Laboratory for High-Energy Astrophysics Seminar Series

2004, April - June

Studying Diffuse Galactic X-ray Emission with XMM-Newton

Dr. Bob Warwick


Thursday, 1 April 2004

NOTE: Special Day and Time: 1:30


We have used XMM-Newton to survey the X-ray sky in a variety of directions in the Galactic Plane, the Galactic Centre and the Galactic Bulge. The observations provide new information on both the discrete X-ray source population and the underlying diffuse Galactic X-ray background in the 0.5-8 keV band. We discuss some early results from this programme which relate to the nature and origin of the relatively hard, diffuse X-ray emission which pervades the inner regions of our Galaxy.

Slides of this presentation

The Last X-ray Eclipse of Eta Carinae

Dr.  Mike Corcoran


Tuesday, 6 April 2004


Eta Carinae, arguably the Galaxy's most massive, luminous and unstable star is a key object in understanding how the most massive stars form and evolve. It may be an analog to GRB hypernova progenitors and perhaps to the first stellar objects to form in the early Universe. Evidence gathered over the last decade indicates that Eta Carinae has a hidden, hot companion star. The nature of this companion, even its existence, is a matter of some controversy, and if the star really exists, its role in the formation and evolution of Eta Car itself is not well understood. The best evidence for the existence of this companion comes from the observation of X-ray eclipses which occur with a well-defined period of 5.54 years. The last X-ray eclipse occurred in the summer of 2003, and was observed by a unique, multi-wavelength, coordinated campaign including space-based and ground-based observatories. I'll present an overview of these observations, and discuss what they tell us about the companion and its influence on Eta Carinae, and what they imply for our understanding of these most massive stars.

X-rays from the First Massive Black Holes

Dr. W. N. Brandt

Pennsylvania State University

Tuesday, 13 April 2004


X-ray studies of high-redshift (z > 4) active galaxies have advanced dramatically over the past few years, largely due to complementary results from Chandra and XMM-Newton. I will review these results and discuss their implications for the first massive black holes and their young host galaxies. In particular, I will focus on constraints on quasar energetics and environments from a systematic Chandra and XMM-Newton study of the highest redshift known quasars, mostly found by the Sloan Digital Sky Survey. I will also present results from the Chandra Deep Field-North survey which (1) allow study of the X-ray emission properties of moderate-luminosity active galaxies at z > 4, and (2) constrain the number density of high-redshift active galaxies with implications for reionization. Finally, I will describe how these studies are laying crucial observational groundwork for future missions including Constellation-X, XEUS, and Generation-X.

Soft Excess Emission from Clusters of Galaxies - the New Face of an Old Enigma

Dr. Richard Lieu

Dept. of Physics, University of Alabama, Huntsville

Tuesday, 20 April 2004

NOTE: Special Time: 11:00


The cluster soft excess phenomenon is being investigated in the era of XMM-Newton. The excess is clearly confirmed among the original sample of clusters where the effect was first claimed to exist, but is also found to be present in a number of new clusters. The thermal interpretation in terms of a large warm baryonic reservoir now suffers from several serious setbacks. (a) The soft excess at cluster centers is tremendously bright (easily 10 times above background), it cannot be of thermal nature in these regions for the same reason why a cooling flow is untenable. (b) The previously reported O VII line emission at the outskirts of some systems is now believed to be a Galactic foreground effect. (c) Strong constraints may now be placed on the amount of cluster warm material by the absence of absorption lines in Far UV and soft X-ray spectra of background lighthouses. (d) By comparing the actual soft excess fluxes with the numerical predictions from models of the Warm-Hot Intergalactic Medium, one finds that even at the nodal points (clusters) the latter falls short of the former by 100 times. Thus the soft excess of clusters remain just as much an enigma as it was a decade ago when discovered. At the cluster cores, however, it is quite plausible that non-thermal emission is the cause, with the electrons in a population of cosmic rays undergoing inverse- Compton interaction with the microwave background, while the primary protons provide the pressure to choke the cooling flow, and replenish the electrons via pion production with the intracluster gas.

Atmospheric Structures in V471 Tauri from UV and X-ray Absorption

Dr.  Fred Walter

State University of New York, Stony Brook

Tuesday, 20 April 2004


The white dwarf in the eclipsing binary system V471 Tau is viewed through the atmosphere of the active K star prior to ingress and after egress. In the far UV and X-rays the surface brightness of the hot white dwarf far outshines the K star emission. This can be used to probe the structure of the extended K star atmosphere along one line of sight on angular scales of 10 micro-arcsec. I will show time series of HST/STIS spectra which show a hot (>250,000K) extended (> 1 K star radius) atmosphere around the K star. We see discrete structures in the velocity-resolved spectra, on spatial scales of less than 100,000 km. The mean velocity is that expected of gas in co-rotation with the K star, but the discrete velocity structures have excursions of up to 70 km/s from the mean. The mean temperature seems to increase with height above the K star photosphere. I will also discuss simultaneous time-resolved CHANDRA LETG spectra designed to probe the lower corona.

