Laboratory for High-Energy Astrophysics Seminar Series:

2003, October-December


X-ray Emission from Massive Stars


Dr. David Cohen


Swarthmore College


October 7



The huge increase in spectral resolution provided by XMM and Chandra has shed new light on several different aspects of the long-standing problem X-ray emission in early-type stars. I will discuss some of the new spectral diagnostics, with an emphasis on line profile analysis, and the surprising things they have told us about X-rays from massive stars. I will also discuss the modeling my group has been doing to try to understand these new results, including the development of a hybrid magnetic-wind model for very young hot stars, and an analytic model of wind clumping and its effect on opacity and line profiles. One of the surprises from the first few years of Chandra and XMM observations is the diversity of behavior seen in the dozen or so hot stars that have been observed thus far.

More information can be found at

Production and Calibration of the First Two HEFT Hard X-ray Optics


Dr. Jason Koglin


Columbia University


October 8, 2:00PM, Room 8



Complete hard X-ray optics modules are currently being produced for
the High Energy Focusing Telescope (HEFT), a balloon born mission
that will observe a wide range of objects including young supernova
remnants, active galactic nuclei, and galaxy clusters at energies
between 20 and 70 keV.  Large collecting areas are achieved by
tightly nesting layers of grazing incidence mirrors in a conic
approximation Wolter-I design.  The segmented layers are made of
thermally-formed glass substrates coated with depth-graded
multilayer films for enhanced reflectivity.  Our novel mounting
technique involves constraining these mirror segments to successive
layers of precisely machined graphite spacers.  I will report the
production and calibration of the first two HEFT optics modules.  I
will also highlight key technologies, particularly our mounting
approach, that may be used to achieve high angular resolution for
future satellite missions.

Accretion Disk Atmospheres


Dr. Mario Jimenez-Garate




October 9, 2:00pm, Room 22



The window on high-resolution X-ray spectroscopy opened by Chandra and XMM-Newton is revealing the nature of the atmospheres and coronae of accretion disks. Observations of X-ray binaries show that dense photoionized plasmas blanket the disk. We compare accretion disk model atmospheres with observed X-ray spectra in order to derive ionization and density structure, opacity, spatial distribution, elemental composition, energetics, thermal stability, and kinematics.

We calculate the X-ray emission line spectrum for a model disk atmosphere surrounding a supermassive Kerr black hole. Our models show that soft X-ray recombination lines are detectable and represent another probe of the flow near the horizon, supporting XMM-Newton observations of MCG-6-30-15 by Branduardi-Raymont et al. (2001). X-ray opacity, Compton scattering, and dynamical effects become crucial for calculating the line spectrum and measuring black hole spin. Via a Monte Carlo, we find that the line ratios will be substantially modified after repeated photoionizations and recombinations inside the atmosphere. The result is the enhancement of the O VIII (871 eV) and C VI (490 eV) line fluxes.

MHD Simulations of the Collapsar Model for Gamma Ray Bursts


Dr. Daniel  Proga


University of Colorao


October 21



I will review MHD simulations of accretion flows onto black holes. My main focus is to explore the connection between accretion flows and related outflows. I will consider applications of such flows to gamma-ray bursts.

Pulsars as Feedback Systems


Dr.  Geoffrey Wright


Astronomy Centre, University of Sussex


October 22, 1:30PM



After many years of research no fully satisfactory description of a radio pulsar's magnetosphere has yet been given. Here an empirical approach is outlined which starts with the (radio) observations and attempts to use them as diagnostics of the physics needed. Perhaps the conventional view of pulsars as steady-state systems needs to be revised.

Status of VHE Gamma-ray Astronomy


Dr.  Rene Ong




October 24, 1:30PM



Significant future progress in high-energy astrophysics is expected to result from a new
generation of sensitive telescopes, both in space and on the ground. Ground-based Cherenkov telescopes have opened the gamma-ray sky at energies above 100 GeV through the detection of a number of sources, including AGN, pulsar nebulae, and supernova remnants. New, more powerful, telescopes will be coming on line during the next few years;
these instruments should provide important complementary capabilities to GLAST. This talk summarizes what we know about the E>100 GeV gamma-ray sky and describes the prospects for exciting science in the near term.

Hurricanes on Neutron Stars: Dynamics of Type I X-ray Bursts


Dr. Anatoly Spitkovsky


KIPAC, Stanford University


October 28



I will present the results of multidimensional simulations of ignition and propagation of thermonuclear burning in the atmosphere of a rapidly rotating neutron star during type I X-ray burst. Due to the effects of Coriolis force, the burning spreads in geostrophically balanced burning fronts with latitude-dependent speeds. I will demonstrate how such fronts propagate on a sphere, and propose a scenario for dynamics of explosion through the burst. The variation of Coriolis force with latitude makes burning hot spots drift against the rotation of the star, contributing to the observed frequency evolution of burst oscillations. Finally, I will discuss how magnetic fields influence the hydrodynamics during the burst and apply this to observations of bursting accreting pulsar SAX J1808.

