Laboratory for High-Energy Astrophysics Seminar Series

2004, January - March

 


Neutron Star Mass and Radius Determinations from RXTE Spectral Data

Mr.  Nikolai Shaposhnikov

GMU

Tuesday, 13 January 2004

Abstract

I will present the results of the analysis of dynamic X-ray burst spectra from a set of LMXBs sources observed with RXTE. The application of the theory of spectral formation during the decay stage of X-ray bursts allows the determine of the fundamental parameters of a bursting compact star. The method implies the fitting of the bolometric flux - color-temperature dependence of burst radiation with the theoretical model which includes the neutron star mass, radius, atmospheric chemical composition and the source distance as parameters. So far the technique was applied to Cyg X-2, 4U 1728-34, 4U 1820-30. The results generally support that neutron stars with burst atmospheres are usually helium-dominated. Superior statistics of the data for 4U 1728-34 also allowed the detection of the effect of a geometry evolution during the X-ray burst with radial expansion, which is due to the strong interaction of a burst with the accretion disk. This effect is even more apparent for the strong radial-expansion burst from 4U 1820-30. The application of new results for the nuclear matter equation of state to the compactness of a star in 4U 1728-34 indicate that the core of the star can contain quark condensate.

To see the slides of this presentation Click Here

Recent results from XMM-Newton (and Mars-Express?)

Dr.  Fred Jansen

ESA/ESTEC

2:30, Wednesday, 14 January 2004

Abstract

An overview of the technical and programmatic status of the XMM-Newton observatory will be provided, along with plans for near future upgrades in the XMM-Newton software and contents of the data archive. A number of recent results ranging from observing solar system objects to Gamma-Ray Bursts will be discussed. These will be placed in the context of the science proposed and future prospects for XMM-Newton science.

To see the slides of this presentation Click Here

Exotic Nuclei in Cosmic Explosions and in Radioactive Beams on Earth

Dr.  Hendrik Schatz

National Superconducting Cyclotron Laboratory, Michigan State University

Tuesday, 20 January 2004

Abstract

With the advent of a new generation of radioactive beam facilities and advances in our theoretical understanding of nuclei far from stability we can now begin to delineate the nuclear processes that govern stellar explosions. These gigantic fireworks include supernovae, novae, and X-ray bursts and are intimately related to fundamental questions of the origin and fate of the elements. I will focus on how the properties of exotic nuclei that exist for mere milliseconds during a stellar explosion, are imprinted on astronomical observations and on the abundances of elements in the cosmos. I will discuss recent advances in astronomy, astrophysical modeling, and in nuclear physics. This will include a discussion of recent experimental results from the National Superconducting Cyclotron Facility at Michigan State University, where we succeeded in studying some of the nuclear processes that occur during stellar explosions.

Explosions from Stellar Collapse

Dr.  Christopher Fryer

Los Alamos National Laboratory

Tuesday, 27 January 2004

Abstract

The collapse of massive stars is believed to be the cause of a number of the most energetic explosions in the universe from supernovae to gamma-ray bursts. Stellar collapse forms nearly all the neutron stars and stellar-massed black holes in the universe and are the dominant contributors to the heavy element abundances in galaxies. They produce gravitational wave and neutrino signals that are detectable for explosions in or near the Milky Way. The last few years has seen considerable progress in our understanding of these events. I will review these different fates, focusing on the progress in recent years.

Probing Extragalactic Star Formation and Black Holes with X-rays

Dr.  Andy Ptak

JHU

Friday, 30 January 2004

Note Special Time: 11:00

Abstract

It is becoming increasingly clear that X-rays are a useful tool for probing star formation. In the local universe, X-ray observations of galaxies often show the immediate (SN, hot gas, high-mass X-ray binaries) and delayed (low-mass X-ray binaries) results of star formation. X-rays are now providing interesting constraints for the star-formation history of the universe at redshifts of order ~ 1, and we have derived the first X-ray luminosity functions of normal galaxies at z>0. Ultraluminous X-ray sources (ULXs) are often observed both locally and in deep surveys, and may be tied to star-formation. At least some ULXs are likely to be intermediate-mass black holes (and may be due to very high-mass star formation). Fe-K emission may be detected from both ULXs and very hot gas in galactic nuclei. Most galaxies in the local universe probably also harbor a supermassive black hole, but are not AGN. In these cases, the physics of the accretion flow may be different than is the case in typical AGN, and Fe-K emission (or the lack of it) provides useful constraints on the nuclear geometry. Ongoing wide-area and optically-selected sample surveys will be discussed, which are critical for determining the (relatively) unbiased statistical X-ray properties of galaxies.

