Laboratory for High-Energy Astrophysics Seminar Series
Schedule: October - December, 2004

O VI Absorption in the Milky Way

Dr. Edward Jenkins


Tuesday, 5 October 2004


Spectra of 153 early-type stars in the disk of the Milky Way recorded by FUSE reveal absorption features from interstellar O VI, a tracer of collisionally ionized gases at temperatures of around 300,000 K. The O VI-bearing material is cooler than hot gases which produce most of the soft x-ray background emission in the Galaxy, but gases in both regimes probably have a common origin. The FUSE survey indicates an average density of O VI that is consistent with earlier results from the Copernicus satellite, but at great distances there is a surprisingly large dispersion in the measurements from one line of sight to another. This behavior indicates that O~VI absorbing regions are not in randomly distributed packets with approximately uniform sizes, but rather that the material is better characterized as a scale-free, power-law distribution of random density fluctuations. A strong correlation between the velocity end points of the O VI features and very strongly saturated lines from less ionized species indicates that there must be some dynamical coupling between the O VI-bearing gas and small amounts of material at much lower temperatures.

Presentation slides

Growth of IMBHs in Globular Clusters and their Gravitational Waves

Kayhan Gultekin


Friday, 15 October 2004

TIME: 1:30


Evidence has been mounting for the existence of black holes with masses from 100 to 10000 M_sun associated with stellar clusters. Such intermediate-mass black holes (IMBHs) will encounter other black holes in the dense cores of these clusters. The binaries produced in these encounters will encounter other objects as well thus changing the orbital characteristics of the binaries. These binaries and their subsequent mergers due to gravitational radiation are important sources of gravitational waves. We will present the results of numerical simulations of sequences of high mass ratio three-body encounters, which will help (1) clarify the nature of the interactions of intermediate-mass black holes in globular clusters, (2) constrain formation models of IMBHs, and (3) help determine what types of detectable gravitational wave signatures are likely.

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A Personal Retrospective and Prospective on High-Energy Astrophysics

Dr. Jonathan Ormes


Tuesday, 19 October 2004


As a resident of building 2 for a third of a century from 1967 until 2000, the speaker will reflect on his career, science and other aspects of working for NASA and the US government. He will comment on topics such as progress in understanding the origin of cosmic rays, the fun to be had studying them, studies of energetic particles and photons in space in the NASA context, management contributions and fads, and prospects for the future. In other words, the speaker expects to ramble through his career talking about whatever has come to mind. Audience participation will be encouraged. Since as of this writing the talk has not been prepared, he reserves the right to change his mind from whatever is written herein. George Pieper, former Director of Space Sciences, used to review abstracts and would reject those without numbers. The following is given to prevent a priori rejection. The speaker anticipates expending about 100 kilocalories preparing the talk. He will probably cool at the rate of 2 x 10^9 ergs/s giving it. It will require you to expend the energy of approximately 20,000 cosmic rays of 10^20 eV each for you to come and enjoy it, but you'd expend about that much no matter what you do during the hour.

Introduction | Presentation slides

Models for the Evolution of X-Ray Binaries in a Young Stellar Population

Dr. Michael Eracleous

Pennsylvania State University

Tuesday, 26 October 2004


I present the results of population synthesis simulations following the evolution of X-ray binaries formed in a burst of star formation. These simulations were carried out in order to explore the evolution of the hard (2-10 keV) X-ray luminos ity of a young stellar population. The results are applicable to the properties of populations of extragalactic X-ray binaries as well as the X-ray properties of starburst galaxies and many LINERs. We find that the integrated 2-10 keV luminosity of the simu lated population reaches a maximum in excess of 10^40 erg/s after approximately 20 Myr (for a star-formation rate of 10 M_sun/year) and remains significant even after the end of star formation and the demise of the luminous OB stars. The results of our simulation are in agreement with recently-derived correlations between the peak X-ray luminosity and the sta r-formation rate. We also find that the cumulative luminosity function is initially fairly flat, in agreement with recent observational results, becoming steeper as the population ages a nd the high-mass X-ray binaries are succeeded by binaries with progressively lighter donor stars. Using the results of the simulations we are also able to track the output of Hydrogen-ionizing far-UV photons from the stellar population. Thus we can plot the track of such a system in the L_x-L_Ha diagram and compare it with recent Chandra observations of the nuclei of nearby galaxies.

Neutron Stars as Gravitational Lenses

Dr. Feryal Ozel

University of Arizona

Wednesday, 3 November 2004

NOTE: Special Day


A neutron star makes one of the most interesting gravitational lenses in the universe: it strongly lenses the emission originating at its own surface owing to its intense gravitational field. After discussing several phenomena that affect a photon's journey away from a slowly or rapidly rotating neutron star, I will show how the self-lensing effects can be observed in the spectra and lightcurves of isolated and accreting neutron stars. These effects not only help us infer the neutron-star masses, radii, and emission geometries but may also help provide clues into the puzzling nature of Type-I X-ray bursters and the elusive millisecond X-Ray pulsars.