Weak Lensing by Large Scale Structure


Dr. Alexandre Refregier




November 5, 1:30pm, Rm. 8



Weak gravitational lensing provides a direct method for mapping the dark
matter in the Universe and for measuring cosmological parameters.
This 'cosmic shear' technique is based of the distortions that large scale
structures induce on the shapes of background galaxies. After describing
the basic principles of this method, I will review the current
observational status of cosmic shear and the corresponding cosmological
implications. I will then describe the prospects that future surveys and
missions offer for weak lensing. In particular, I will show how weak
lensing can be used to constrain models beyond the standard LambdaCDM
paradigm, such as those containing dark energy and non-gaussianity. I
will finally discuss the observational challenges which have to be met for
the promise of cosmic shear to be fully realized.

The Most Successful X-ray/TeV Campaign on the TeV Blazar Mrk421


Dr. Giovanni Fossati


Rice University


November 6, 1:30pm, Rm. 8



We present the result of a week-long simultaneous observations of the
blazar Mrk421 in X-rays (RossiXTE) and TeV gamma-rays (Whipple and HEGRA).
The source shows strong variations in both bands, which are highly
correlated, with no evidence of a non-zero interband lag.  This lends
further support to the standard model in which the same electrons
produce the X-rays by synchrotron emission and then scatter photons
to produce the gamma-rays by inverse Compton scattering.  During the
best observed individual flare the gamma-rays flux scales approximately
quadratically with X-ray flux, suggesting that the scattering occurs
in the Thomson regime.  The details of the relationship between X-ray
and TeV fluxes and spectra set severe constraints on the values of some
fundamental physical parameters, namely seemingly "pushing" them towards
what were regarded as extreme values, e.g. jet Lorentz factor >100.
We will also discuss the highlights of the 2002 X-ray/TeV campaign on
PKS1959+650, when for the first time a TeV "orphan" flare was observed.
We will discuss the astrophysical consequences of these new findings in
the context of the competing models for the jet emission in blazars.

Using Millisecond Signals to Probe the Variability Modes in Neutron Star LMXBs


Dr. Wenfei Yu


University of Amsterdam


November 7, 1:30pm, Rm. 8



RXTE observations have shown that the kilohertz QPO frequency in low magnetic field neutron star low mass X-ray binaries is either positively or negatively correlated with
the X-ray flux or X-ray colors on time scales of hundreds of seconds to hours, depending on source and source state. This supports the neutron star accretion disk-magnetosphere interaction theory if 1) the X-ray flux on these time scales represents the mass accretion rate, and 2) the kHz QPO frequency is an indicator of the orbital frequency at the inner disk edge. In this talk I will show the correlation between the kHz QPO frequency and the X-ray flux on the short time scale of a certain QPO or noise component which is simultaneously observed with the kHz QPOs. Comparing the correlation with that holds on time scales of hundreds of seconds to hours, we are able to discriminate the variability modes of the QPOs and noise components from that of longer time scales, which is probably a disk accretion mode. This phenomenological approach has been proved successful in identifying mHz QPO in 4U 1608-52 and normal branch oscillation (NBO) in Sco X-1 as modes inside the inner disk edge.

The Violent Solar Fall of 2003


Dr. Scott McIntosh




November 18



We will look at the erratic and explosive behavior of Solar Active
 Region 10486 in many energy bands, from many orbital platforms as it
crossed the Solar disk from mid-October to early November of this
year. This active region produced four of the ten largest solar flares
ever recorded and we will discuss what made it so interesting and so
explosive. We will discuss the Space Weather resulting from these huge
events, their impact on the near-Earth environment and their potential
influence on our everyday lives. Furthermore, a ``prediction'' of the
resulting powerful, some geo-effective, events should have been easy
to make long before the active region had crossed the Sun's East limb.

High-Energy Activity in Galaxies


Dr. Stefan Immler


Pennsylvania State University


November 20, 1:30pm, Room 8



Recent X-ray observations of nearby galaxies with Chandra
and XMM-Newton have provided a wealth of information on
the high-energy activity in galaxies. I review some of the
highlights regarding the X-ray source population, ultra-luminous
X-ray sources, young supernovae and the hot interstellar medium.
By contrast, the ultra-deep Chandra surveys of the HDF-N and
CDF-S, allow, for the first time, a study of the X-ray properties
and evolution of galaxies over cosmologically interesting distances.
These data suggest that the X-ray production mechanisms for elliptical
galaxies at z=0.06-1.5 are similar to those operating locally, and they
give independent evidence that the bulk of the stellar mass was formed
at z>1.5.

What governs the properties of AGN?