Slides of this presentation

X-ray Views of Stellar Mechanical Energy Feedback

Dr.  You-Hua Chu


Tuesday, 27 April 2004


Massive stars inject mechanical energy into the interstellar medium (ISM) vis fast stellar winds and supernova ejecta at velocities of thousands of km/s. The dynamical interactions generate gas at X-ray-emitting temperatures, 10^6 to 10^8 K. Thus, X-ray observations of the hot gas are essential in the study of stellar mechanical energy feedback. In the first part of my talk, I will present X-ray views of shocked fast stellar winds in a range of astronomical objects. In the second part, I will use the energy budget in a superbubble to illustrate that only a small fraction of stellar mechanical energy is retained as the thermal and kinetic energies of the ambient ISM.

Slides of this presentation

X-ray Polarimetry with Gas Pixel Detectors

Dr. Ronaldo Bellazzini

National Institute for Nuclear Physics (Italy)

Thursday, 30 April 2004

NOTE: Special Day, Place: Room 22, and Time: 10:30


A new class of instruments promises to bring high efficiency also t X-ray polarimetry, the last unexplored field of X-ray astronomy. It will then be possible to resolve the internal structures of compact sources which otherwise would remain inaccessible, even to X-ray interferometry. These instruments derive the polarization information from the track of the photoelectrons imaged by a finely subdivided gas detector. A custom CMOS array of active pixels directly used as the charge collecting anode of a GEM amplifying structure has been developed and built. With these detectors a polarization instrument with an order of magnitude greater sensitivity is possible, which allows the direct exploration of the most dramatic objects of the X-ray sky.

Slides of this presentation

The Anomalous low state of Hercules X-1

Dr.  Martin Still


Tuesday, 4 May 2004


During the last months of 2003, the 35 cycle of the X-ray pulsar Hercules X-1 decayed. The object now resides in a deep low state, resembling a persistent ADC source. While the mechanism that forces state changes is almost certainly variations in accretion disk structure, the engine ultimately driving structural evolution remains unknown. Nevertheless, anomalous low states provide us with an opportunity to test and confirm three independent predictions. I present recent and archival RXTE and XMM-Newton data revealing that: 1) disk evolution is driven, at least partially, by the radiation pressure of reprocessed X-rays from the central source, 2) The companion star is a source of Compton reflected X-rays, and 3) the accretion flux from the (currently obscured) neutron star remains approximately constant during the low state. Monitoring of Compton reflection during several orbital cycles allows crude estimates to be made of accretion disk structure.

Slides of this presentation

The Chandra View of M101

Dr.  K. D. Kuntz


Tuesday, 11 May 2004


X-ray studies of objects within our own Galaxy are hampered by absorption and distance uncertainties. Thus, X-ray studies of other galaxies can significantly complement the study of our own. The M101 Ms collaboration has been working on a "complete" X-ray study of the nearby face-on galaxy M101, which is similar to the Milky Way in many respects. I will discuss both the diffuse emission, which appears to be characteristic of late type spirals, and the emission from the various point source populations. The latter topic is becoming particularly interesting as we are beginning to determine the optical counterparts of the X-ray sources with high resolution multi-band HST imaging. Further, M101 has had two transient ULX for which we have tentative optical identifications.

Slides of this presentation

Deep surveys of massive black holes with LISA

Dr. Alberto Vecchio

University of Birmingham (UK)

Tuesday, 18 May 2004

Special Seminar, 11:00


Binary systems of massive black holes will be detectable by the Laser Interferometer Space Antenna (LISA) throughout the entire Universe. Observations of gravitational waves from this class of sources will have important repercussions on our understanding of the behaviour of gravity in the highly non-linear relativistic regime, the distribution and interaction of massive black holes at high redshift and the formation and evolution of cosmic structures. Here we discuss the expected quality (and shortcomings) of LISA deep surveys.

A Self-Consistent Model for Relativistic Outflows from Advection-Dominated Accretion Disks with Shocks around Black Holes

Dr. Truong Le


Tuesday, 18 May 2004


Relativistic outflows (jets) of matter are commonly observed from systems containing black holes. In these systems, the accretion rate is low, radiative cooling is inefficient, and the gas in the disk cools via the emission of high-energy particles in the jet. The strongest outflows occur in the radio-loud systems, which contain hot, tenuous accretion disks that have generally been modeled using the ADAF scenario. However, there has been some ongoing inconsistency between the theory and the observations because the original ADAF model does not include any outflow, despite the fact that the temperature in the disk is so high that the gas is gravitationally unbound. Up until the present time, no comprehensive model for the disk structure and the associated outflow has yet been produced.
In this talk we will discuss a new, self-consistent model describing the structure of an inviscid, advection-dominated accretion disk, including a standing shock in which particles are efficiently accelerated up to relativistic energies before escaping from the disk. The shock is supported by the centrifugal barrier in the disk. This is the first model to successfully explain the outflows using a well understood microphysical particle acceleration mechanism. The theoretical analysis parallels the early study of cosmic-ray acceleration in supernova shock waves. Using the mass and accretion rate for the black hole in M87, we demonstrate the self-consistency of the model by comparing the predicted jet kinetic luminosity with the observations. We also describe in detail how the particles are advected, diffused, and accelerated within the disk, resulting in a nonthermal distribution of relativistic particles with a power-law tail at the highest energies.

