star. The
planet's 24 micron flux is 55 +/- 10 micro-Jy (1 sigma), with a
brightness temperature of 1130 +/- 150 Kelvins, confirming the predicted
heating by stellar irradiation. The secondary eclipse occurs at the
midpoint between transits of the planet in front of the star (to within
+/- 7 min, 1 sigma), which means that a dynamically significant orbital
eccentricity is unlikely.
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Observations of Galactic gamma-ray sources with H.E.S.S.
Dr. David Berge
MPI-K Heidelberg
Tuesday, 19 April 2005
Abstract
H.E.S.S. is a new system of four imaging atmospheric Cherenkov telescopes
operating in full strength since December 2003 in Namibia. During its
first two years of running, a number of Galactic gamma-ray sources
have been observed. The talk will review the most important results,
including highlights like spatially resolved gamma-ray emission from
shell-type supernova remnants (RX J1713.7-3946 and RX J0852.0-4622)
and a gamma-ray survey of the inner part of our Galaxy performed with
the H.E.S.S. telescopes in 2004.
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Scaling black holes: Galactic binaries, UltraLuminous X-ray Sources and
Active Galaxies
Dr. Christine Done
Univ. Durham, UK
Wednesday, 20 April 2005 at 12noon
*Bldg 2, Room 22*
Abstract
Black holes come in a variety of size scales, from the stellar mass
objects in binary systems in our own galaxy, to the supermassive black
holes which power the Active Galactic Nuclei, with the ultra-luminous
X-ray sources lying somewhere between. Theoretically, the accretion
flow onto a black hole should be fundamentally similar in all these
objects, depending mostly on L/L_Edd, with only a rather weak dependence
on black hole mass. I will review recent advances on accretion flows in
the galactic objects, show how these can be applied to understand some
puzzling aspects of the ultraluminous sources, and then relate them to
observations of the supermassive black holes.
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Shedding X-Ray Light on Dark Matter
Dr. David Buote
UC Irvine
Tuesday, 26 April 2005
Abstract
I review constraints on dark matter from X-ray observations of galaxies
and clusters of galaxies with Chandra and XMM-Newton, and discuss their
implications for cosmology. I also briefly discuss our program to study
substructure in galaxy clusters and present preliminary constraints
supporting the standard cosmological model having both dark matter and
dark energy.
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The Quasar Fundamental Plane: the Host-Nucleus Connection
Dr. Tim Hamilton
Shawnee State University
Tuesday, 3 May 2005
Abstract
We have found a relationship between a quasar's host galaxy properties
and its nuclear luminosity. When nuclear luminosity (optical or x-ray) is
plotted against the half-light radius and effective surface magnitude of
the host, we find quasars lie on a thin plane within this parameter space.
This "fundamental plane" of quasars explains 96% of the variance in the
optical sample and 95% in the x-ray sample.
Dividing the quasars into radio-loud or radio-quiet, elliptical or spiral
classes, the fundamental plane rotates about a fixed axis. The different
classes are characterized by different angles, with radio-loudness having
the strongest effect.
The origins of the quasar fundamental plane may lie in the normal galaxy
fundamental plane and the fueling mechanism of the active nucleus.
Then this relationship could be used to test active galaxy unification
models by comparing the fundamental planes of different AGN types.
We have already found a similar plane for Low-Luminosity AGN and can
make some initial comparisons. Further work might find application to
galaxy evolution.
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X-ray observations of AGN with INTEGRAL and other Satellites
Simona Soldi
INTEGRAL Science Data Centre
Thursday, 5 May 2005 at 1:30pm
*Bldg 2, Room 8*
Abstract
Hard X-ray emission from AGN in crowded fields as well as in isolated
regions of the sky can be studied thanks to the imaging and spectral
capabilities of the INTEGRAL satellite. We show some of the AGN observed
by INTEGRAL during the regular scans of the Galactic Plane. We investigate
the shape of the continuum emission and the presence of an exponential
cutoff which could be the signature of Comptonization processes.
We present then INTEGRAL observations of the high latitude quasar
3C 273, one of the most studied AGN. The observation of June 2004 was
performed during a particularly low state of the synchrotron emission with
simultaneous multiwavelength observations. We study possible correlations
between the X-ray emission as seen by recent and past X-ray satellites
and the emission at other frequencies, using and updating the 3C 273's
on-line database (http://obswww.unige.ch/3c273/).