To see the slides of this presentation Click Here

Highlights From the First Year of the INTEGRAL Mission

Dr.  Chris Shrader

NASA/GSFC/LHEA

Tuesday, 3 February 2004

Abstract

The INTEGRAL (International Gamma-Ray Astrophysics Laboratory), has now been operating successfully in orbit for about 15 months. INTEGRAL carries two main gamma-ray instruments, the spectrometer (SPI) - optimized for high-resolution gamma-ray line spectroscopy (20 keV-8 MeV, E/dE ~ 500), and the imager (IBIS) - optimized for high-angular resolution imaging (15 keV-10 MeV, FWHM ~ 12 arcmin). Two monitor experiments, JEM-X in the ( 5-35 keV) X-ray band, and OMC in optical (V-band) complement the payload. To date, ~50 individual Guest Observer programs have been carried out, and through the Core program observations, an exposure of ~5 Msec in the central region of the Galaxy has been obtained. I will review the status of the mission and review a number of early science highlights, including updates on updates on key areas of study such as gamma-ray lines, Galactic diffuse continuum emission, gamma-ray emission from compact Galactic objects, AGN, and gamma-ray bursts.

To see the slides of this presentation Click Here

Black Hole Power: Seen One, Seen Them All?

Dr.  Martin Gaskell

Joint Seminar with Code 680, Note Special Date and Place

Room 183 in Building 21, 3:30, Wednesday, 4 February 2004

Abstract

It has long been agreed that quasars and other active galactic nuclei (AGNs) derive their ultimate power from the accretion of matter onto black holes, but there has been much long-standing debate about the mechanisms by which accretion energy is converted into the observed continuum radiation. Two of the most fundamental AGN questions are: (a) what produces the observed overall X-ray to far IR spectral shape? and (b) what is the cause of temporal variability in different wavebands? I present new results showing that after correction for reddening, differences in the overall shape of the continuum are much smaller than has hitherto been believed, and that the variability properties of different classes AGNs are very similar. These results argue that, contrary to some previous suggestions, the underlying mechanisms producing the continuum in AGNs, and the variability of the continuum, are very similar over an enormous range of luminosity.

X-ray Spectroscopy of Low-Mass X-ray Binaries

Dr. Adrienne Juett

MIT

Friday, 6 February 2004

Note Special Time: 1:30

Abstract

The high-resolution spectral capabilities of Chandra and XMM allow us for the first time to directly measure the absorption edges in X-ray spectra. I will review my thesis work which has used absorption edge spectroscopy of low-mass X-ray binaries (LMXBs) to study both the local material in these systems and the interstellar medium. I have used high-resolution spectroscopy to show that a residual feature in the low-resolution spectra of four LMXBs could be explained by an unusual Ne/O abundance ratio in the absorption edges. Given the low optical luminosities of these systems and the Ne overabundance, I then suggested that these sources were all ultracompact binaries with C-O or O-Ne-Mg white dwarf donors. In addition, I have used photoelectric absorption features in the Chandra/HETGS spectra of seven bright X-ray binaries to study the detailed spectroscopic structure of oxygen absorption in the interstellar medium (ISM). This represents the highest-resolution X-ray spectral study of interstellar oxygen absorption ever performed, revealing previously undetected features and demonstrating the inadequacy of existing models for grating data.

To see the slides of this presentation Click Here

Cassiopeia A: X-ray Windows into Supernova Explosions and Nucleosynthesis

Dr. Una Hwang

Monday, 9 February 2004

Note Special Time: 11:00

Abstract

Young supernova remnants offer the opportunity to probe the explosions that formed them through X-ray emission from their shock-heated gas. The newest generation of X-ray observatories have provided the means to consider and begin detailed quantitative studies of the explosion dynamics and nucleosynthesis in such remnants. Cassiopeia A, the remnant of the most recent known Galactic core-collapse supernova, is one of the best candidates for such studies by virtue of its brightness, size, and well-constrained age and distance. I will present current results based on spectral and imaging observations with the Chandra X-ray Observatory, interpreted in the context of models accounting for the basic hydrodynamics and plasma physics. These results include a factor of two asymmetry in the distribution of explosion energy, the nucleosynthesis of Fe, and an assessment of the mixing between ejecta layers.