Slides of this presentation

Laboratory for High Energy Astrophysics Boldt Lectureship Series

How Black Holes Shape their Cosmic Environments

Dr. Mitchell Begelman

JILA, University of Colorado

Monday, 24 May 2004

Bldg. 26, Room 205

2:00 PM, followed by reception


In addition to radiation, accreting supermassive black holes produce copious outflows of fast plasma and energetic particles. These forms of kinetic energy output can profoundly affect a black hole's environment, and may even regulate the growth of both the black hole and its galactic host. Thanks to recent Chandra and XMM observations, clusters of galaxies are proving to be valuable laboratories for studying this interaction. I will discuss what we have learned recently about energetic feedback from black holes to their surroundings, from both observations and theoretical modeling.

Connecting Cosmic Rays and Photon Emission in Supernova Remnants

Dr. Don Ellison

North Carolina State University

Wednesday, 26 May 2004

Room 8, Note Special Time: 3:00


The long held belief that supernova remnants (SNRs) produce the majority of galactic cosmic rays remains unproven. If efficient diffusive shock acceleration is occurring in young SNRs, it will produce distinct morphological and spectral signatures observable with current and future radio, X-ray, and gamma-ray observatories. The nonlinear theory of diffusive shock acceleration is reviewed and we describe a simplified model of SNR structure and evolution which combines a hydrodynamic simulation with particle acceleration. The particle and photon emission from this model is presented and we discuss some recent observations which may indicate the presence of the efficient production of cosmic ray ions.

Slides of this presentation

Dark Energy Search with Supernovae

Prof. Yun Wang

University of Oklahoma

Tuesday, 8 June 2004


Type Ia supernovae provide us with the most sensitive probe to dark energy in the universe. To determine the nature of dark energy from observational data, it is important that we use model-independent and optimal methods. We should probe dark energy using its density (allowed to be a free function of cosmic time) instead of its equation of state. We should minimize gravitational lensing effect on supernovae by flux-averaging. We need to include complementary data (for example, from the Cosmic Microwave Background [CMB] and large scale structure [LSS]) in a consistent manner to help break the degeneracy between the dark energy density and the matter density fraction. We should push for ambitious future supernova surveys (such as the Joint Dark Energy Mission [JDEM]) that can observe a large number of supernovae at the highest possible redshifts. I will discuss these and other issues that will be important in our quest to unravel the mystery of the nature of dark energy, as well as constraints on dark energy from current supernova, CMB, and LSS data. A precise measurement of dark energy density as a free function of cosmic time from JDEM will have a fundamental impact on particle physics and cosmology.

Slides of this presentation

The Secret Lives of Cool Stars: Radio and X-ray Views of Coronal Structure and Variability

Dr. Rachel Osten

NRAO, Charlottesville

Tuesday, 15 June 2004


I will discuss recent multi-wavelength observations of active cool stars, with an emphasis on how radio and X-ray observations provide independent but complementary views of the structure and variability of coronal outer atmospheres. I will pay particular attention to multi-wavelength studies of stellar flares and the evolution of flaring plasma, and show that stellar flares are different enough from solar flares that the solar paradigm may not be applicable. In the last few years, both persistent and variable radio and X-ray emission from ultracool stars/brown dwarfs has been discovered, and I will discuss current efforts to understand "coronal" activity on these ultracool objects.

Slides of this presentation

Detecting the New Ether Wind

Prof. C. J. Martoff

Temple University

Thursday, 17 June 2004

Special Time: 3:00


Many theories of the galactic halo include a component consisting of Weakly Interacting Massive Particles which does not share the galactic rotational motion. As a result, one would expect a net directional flow of WIMPs relative to an earthbound lab. The DRIFT experiment attempts to detect the WIMPs and measure their directional distribution. This talk will concentrate on the detection issues involved, including collaborative work with GSFC.

X-ray Emission due to Solar Wind Charge Exchange with Interstellar and Geocoronal Neutrals

Dr. Ina Robertson

University of Kansas

Tuesday, 22 June 2004

Joint Code 660 and Code 690 Seminar


X-rays are generated throughout the heliosphere and the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and interstellar and geocoronal neutrals. In this seminar I will first discuss solar wind charge transfer with geocoronal hydrogen. The highest production rate of this type of radiation is in the cone region of the magnetosheath, where the solar wind density is high. This might be observable from an observation point outside the geocorona. I will next discuss solar wind charge exchange with interstellar neutrals. We estimate that heliospheric X-ray emission accounts for roughly half of the observed soft X-ray background intensity. I will discuss the impact this estimate might have on our view of the local bubble.

Slides of this presentation