Laboratory for High-Energy Astrophysics Seminar Series

2004, July - September

Probing Colliding Wind Binaries with High-Resolution X-ray Spectra

Dr. David Henley

Birmingham (UK)

Thursday, 15 July 2004


Winds from massive stars have profound effects on their environments and on the stars' own evolution. Most massive stars reside in binaries, in which their winds collide highly supersonically and produce copious X-rays. High-resolution X-ray spectra enable us to probe in detail the dynamics and structure of the wind-wind collision region, and can give us information on the shape, orientation and velocity structure of the wind-wind collision, as well is its temperature and density structure, providing us with a new tool for understanding mass-loss from massive stars. We present results of Chandra grating observations of the key colliding wind systems Gamma Velorum, WR 140 and Eta Carinae. Comparisons of these observations with model X-ray line profiles are providing new insights into the nature of the wind-wind collisions in these systems.

Slides of this presentation

Radio-loud AGN and GLAST: Looking Ahead to a Bright Future

Dr. Rita Sambruna


Tuesday, 27 July 2004


Radio-loud Active Galactic Nuclei (AGN) are characterized by powerful relativistic jets originating from the accretion process onto the central supermassive black holes. The kpc-scale jets mark the site of the most powerful particle acceleration processes, and despite several decades of intense research, there still are many open questions, including: What are the emission processes in jets at the various wavelengths? How is the accretion power transported along the jet from small to large scales? How do jets propagate into the interstellar space? Equally mysterious is the origin of the weak broad-band emission from the nuclei of low-power jetted sources, for which radiatively inefficient accretion geometries have been proposed in addition to beamed radiation from a misoriented jet. After reviewing recent progress on our understanding of large-scale jets and the nature of the central engines in low-power radio galaxies, I will discuss the exciting prospective of future GLAST observations of these sources. Detections at gamma-rays of both the large-scale jet and the isotropically emitting central powerhouse in radio galaxies with GLAST are expected to provide a major breakthroug in our understanding of the physical properties of radio-loud AGN.

This work is supported by NASA and NSF funds.

Slides of this presentation

The Accretion History of the Universe

Dr. Richard Mushotzky


Tuesday, 24 August 2004


The Chandra data on the sources of the x-ray background have provided fundamentally new information on where, when and how many radiating massive black holes there are in the universe. Roughly 2/3 of all Chandra selected AGN are not found in optical surveys, while the Chandra observations find virtually all optically selected AGN. The Chandra sources have a different redshift evolution than optically selected AGN peaking at z~1 rather than z~2.5. They show strong luminosity evolution at z<1 and negative evolution at higher z. The host galaxies are luminous and red, probably mostly giant E galaxies at all redshifts. The Chandra sources also trace large scale structure in a manner indicating that they are highly biased tracers of structure.

Slides of this presentation

X-rays from Starbursts

Dr. Massimo Persic

INAF/Osservatorio Astronomico di Trieste

Thursday, 2 September 2004



X-ray emission processes in starbursts (SBs) are assessed in order to identify and characterize the main spectral components. Main predictions are that less than 1 keV emission is dominated by thermal galactic wind emission, while 2-10 keV emission is mostly contributed by HMXBs and related sources (which typically have 2-10 keV flat spectra). Based on these results, and using a sample of local as well as distant star-forming galaxies for which independent estimates of SFR are available, we show the existence of a linear correlation between the SFR and the SB's 2-10 keV luminosity, which holds over 5 decades in each quantity. The 2-10 keV luminosity associated with a SB provides, therefore, a tool to measure the SFR there.

Slides of this presentation

Small Stars in a Big Backyard: Neutron Stars as Probes of their Environments

Prof. Bryan Gaensler

Harvard University

Tuesday, 14 September 2004


Neutron stars are extraordinary physical laboratories, providing beautiful experimental data on everything from general relativity to the equation of state of degenerate matter. But because the properties of neutron stars can be so accurately measured, these objects can also act as exquisite probes for studying the complicated astrophysical environments in which they are embedded. I will present a multi-wavelength observational programme, which demonstrates the new insights which neutron stars are now providing into the physics of supernova explosions, the structure of relativistic shocks, and the properties of the diffuse interstellar medium.

Slides of this presentation

Recent Progress in Iron K Line Modeling

Dr. Tim Kallman


Tuesday, 21 September 2004


Iron K lines are of great interest in X-ray astronomy. They are emitted by many classes of objects, and are usful for their value in determining abundances, obscuration and relativistic effects. As observations improve so does the importance of the diagnostic information obtained from these lines. Until recently, much of the atomic data used in interpreting iron K line observations was based on calculations which were quite primitive, by today's standards. In this talk I will describe a recent computational campaign to improve and update the atomic data used to model iron K lines and other associated observables. This has culminated in a new set of models for these lines. I will discuss the methods and results, and I will illustrate the application of these results with several examples.

Slides of this presentation

The Local Bubble:Some Like It Hot?

Dr. Barry Welsh

UC Berkeley

Tuesday, 28 September 2004


New data gained from both ground-based and satellite-borne instruments are challenging the long-held view of the physical nature of the Local Bubble -- a very low density cavity in the interstellar gas that surrounds our Sun to a distance of ~100pc in most galactic directions. A central question in this debate is the temperature and ionization state of the local interstellar gas. We pose the question as to why "some like it hot"!

Slides of this presentation

Interpreting the High Frequency QPO Power Spectra of Accreting Black Holes

Dr. Jeremy Schnittman

Wednesday, 29 September 2004

Note special Day, Room: 22, and Time: 3:00


Recent RXTE observations have shown evidence for quasi-periodic oscillations (QPOs) at frequencies with a 2:3 ratio in a number of accreting black hole systems. In the context of a relativistic hot spot model, we investigate different physical mechanisms to explain these high frequency QPOs. The locations and amplitudes of the QPO peaks are determined by general relativistic ray-tracing calculations. The inclusion of multiple hot spots with finite lifetimes and distributed over a range of geodesic orbits allows us to explain the widths of the various peaks. A simple model for electron scattering in the corona helps account for the damping of higher frequency harmonic modes. Finally, the complete model is used to fit the observed power spectra of both type A and type B QPOs seen in XTE J1550-564, giving confidence limits on each of the model parameters.