To see the slides of this presentation Click Here

The Theory of Colliding Stellar Winds

Dr. Julian Pittard

Monday, 9 February 2004

Note Special Time: 1:30

Abstract

The collision of the hypersonic winds in early-type binaries produces shock heated gas, which radiates thermal X-ray emission, and relativisitic electrons, which emit non-thermal radio emission. In this review I present our current understanding of the emission in these spectral regions and discuss models which have been developed for the interpretation of this emission. Throughout I highlight processes which affect the resulting emission and conclude with ideas for future research.

To see the slides of this presentation Click Here

Initial Results from the Chandra Multiwavelength Plane (ChaMPlane) Survey

Dr.  Josh Grindlay

Harvard-Smithsonian Center for Astrophysics

Tuesday, 10 February 2004

Abstract

The high resolution Chandra images of the galactic plane being acquired in numerous individual observation programs are being analyzed systematically for their point source content and identifications with optical counterparts. ChaMPlane is the multi-year archival analysis project to do this, with the primary goal to constrain the populations of accreting compact binaries and compact objects (CVs, quiescent low mass X-ray binaries, and Be HMXBs). We present initial results for total source logN-logS distributions vs. galactic center offset and for X-ray colors to constrain spectral models. A large optical survey is also underway to measure colors and spectra of candidate IDs; initial results are summarized. Finally, a deep pointing program for Chandra and HST/ACS observations of two low-extinction fields, Baades Window and Stanek's Window, is described. Initial results are presented for the BW field which may suggest a high CV space density in the galactic bulge.

Slides of this presentation

X-ray emission from young stars - Is it really thermal?

Dr. Kenji Hamaguchi

Wednesday, 11 February 2004

Note Special Time: 1:30

Abstract

X-ray emission from normal stars is believed to be thermal. Emission lines in their spectra are often equivalent to sub-solar and have anomaly of elemental abundance. The anomaly is interpreted by mechanism of elemental selection around stellar surface. However, I see some young stars with extremely low apparent metal abundance in X-rays, which does not seem to be explained with the above framework. I therefore hypothesize that the X-ray emission needs non-thermal process as an alternative X-ray emission mechanism. In this talk, I focus on three strange phenomena observed in young stellar X-rays: 1). extreme low elemental abundance of a young magnetic B star, 2). apparent abundance variation of an extremely young star and 3). anomalous X-ray variation during an eclipse of a young binary system. I discuss possible non-thermal mechanism on stars.

Iron K line observations of Active Galactic Nuclei

Dr. James Reeves

Friday, 13 February 2004

Note Special Time: 10:30

Abstract

Spectroscopy in the iron K-shell band in Active Galactic Nuclei will be reviewed from recent XMM-Newton and Chandra observations. These new observations show that the standard paradigm whereby a broad iron K line results from reflection off the inner regions of an optically thick accretion disk to be over simplistic. "Narrow" iron K lines, with typical velocity widths of a few thousand km/s, are commonly found in Seyfert spectra, and may originate anywhere from the outer accretion disc or broad line region to the parsec scale molecular torus. Further emission components, from highly ionized gas are also found, either from a photoionized disc or a blend of multiple line components. Evidence for the redshifted part of a disk emission line is uncertain in many of the Seyfert 1s, and can be confused with the strong absorption that is often present. I also present evidence for a new phenomenon in some AGN, in the form of high velocity X-ray outflows, which can require outflow velocities of the order 0.1c and outflow rates as high as several solar masses of material per year. Finally I discuss the need for higher spectral resolution, with calorimeter based detectors such as Astro-E2 XRS, to resolve many of the issues presented here.