Dr. Dirk Grupe


Ohio State University


November 24, 1:30pm, Rm 8



Active Galactic Nuclei (AGN) are characterized by a number of optical and
X-ray properties which are related: steep soft X-ray spectra correlate with
strong optical FeII emission correlates and anti-correlate with the widths
of the Broad Line Region emission line widths, namely the width of H-beta,
and the strength of the Narrow-line Region (NLR) emission lines. All these
properties seem to be govern by one underlying physical parameter, often
called Eigenvector 1 or the First Principal Component.

In my talk I will present the study on a sample of 110 soft X-ray AGN
selected from the ROSAT All-Sky Survey. Half of these sources turn out to be
Narrow-Line Seyfert 1 galaxies(NLS1s), which are relatively rare among
optically selected samples. NLS1s are AGN which show the strongest FeII
emission, steepest X-ray spectra and weakest NLR emission. They are
therefore at one extreme end of the Eigenvector 1 relation.
I will discuss possible interpretations of Eigenvector 1 such as the mass
of the central black hole and the Eddington accretion ratio.

UV and X-ray Observations of the Warm-Hot Intergalactic Medium


Dr. Smita Mathur


Ohio State University


November 25



Hydrodynamic cosmological simulations predict that most of the baryons at low
redshift reside in warm-hot intergalactic medium, WHIM. One of the few
prospects for detecting this shock heated, low density gas is via the "X-ray
forest" of absorption lines it should produce in quasar spectra. Such
observations are now possible with Chandra X-ray Observatory. I will describe
recent efforts to trace the WHIM and determine its physical properties, with
Chandra as well as with HST and FUSE, and discuss the implications towards the
missing baryon problem. I will also outline future prospects with

Data Analysis for LISA Capture Sources


Dr.  Curt Cutler


Albert Einstein Institute, Potsdam


December 8, 11:00am, Room 8



Captures of compact stars by massive black holes represent
an important source for LISA. I describe a simple, analytic model of
the inspiral gravitational waveform and use it to 1) describe how
the signal-to-noise builds up over time, 2) estimate how
accurately the system's physical parameters can be extracted, and
3) (begin to) estimate the sensitivity of hierarchical search methods,
for realistic computing power.

The Other Microlensing and the Structure and Composition of Galaxy Halos


Dr.  Christopher Kochanek


Ohio State University


December 9



Microlensing originally meant the microlensing of the images of gravitationally
lensed quasars by the stars in the lens galaxy rather than the now more familiar
case of Galactic microlensing. We will discuss how quasar microlensing can
be used to measure the surface density and composition of the lens galaxy
halo and the mean mass of its stars. In addition, we measure the size of
the quasar accretion disk and the mass of its black hole. The only
limitation to the method is the sociological problem of conducting
monitoring experiments on optical telescopes.

Galaxy Clusters, Dark Energy and Structure Formation


Dr. Joe Mohr


University of Illinois


December 15



The recent WMAP observations of the cosmic microwave background (CMB)
provide compelling evidence that we live in a geometrically flat, dark
energy and dark matter dominated universe. One remaining challenge
is understanding the nature of the dark energy. Large galaxy cluster
surveys provide a means of precisely measuring the equation of state of
the dark energy-- with far higher precision than is possible with CMB
anisotropy experiments and in a way that is complementary to supernova
distance measurements. One funded cluster survey will be carried out
at mm wavelengths with an 8m mm-wave telescope located at the South
Pole, and another proposed survey would carry out an X-ray survey for
clusters. The design of these surveys and the optical and near-infrared
followup to obtain cluster redshifts is critical to minimizing systematic
errors in measurements of the amount and nature of the dark energy.

To date, galaxy clusters have performed well as cosmological
probes. Recent X-ray and near-infrared analyses of cluster baryon
fractions are in excellent agreement with WMAP measurements of the matter
density. Small scale X-ray and SZE surveys for galaxy clusters further
support a low matter density universe. The primary challenge to using
clusters to precisely study the dark energy comes in understanding the
relationship between cluster mass and observables like the X-ray flux,
SZE flux, and near-infrared galaxy light. Tight mass-obserable relations
exist in the local universe. Recent Chandra observations indicate those
relations persist to intermediate redshift, but the redshift evolution
of those scaling relations is complex. We have recently shown that large
cluster surveys can overcome these challenges. That is, surveys yielding
tens of thousands of clusters contain enough information to study the
dark energy while simultaneously solving for the evolution of cluster
mass-observable relations.

Gamma-ray Sources from Regions of Star Formation


Dr.  Diego Torres




December 16



We discuss different scenarios by which regions of star formation both in our galaxy and beyond can become gamma-ray emitters. Particular emphasis is placed on the expectations for these to become detectable sources for the GLAST mission.

To see the slides of this presentation Click Here


Page Author: Dr. Robin Corbet

Last Update: 2003-04-01