October 13, 2004
The Evolving AGN Population in the Chandra Deep Fields Abstract: Observations of the Chandra Deep Fields (CDFs) have resolved the vast majority of the X-ray background below 8 keV, the bulk of which appears to be active galactic nuclei (AGN) at moderate redshift, and the focus has now shifted toward understanding the nature of these resolved sources. The CDFs stand to offer many more insights in this regard because of their excellent multi-wavelength coverage. I will briefly review what we have learned about the AGN population to date, highlighting recent results on the host-galaxy morphologies of this population from GOODS ACS imaging data.
Institute for Advanced Studies
Mass Ejection from Quasars: New Perspectives from the X-rays Abstract: Gas outflows are believed to be ubiquitous in quasars and are likely to be launched from the innermost regions of these objects. Recent works suggest that they may play a major role in the formation of host galaxies and their environment. Nevertheless, the physics of quasar outflows is ill-constrained and several of their key properties, such as the mass loss rate, are highly uncertain. These uncertainties preclude a more detailed investigation of their effect on small (black hole) and large (e.g., galactic) scale phenomena. Here I present a new quantitative model which self-consistently accounts for the structure, dynamics, and spectral features of highly ionized flows in quasars. I apply this model to the X-ray outflow in NGC 3783 and identify, for the first time, the physical mechanism responsible for its acceleration. A quantitative comparison of model predictions to the 900 ksec Chandra/HETG spectrum of NGC 3783 reveals many of the physical properties of the outflowing gas. The implications of the new results for the study of quasars and their interaction with the environment will be discussed.
X-ray Study of the Local Hot Gas Recently, a number of local (z ~ 0) X-ray absorption lines were detected with Chandra and XMM-Newton along several lines-of-sight. While such detections indicate the existence of a large amount of hot gas around our Galaxy, current observations cannot reveal its true origin, i.e., whether this gas is located in the distant halo or in the Local Group. I will discuss several models and diagnostic observations to solve this puzzle, and I will also discuss the implications for the local baryon distribution.
The Big Dig: Exercises in Excavation using Relativistic Jets
The role of relativistic jets in structure formation and the thermodynamic balance of the interstellar and intergalactic medium has seen a surge in popularity in recent years. However, we are still lacking some of the most fundamental knowledge about the nature of jets themselves that are critical when evaluating their role in feedback. Starting from the discovery of the scale invariant properties of jet cores and of a robust correlation between the kinetic power of jets and their radiative properties, we have developed a number of tools to study the nature of these flows and to estimate their bulk speed and power. Based on these new results, I will present a sketch of how this interaction might work in AGNs and in X-ray binaries.
Smithsonian Astrophysical Observatory
Neutron Star Masses from Quiescent X-ray Observations and Phase Resolved Optical Spectroscopy
The description of the relation between the pressure and density of ma tter comprised by a neutron star (the equation of state; EoS) is one of the ultimate goals of neutron star studies. The EoS is determined by the physics of the strong interactions between fundamental particles and therefore is of great relevance in high--energy and particle physics. For each EoS stellar structure theory predicts the neutron star mass-radius relation and provides a firm upper limit on the mass of a neutron star above which it collapses into a black hole. Therefore, measuring a high mass for even one neutron star would imply the firm rejection of many proposed EoSs. Optical spectroscopy can provide strong limits on the masses of the neutron stars in low-mass X-ray binaries. I will report on recently obtained phase resolved spectroscopic observations of an eclipsing neutron star X-ray binary using the Very Large Telescope. Another way to constrain the EoS is via neutron star cooling after accretion has halted in so called transient neutron star systems. It has been shown that soft thermal X-rays are generated by the neutron star crust and core since they are heated via pycnonuclear reactions in the crust during the accretion phase (Brown, Bildsten, \& Rutledge 1998). The crust radiates thermally in (soft) X-rays during the long periods of quiescence, cooling the neutron star. If the observed soft thermal X-ray emission is indeed due to the cooling neutron star, its radius and mass can be determined using neutron star atmosphere models to fit the observed X-ray emission. Such observations could constrain the EoS. However, it is also possible that, due to enhanced neutrino cooling processes in the core the neutron star is so cold that the cooling neutron star produces little or none of the observed X-rays. We know from neutron star cooling theory that neutron stars with masses above ~1.6 Msun can cool down much faster via enhanced neutrin o cooling processes than 1.4 Msun neutron stars. Hence, finding that the neutron star is cool even though it has had (long) accretion episodes infers that the neutron star is massive, again ruling out a suite of EoSs. I will provide an update on the status of the constraints on the neutron star masses using X-ray observations of quiescent neutron stars.
Can we determine the grain composition of the ISM using Chandra and Astro E2 ?
I will discuss the ability of the Astro E2 XRS and Chandra HETGS to directly probe the grain composition of the ISM. Using experimental data taken at the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Light Source at Lawrence Berkeley National Laboratory, I will discuss our current prospects for (1) distinguishing gas from dust phase absorption, and (2) determining the chemical composition of interstellar grains, through the details of X-ray absorption fine structure.