Slides of this presentation

Ongoing Results of the CHIPS Mission

Dr.  Mark Hurwitz

Space Sciences Laboratory, University of California - Berkeley

Tuesday, 17 February 2004

Abstract

CHIPS, the Cosmic Hot Interstellar Plasma Spectrometer, is NASA's first University-Class Explorer. CHIPS is surveying the sky for diffuse emission in the relatively unexplored but energetically important band between 90 and 260 Angstroms. An overview of the instrument and satellite development will be presented. Although several emission features of highly ionized iron were expected to be detected, only a single iron line (Fe IX 171.1 Angstrom) has been securely identified in the data. Interpreted with a collisional ionization equilibrium, Raymond & Smith type plasma model, the CHIPS results indicate an emission measure about a factor of 10 below "canonical" parameters for the local hot bubble, and a surprisingly low temperature around 650,000 K. The divergence of these results from the conventional picture may indicate that the the diffuse EUV emission and local component of the diffuse SXR emission do not share a common origin.

Slides of this presentation

Gaseous Cool Halos of Spiral Galaxies

Dr. Eric Miller

University of Michigan

Friday, 20 February 2004

Note Special Time: 10:30

Abstract

In a currently emerging picture, extra-planar gas plays an important role in the evolution of spiral galaxies as material is fed to the disk from a galactic fountain or extragalactic accretion. The high-velocity HI clouds (HVCs) of our own Galaxy possibly arise from these processes, but the distances, masses, and origins of these clouds are unknown. The mass and space distributions of HVCs can be obtained from very deep observations of other galaxies at known distance, and we have used the VLA to search for anomalous HI clouds in two nearby spirals, M 83 and M 51. Using a new source detection algorithm, we detect several HVC candidates in the two galaxies, and for the first time we place statistical limits on the mass distribution of the HVC ensemble. We also detect an extended HI disk that is kinematically distinct from the normal HI disk, and we interpret this as a result of a galactic fountain. In this talk, I will present the results, which require the presence of both a galactic fountain and companion stripping/accretion. I will also discuss the necessity for multi-wavelength observations in this type of study, as fountain gas is expected to cool from a highly-ionized to neutral phase and should be visible in emission from the radio to X-ray regimes.

Missing AGN: In Search of the Intrinsic Population

Dr.  Belinda Wilkes

Harvard-Smithsonian Center for Astrophysics

Tuesday, 24 February 2004

Abstract

Recent radio, Infrared (IR) and X-ray surveys have uncovered a new subset of the population which lacks the classic blue colors and often the strong broad emission lines traditionally required to secure the classification of Active Galaxy (AGN) or quasar. However their high X-ray and/or IR luminosities are hard to explain without invoking the super massive central black holes characteristic of AGN and quasars, leading to the suggestion that their optical emission is largely hidden from view by obscuring material in these "buried" AGN. This raises many questions concerning not only the nature of the obscuring material, if indeed this is the correct explanation, but also how large a fraction of the population is buried in this way and so missed by many previous surveys, how best to find them and how much of the energy budget of the universe is generated by accretion. I will review the standard AGN model with reference to the spectral energy distributions we observe. I will then discuss the mounting evidence for missing AGN and the multi-wavelength properties of a number of examples selected in IR and X-ray surveys, relating them to their better-known cousins. Finally I will look at the prospects of finding more via multi-wavelength surveys such as ChaMP, SWIRE and GOODS.

Slides of this presentation

Ultra-luminous Compact X-ray Sources

Dr. G. C. Dewangan

Friday, 5 March 2004

Note Special Time: 2:30

Note Special Place: Visitors Center

Abstract

Until recently, it was generally believed that compact X-ray sources, associated with black holes, exist in nature in two broad classes - X-ray binaries with stellar mass (about 3-20 solar mass) black holes and active galactic nuclei with super-massive (about 10^6 - 10^10 solar mass) black holes. In the last few years, the discovery of ultra-luminous X-ray sources (ULXs), with luminosity intermediate to that of X-ray binaries and active galactic nuclei, has led the possibility of detection of intermediate mass black holes (about 10^2 - 10^4 solar mass). In this talk, I will discuss the emerging nature of ULXs from XMM-Newton and Chandra X-ray observations.