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The View of the Sky through the Eyes of a SHARC
Attila Kovacs
Caltech
Tuesday, 10 May 2005
Abstract
The SHARC-2 array, developed in collaboration at Caltech and Goddard
Space Flight Center, has brought us new images of the sky at 350um with
unprecedented quality and depth. Its success owes to its innovative design
and a novel approach to sub-mm data reduction, which may be adapted to
a range of new large format bolometer arrays in the far-infrared and
sub-millimeter. With its 350um 'eyes' it is specially suited to the study
of dust in the ISM, which enabled us to pursue various science objectives,
both within and outside the boundaries of our own Galaxy. Snapshots of
these results will be presented, with a special focus on the study of
the distant (z~1-3) universe.
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Gamma-Ray Burst Modeling
Demos Kazanas
GSFC
Tuesday, 17 May 2005
Abstract
We report on progress with more detailed calculations based on the
'Supercritical Pile' model of Gamma Ray Bursts (GRB). This model
was proposed as a means for converting the energy stored in the form of
relativistic protons in the Relativistic Blast Wave (RBW) of a GRB into
electrons and then to photons using the photon-proton pair production
reaction. It was also argued on the basis of semi-analytical arguments
that the threshold of this reaction is responsible for the peak of the
GRB nu F nu spectra at ~1 MeV, as required by observation. The name of
the model derives from the fact that it depends on certain thresholds
very similar to those of a nuclear pile (critical mass). We simulate the
spectral and temporal evolution of a GRB by following the distributions
of photons, electrons and protons as implied by this model and test
for the kinematic and dynamic thresholds which are found in agreement
with the analytic estimates. We can now produce, under some simplified
assumptions, the evolution of prompt GRB spectra from IR to TeV gamma
rays which can be compared to observations.
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QPOs and Nonlinear Resonance: What is the Link?
Paula Rebusco
MPA
Thursday, 19 May 2005
Time: 1:30pm, Location: Bldg 2, Rm 8
Abstract
Kilohertz quasi-periodic variability is observed in the X-ray emission
of many Galactic black hole and neutron star sources. Kluzniak and
Abramowicz noticed that twin peak HFQPOs are in rational ratios, and
that most often the ratio is 3:2. In particular, all four microquasars
in which twin peak HFQPOs are observed, show with a rather impressive
accuracy the fixed 3:2 ratio. In neutron star sources, where twin peak
HFQPOs vary and ratios are not fixed, the ratios cluster very close
to 3:2. An analytic calculation was done by considering the possibility
of non-linear parametric resonance in nearly circular, nearly Keplerian,
nearly equatorial plane motion of test particles around compact objects.
The obtained solution confirms the importance of the 3:2 ratio and it
agrees with a previous numerical simulation. With a particular choice of
tunable parameters, it fits accurately the observational data for Sco X-1.
Why the resonant model is so promising? What is still missing?
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Radiation-driven Winds Near BHs
Anton Dorodnitsyn
Space Research Institute, Moscow
Tuesday, 24 May 2005
Abstract
The critical aspect of any radiation-driven wind theory is how the
momentum is transferred from the radiation field to matter. One of the
most favorable mechanism which is believed to work in winds launched from
luminous accretion disks is the acceleration of plasma by the absorption
of the radiation flux in lines of abundant elements. In a standard theory
only gradients of the flow velocity are taken into account in Sobolev
approximation, when calculating the radiation pressure force. This theory
was developed to explain winds from early-type stars, for which it is
in a good agreement with observational data, and now is extensively
used to explain winds in AGNs, YSO, and binaries. We report on our
recent studies of line-driven winds near compact objects. The crucial
difference from the standard case of O-type star wind consists of the
following: if a wind is settled not too far from BH (within 50 rg) both
velocity gradient and gradient of the gravitational potential should
be taken into account when calculating the radiation force. The first
stands for the Sobolev effect the latter takes into account gravitational
redshifting. We develop a theory of such flows from first principles,
considering one dimensional wind from accretion disk around Schwarzschild
BH. We show, that taking into account gravitational redshifting can indeed
significantly increase the efficiency of acceleration. We discuss the
possible connection of our studies with recently detected absorption
features in X-ray spectra of some quasars. The observational signature
of such flows may have very important implications in future studies of
parameters of putative BHs in central engines.