The X-ray properties of high-redshift galaxy clusters
We present a sample of 11 high-redshift galaxy clusters at 0.6 < z <1.0 observed with Chandra and/or XMM. Several of the clusters are discussed in some detail, including an early-stage merger between two massive clusters, and an apparently relaxed cluster which is extremely hot and massive. The properties of the sample are investigated, and compared with those of other samples in the local universe and at high redshift. The distribution of gas in high-redshift systems, and its metal abundance, are found to be consistent with those in local systems. The scaling relations between cluster X-ray temperatures and their masses and X-ray luminosities are found to evolve. This evolution is consistent with simple self-similar models of galaxy clusters in which their properties reflect those of the universe at the epoch at which they are observed.
University of Virginia
Dynamics of Black Holes and Binaries in the Galactic Center
Dynamical friction drives stellar-mass black holes into the central few parsecs around Sgr A*, perhaps constituting 10% of the total stellar mass in that region. If binaries populate our Galactic nucleus, as they do the dense cores of globular clusters, some fraction of the stellar-mass black holes may acquire companions as a result of binary-single exchange interactions. Such a binary may capture a second black hole in a subsequent exchange, yielding a double black-hole binary. I will describe preliminary results on the dynamical production of black-hole binaries in the Galactic center, addressing these objects as potential sources of X-rays and gravitational radiation. Extrapolation of these results to other galactic nuclei will also be discussed.
Globular Cluster X-ray Sources: the Keys to Cluster Dynamical Evolution
The extreme stellar densities in the cores of globular clusters are
expected to result in a number of interesting dynamical effects
because of the relatively high frequency of close encounters (and
even mergers) between cluster members. For example, it has been
known for decades that globular clusters are a favored environment
for X-ray binaries, with formation rates per unit mass exceeding
those in the Galactic disk by orders of magnitude. These X-ray
binaries, as well as other close binaries, play a pivotal role in a
cluster's evolution. Even a modest population of binaries contains a
potential reservoir of binding energy that easily exceeds the kinetic
energy of all single stars in the cluster.
The interplay between stellar dynamics and stellar evolution, as external and internal factors modifying the binary properties, is highly complex, and many details of these processes are not well understood. Chandra observations of globular clusters are ideal for finding large numbers of close binary systems. Identifying the nature of these systems allows us to asses the effects of the parent cluster's physical properties on its different binary subpopulations (X-ray binaries, cataclysmic variables, etc.). I will discuss this ongoing effort and explore the link between the number of X-ray sources and the encounter frequency of a cluster. I will show how this allows us to determine the total number of low-mass X-ray binaries in the Galactic globular cluster system.
Institute for Advanced Studies
The Hydrodynamics of Dead Collapsars
We present a numerical investigation of dead, or relic, collapsars and the environmental impact that GRB activity has on the host galaxy.
Simulations of AGN Feedback in the Cosmological Context
I will present 3D adaptive mesh refinement (AMR) FLASH code simulations of dissipation of AGN-induced gas motions and waves in clusters of galaxies. These simulations are motivated by recent detections of sound waves and weak shocks in the Perseus and Virgo clusters. I will discuss dissipation of these waves and heating of the intracluster medium (ICM), the detectability of the waves with Chandra and the energy transfer between the buoyant bubbles and the ICM. Results from high resolution cosmological AMR simulations of AGN heating, including viscosity and conductivity as well as simulated Chandra observations, will also be discussed.
Pulsar under Stress: the Magnetosphere of Binary Pulsar J0737B
Recently discovered binary pulsar system J0737-3039 is a unique labora tory for probing the structure of pulsar winds and pulsar magnetospheres. The wind from millisecond pulsar A compresses the magnetosphere of pulsar B and confines it within B's light cylinder radius. We present numerical models of the interaction between the relativistic wind from A with the magnetosphere of pulsar B, modeled as inclined rotating magnetic dipole. Using a particle-in-cell code we obtain the 3D time-dependent shape of the B's magnetosphere including the stand-off shock and the magnetosheath, which is reminiscent of a terrestrial magnetosphere. For large inclination angles between magnetic and rotational axes of B the shape of the magnetosheath is skewed by B's rotation -- the shocked wind plasma streams into B's magnetosphere preferentially on the side that is moving in downwind direction. These "cusps" introduce time-averaged asymmetry in the shape of the magnetosheath which is potentially responsible for asymmetry in the morphology of A's eclipse by B. We also consider the properties of the B pulsar related to the confinement of its magnetosphere by the wind: B's spindown is affected by the wind torque and by the shape of the magnetosheath. Understanding the pulsar spindown in a different environment where the light cylinder is obstructed is a crucial piece of pulsar magnetospheric puzzle. We also compute the evolution of the polar cap size with rotation of B and use it to explain the changes in the observed pulseshape of B with orbital phase.
Numerical Simulations of the Collapsar Model for Gamma-Ray Bursts
It is generally believed that long soft Gamma-Ray Bursts (GRBs) are made from the death of massive stars. I summarize the basic aspect of the collapsar model, a favored model for the central engine of GRBs. Numerical simulations of relativistic jets in collapsing massive stars and observational implications of these results are presented. I will also report on recent simulations using a newly developed special relativistic hydrodynamics code with adaptive mesh. The collapse of rotating massive stars and formation of accretion disks and relativistic jets are studied in these simulations.
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