Presentation slides

Radio-free X-ray Cluster Cores

Dr. Megan Donahue

Michigan State University

Tuesday, 9 November 2004


We now know that X-ray emitting gas in the cores of clusters of galaxies exhibits more complex phenomena than predicted by the earliest models. Either we don't understand how gravitationally-heated gas radiates energy while that gas relaxes in a cluster's gravitational potential well or additional physical processes must be included in the accounting of heating and cooling of such gas. Most of the cores with radiative cooling times shorter than a Hubble time also exhibit radio sources and luminous optical emission line filaments. In order to investigate the contribution of radio sources and processes related to star formation to the core environments, we have observed with Chandra several cluster "cooling flows" that do not exhibit powerful radio sources or optical emission line filaments. We compare the X-ray properties of these low redshift clusters to a uniformly-analyzed sample of X-ray luminous clusters at similar redshift. We investigated profiles of X-ray surface brightness, temperatures, metallicity, and entropy. I will briefly review recent developments in this subject and present our results in context.

Presentation slides

Feedback from Massive stars: Proplyds, Jets, and Triggered Star Formation in Orion and Carina

Dr. Nathan Smith

Univ. Colorado

Tuesday, 16 November 2004

NOTE: Special Time - 1:30pm


Star formation in nearby regions like Taurus-Auriga has been studied extensively, but may not be representative of the conditions under which most stars (including our own solar system) may have formed. Feedback from nearby massive stars can have a profound impact on the star formation process, and I will review some recent observational results of studying externally-irradiated protoplanetary disks, jets, and embedded sources behind advancing ionization fronts in HII regions. Two regions in particular -- the Orion Nebula and the Carina Nebula -- are excellent laboratories for studying these phenomena in detail.

Supernova Asymmetries and Their Effects on Nucleosynthesis

Dr. Christopher Fryer


Tuesday, 16 November 2004


Supernovae are among the most energetic explosions in the universe. It is in these energetic explosions that most of the heavy elements in the universe are produced. To predict these nucleosynthetic yields, we must understand the explosion mechanism itself, a problem that has plagued theorists for the last 4 decades. One of the latest twists on our understanding of supernovae is their high level of observed asymmetry. Asymmetries in the explosion mechanism will affect both the explosive and r-process yields in these explosions. Here I will review our current understanding of supernovae and their asymmetries and discuss how these asymmetries change our understanding of the nucleosynthetic yields produced in these explosions.

Synthetic LISA: Simulating the Future of LISA Data Analysis

Dr. Michele Vallisneri


Note Special Day and Time:

Thursday, 18 November 2004 - 1:15pm


The NASA/ESA Laser Interferometer Space Antenna (LISA) will extend the reach of ground-based detectors to entirely new classes of gravitational-wave sources, allowing unprecedented astrophysical studies and high-precision tests of general relativity. It will also introduce new and peculiar complications in the reduction and analysis of experimental data, such as the suppression of laser phase noise by several orders of magnitude, the nontrivial signal and noise transfer functions, and the resolution of many simultaneous continuous signals. I discuss the status and the outstanding challenges of LISA data analysis and detector characterization, and I explore the role of simulation in developing and testing new strategies and techniques.

Presentation slides

Multiwavelength Observations of Ultraluminous X-Ray Sources

Dr. Philip Kaaret

University of Iowa

Tuesday, 23 November 2004

Note Special Time: 1:30pm


The discovery of X-ray sources in external galaxies with apparent luminosities exceeding the Eddington luminosity for stellar-mass black holes has generated great interest due to the possibility that they may be intermediate-mass black holes. Multiwavelength observations of these ultraluminous X-ray sources (ULXs) may help understand their physical nature. Enabled by the arcsecond X-ray source positions now available from Chandra, there has been significant recent progress in the identification of optical and radio counterparts to ULXs. I describe recent optical and radio observations of ultraluminous X-ray sources with particular attention to the measurement of the true luminosity of a ULX via observation of a surrounding photoionized nebula.