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Chaos in Spiral Galaxies
Frank H. Shu
National Tsing Hua University, Taiwan
Thursday, 26 May 2005
Time: 10:30am, Location: Bldg 21, Rm 183
Abstract
The visible spiral structure of disk galaxies is often quite disorderly,
with spurs, branches, and feathers making ragged what theoreticians
ideally describe as a two-armed, grand-design, spiral. In the
most notorious cases, the blue-light pictures associated with newly
formed stars from the gaseous interstellar medium are observationally
"flocculent." The mystery deepens when infrared images, such as those
from the Spitzer Space Telescope, show that the distribution of older disk
stars is often arranged in beautiful two-armed grand-design spirals in the
same galaxies where the blue-light images are chaotic. One interpretation
is that the irregular nature of population I spiral-structure demonstrates
that the dynamics of interstellar gas is largely decoupled, perhaps by
numerous supernovae explosions, from the regular gravitational forcing
provided by the smooth spiral arms of the background disk stars. In this
talk, I advance an older and opposite concept: extra structural features
appear not because the gas is decopuled from the background spiral
forcing, but because it is too well coupled. In the nonlinear regime,
branches, spurs, and feathers can arise, respectively, because of the
action of ultraharmonic resonances, reflections of density waves off sharp
edges created by nonlinear dredging, and gravitational instabilities -
perhaps mediated by magnetic fields - behind the compression zones of
galactic shocks. In particular, chaos, or flocculence, is produced
in the presence of overlapping resonances. Thus, disorderly effects
in galaxies can be due to orderly causes, a well-known result, by now,
in other areas of nonlinear dynamics.
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Blazars in the VERITAS Era
Deirdre Horan
VERITAS
Tuesday, 21 June 2005
Abstract
Blazars are the main class of active galactic nuclei detected at
gamma-ray energies. The EGRET experiment on the Compton Gamma Ray
Observatory made firm identifications of 66 blazars and recent results
have associated many of the unidentified EGRET sources with known
blazars. At higher energies (E>300 GeV), ground-based gamma-ray
telescopes have made firm detections of seven blazars with a number of
other detections claimed at lower levels of significance. In order to
improve our understanding of the gamma-ray emission from blazars, we
must detect and study more of these objects at very high energies.
I will describe the current observational status of blazars at TeV
energies and will discuss search strategies that can be employed to
increase the sample at these very high energies. With the next
generation of space and ground based gamma-ray observatories coming on
line, we can expect to identify and discover many more gamma-ray
blazars and significantly improve our understanding of the physics of
these powerful objects.
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SPEAR Reveals the Far Ultraviolet Galaxy
Jerry Edelstein
UC Berkeley, SSL
Tuesday, 28 June 2005
Abstract
SPEAR has obtained unprecedented hyper-spectral images of diffuse
FUV (900-1700) emission from most of the Galaxy. We describe this
sounding-rocket, its orbital micro-sat mission, and results. Observations
include supernova remnants, super-bubbles and the general ISM. A plethora
of spectral lines have been detected from high-energy gas (e.g. HeII,
CIII, CIV, OIII, OIV, OVI, and SiIV) and from cooler gas (e.g. SiII,
AlII and FeII). The gas is likely not in equilibrium. We detect strong
H2 fluorescence emission over the sky that can directly measure the
H2 destruction rate and association with NHI and CO. Our maps of the
bright FUV continuum, disentangled from the fluorescence contribution,
will probe dust properties and the dust-gas association.
The FUV interstellar radiation field, a controlling agent of stellar
formation through the heating of the cold ISM, can be directly
measured. The uncertain reddening to bright stars can be tested.
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The Dark Side of Interstellar Clouds
Isabelle A. Grenier
Service d'Astrophysique, CEA Saclay
Thursday, 30 June 2005
Location: Bldg 2, Room 8 - Time: 1:30pm
Abstract
Large amounts of dark gas, not accounted for in the radio HI and CO
line surveys, have been found in infrared and gamma rays above 100 MeV,
at the interface between the atomic and molecular clouds in the solar
neighbourhood. This gas is traced in gamma rays by its interaction with
ambient cosmic rays and at mm and sub-mm wavelengths by its dust content.
The dark clouds are rather diffuse, with volumes and average volume
densities closer to that of the dense HI clouds than the CO cores. The
diffuse H2 or dense HI nature of this gas is open. The local measurements
imply a total dark-gas mass in the Milky Way at least comparable to
the molecular mass detected in CO. The column-densities and large
angular extent of the local dark clouds imply severe revisions of the
gamma-ray interstellar emission model to high latitudes, therefore of
the extragalactic gamma-ray background intensity and of the EGRET source
catalogue. New estimates will be discussed.
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Jerry Bonnell
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