From Cores to Cosmology: Implications of Recent X-ray Observations of Elliptical Galaxies

Dr. Mike Loewenstein

NASA/GSFC/LHEA

Tuesday, 9 March 2004

Abstract

The oldest and most massive galaxies in the universe are elliptical galaxies. They are thus important probes of galaxy formation processes, and provide a bridge from galactic to large-scale structure. I present estimates of the dark matter distribution in the elliptical galaxy NGC 4636, based on data from the Chandra and XMM-Newton X-ray Observatories, with an emphasis on the implications of the amount of dark matter in the core for the nature of dark matter. In addition to containing most of the stellar mass in the universe, elliptical galaxies host most of the total density in supermassive black holes. I show how combining black hole demographics derived from HST and SDSS, with recent calculation of the evolution of the hard X-ray luminosity function of AGN, can be used to constrain the accretion evolution and growth of supermassive black holes. This has implications for heating the intracluster and intergalactic medium.

Unsolved Mysteries of High Velocity Clouds

Dr.  Robert Benjamin

University of Wisconsin - Whitewater

Tuesday, 23 March 2004

Abstract

For four decades, the two principal mysteries of High Velocity Clouds have been: how far away are they and where do they come from? Although these aren't answered questions, significant progress has been made in recent years. In this talk I focus on three unsolved mysteries of High Velocity Clouds: what surrounds HVCs, what makes up HVCs and are HVCs important?

Slides of this presentation

Self-assembly of Multifunctional Particles for Nanostructures

Dr.  Peter Searson

Department of Materials Science and Engineering, Johns Hopkins University

Joint Seminar with Code 680, Note Special Date and Place

Room 183 in Building 21, 3:30, Thursday, 25 March 2004

Abstract

Nanometer size particles, such as nanorods or nanowires exhibit many unique properties associated with their inherent shape anisotropy. The introduction of multiple segments along the length of a nanowire can lead to further degrees of freedom associated with the shape of each segment and the coupling between the layers. Nanoporous segments with high surface area can be introduced into the nanowires by dealloying one component of a binary alloy. The unique properties of multisegment nanowires can also be exploited in suspensions where the manipulation and assembly of nanometer scale particles has become an important tool in nanotechnology. The ability to bind molecules with different functionalities to different components in multisegment nanowires introduces an additional degree of freedom that is important in drug delivery and self-assembly of three dimensional architectures for circuits or assembling scaffolds for tissue engineering.

The Elemental Abundances in the Intracluster Medium as Observed with XMM-Newton

Dr. Takayuki Tamura

ISAS/JAXA

Friday, 26 March 2004

Note Special Day: Friday, 26 March and Time: 2:00

Abstract

We measured the elemental abundances and their spatial distributions in the intracluster medium using XMM-Newton EPIC and RGS data of a number of galaxy clusters. The sample mainly consists of X-ray bright and relaxed clusters with a cD galaxy. Along with detailed Si, S and Fe radial abundance distributions within 300-700 kpc in radius, the O abundances are accurately derived in the central region of the clusters. The Fe abundance maxima towards the cluster center, possibly due to the metals from the cD galaxy, are spatially resolved. The Si and S abundances also exhibit central increases in general, resulting in uniform Fe-Si-S ratios within the cluster. In contrast, the O abundances are in general uniform over the cluster. The mean O to Fe ratio within the cluster core is sub-solar, while that of the cluster scale is larger than the solar ratio. These measurements indicate that most of the Fe-Si-S and O in the intracluster medium have different origins, presumably in supernovae Ia and II, respectively. Based on these measurements we discuss the past star formation history in clusters.

LMXBs as Gravitational-Wave Sources

Dr. Nils Andersson

Univ of Southhampton

Tuesday, 30 March 2004

Abstract

In the last few years it has become clear that the LMXBs harbour rapidly spinning neutron stars. This is natural since the neutron stars should spin up due to accretion from the companion, but these systems still pose some difficult questions for theorists. In particular, the data suggests that the fastest spinning system has a rotation frequency of 620 Hz which is significantly below the break up limit for most modern neutron star equations of state. This has led to the suggestion that the spin-up torque may be counteracted by the emission of gravitational waves, which is interesting since it would make these systems detectable with advanced gravitational-wave detectors.

In this talk I will discuss the three proposed ways in which the neutron stars in LMXBs may radiate gravitational waves, namely i) accretion induced assymetries in the crust, ii) unstable r-mode oscillations, and iii) free precession due to toroidal magnetic field stresses. I will describe our current understanding of these mechanisms, assess their viability, and discuss the challenges that need to be overcome in order to improve the chances of detecting the emitted gravitational waves.

Slides of this presentation