Signatures of relativistic hadrons in leptonic blazar jet models

Dr. Markus Boettcher

Ohio University, Athens

Tuesday, 30 November 2004


The broadband spectral energy distributions and spectral variability of BL Lac objects and gamma-ray loud quasars (blazars) have been modelled very successfully with relativistic jet models in which most of the radiation is produced by relativistic electrons (and positrons) in a jet directed at small angles with respect to our line of sight. In the first part of my talk, I will give a review of recent observational and theoretical modeling work employing these purely leptonic jet models. In the context of such models, very-high-energy $\gamma$- ray flares of TeV blazars are generally accompanied by simultaneous flaring activity in X-rays. However, the recent observations by the Whipple collaboration of an ``orphan'' TeV flare of 1ES~1959+650 (without simultaneous X-ray flare) is virtually impossible to reconcile with the standard leptonic jet models. In the second part of this talk, I will thus describe a recently proposed model in which the ``orphan'' TeV flare may originate from relativistic protons, interacting with an external photon field supplied by electron-synchrotron radiation reflected off a dilute reflector. While the external photons will be virtually ``invisible'' to the co-moving ultrarelativistic electrons in the jet due to Klein-Nishina effects, their Doppler boosted energy is high enough to excite the $\Delta$ resonance from relativistic protons with Lorentz factors of $\gamma_p \sim 10^3$ -- $10^4$. This and similar diagnostics of relativistic protons may be combined to constrain the elusive number and characteristic energy of relativistic protons in the jets of blazars.

Presentation slides

Thermonuclear Probes of Neutron Stars: A Path to the Equation of State

Dr. Tod Strohmayer


Tuesday, 7 December 2004


Observations with NASA's Rossi X-ray Timing Explorer (RXTE) have resulted in the discovery of fast, coherent X-ray intensity oscillations (hereafter, ``burst oscillations'') during thermonuclear X-ray bursts from 13 low mass X-ray binaries (LMXBs). Although many of their detailed properties remain to be fully understood, it is now beyond doubt that these oscillations result from spin modulation of the thermonuclear burst flux from the neutron star surface. Among the new timing phenomena revealed by RXTE the burst oscillations are perhaps the best understood, in the sense that many of their properties can be explained in the framework of this relatively simple model. Because of this, detailed modelling of their properties can be an extremely powerful probe of neutron star structure, and thus the equation of state (EOS) of ultradense matter. I will describe recent efforts to constrain neutron star structure in this way. I will also discuss the use of high resolution burst spectroscopy of rotating neutron stars to constrain the EOS. As a bonus one should also get a broad overview of some new theoretical work on X-ray bursts.

Young Neutron Stars and Their Wind Nebulae

Dr. Pat Slane

Harvard-Smithsonian CfA

Tuesday, 14 December 2004


Young neutron stars probe some of the most extreme physical environments in the Universe. Their rapid rotations and large magnetic fields combine to accelerate particles to extremely high energies, producing energetic winds that result in the slow spin-down of the stars and generate nebulae of synchrotron-emitting particles spiraling in a wound-up magnetic field. The structure of these nebulae is determined by the energy input from the central pulsars as well as the structure and content of the medium into which they expand. In the centermost regions, relativistic outflows in the form of rings and jets are formed; the geometry of these emission regions reveals the orientation of the pulsar spin axes and can provide information on the formation of kicks imparted in the moments following their formation. Their large-scale structures reveal details of the magnetic field and signatures of interaction with the ejecta from the explosions that gave them birth. The stellar interiors are characterized by conditions and physical processes otherwise observed only within the nuclei of atoms. Our incomplete understanding of this structure prohibits a definitive determination of the cooling rate of these stars. X-ray observations provide the necessary comparisons with model calculations, thus providing constraints on hadronic physics at high densities. In this talk I will summarize recent advances in our understanding of the nebulae produced by young neutron stars, as well as ongoing efforts to constrain models for neutron star cooling.

Multiwavelength Variability of Very High Energy Gamma Ray Blazars

Dr. Abe D. Falcone

Purdue University / Whipple Observatory

Note Special Date: Monday, 20 December 2004
Special Time: 1:30 pm


By using ground-based observatories, some active galactic nuclei belonging to the blazar class have been observed to emit very high energy (>300 GeV) gamma rays. Traditionally, blazars have been classified as either flat spectrum radio quasars, which have broad emission lines, or BL Lacs, which are featureless or have very narrow lines. At this time, the small list of TeV emitting blazars is restricted to nearby BL Lacs. In particular, all of the known TeV emitting blazars fall into the category of high-frequency-peaked BL Lacs, which have the first peak of the characteristic two-component spectral energy distribution (SED) in the X-ray energy region. By studying the variability of these blazars, which have TeV flaring timescales shorter than those at any other wavelength, it is possible to place strong constraints on properties such as the size of the emission region, the strength of the magnetic field at the acceleration site, and the Doppler factor in the jet. The commonly invoked mechanisms for the high energy gamma ray emission are inverse compton processes (with either external seed photons or with seed photons from the electron synchrotron emission), proton cascades, and/or proton synchrotron. By observing the extreme high energy region of the SED and combining this with data at other wavelengths, the acceleration mechanism/s can be constrained. Additionally, inferences can be made about the extragalactic background. Currently operating telescopes have made significant progress with studies of TeV blazar variability. The next generation of more sensitive instruments, including VERITAS, will increase the source statistics, and therefore our understanding, of these enigmatic objects.

Page Author: Jerry Bonnell