Chandra Fellows Symposium 2006

Jan-Uwe Ness

The SSS Phase of RS Ophiuchi Observed with Chandra and XMM-Newton I. Data and Preliminary Models

Abstract:The recurrent nova RS Oph reoccurred after 21 years of silence on 12 February 2006. In contrast to 1985, much denser coverage in X-ray observations was achieved. Swift observed RS Oph up to several times a day during the most interesting phases, while Chandra and XMM-Newton took snapshots with high spectral resolution. I will introduce the Chandra grating observations that were obtained at strategic times of the lightcurve, covering very different phases of the evolution. The first observation revealed details about the early phase which is dominated by the ejected material colliding with the wind and outer atmosphere of the companion red giant (IAUC 8683). Two more observations were taken during the time when the burning white dwarf became visible through the expanding shell (Super-Soft-Source, SSS, phase). The ascent into permanently transparent emission was accompanied by a high degree of variability (ATel 801), which can also be seen in the Chandra lightcurve. Chandra also observed the nebular phase observing the response of the surrounding material to the strong radiation and the kinematics (recombining plasma and plasma in collisional equilibrium). I will present all data up to and including the most recently available at the time of the symposium and will discuss the complete evolution of RS Oph from early outburst to late decay.

Abstract:We discuss the interaction of supermassive black holes with the cluster environment. Hot cluster gas, the dominant baryonic component in clusters, provides a record of activity of AGN/supermassive black holes within the cluster. The hot gas shows evidence of shocks and buoyant bubbles of relativistic plasma, both produced by supermassive black hole outbursts. In addition, filamentary structures seen in the X-ray gas distribution show a complex relation between the AGN produced plasma and the thermal gas. We focus on the Chandra observation of M87 and other clusters that show evidence for outbursts with energies up to $10^{62}$ ergs.

Jets from Quiescent Stellar Black Holes.

Abstract:The production of synchrotron-emitting relativistic outflows appears to be a ubiquitous feature associated with accretion onto stellar mass black holes down to a few per cent of the Eddington rate. There is mounting evidence, primarily from large scale structures, that the kinetic power of such jets can account for a sizable fraction of the total accretion energy budget. Yet the overwhelming majority of Galactic stellar mass black holes are thought to be accreting at highly sub-Eddington rates. I will report on recent multiwavelength observations of nearby quiescent black holes and discuss their implications for testing the survival of jets at low accretion rates.

Abstract:Understanding the Nature of Ultra-Luminous X-ray Sources.

Ultra-luminous X-ray sources (ULXs) are off-nuclear X-ray point sources with luminoisities of 1E39 - 1E41 erg/sec. They can be intermediate mass black holes (IMBHs) of 1E2 - 1E5 solar masses, or stellar mass black holes with special emission mechanisms. I will talk about the works my collaborators and I have done in attempts to understand their nature. These include (1) a ROSAT HRI survey to reveal their connection with low-mass X-ray binaries, high-mass X-ray binaries, and star formation activities; (2) studies on variability of their fluxes, with particular attention to periodic variations and quasi-periodic oscillations; (3) in-depth studies of their optical counterparts and environments with Hubble Space Telescope and large ground-based telescopes. I will conclude with the possible approaches to promote our understanding of ULXs with available instruments.

Abstract:X-ray and optical observations of quadruply lensed quasars can provide a micro-arcsecond probe of the lensed quasar as well as provide information on the ratio of baryonic to dark matter in the lensing galaxy. We utilize X-ray observations of ten lensed quasars recorded with Chandra as well as corresponding optical data recorded by either Hubble or ground-based optical telescopes. These are analyzed in a systematic and uniform way with emphasis on the flux-ratio anomalies that are found relative to the predictions of smooth lens models. A comparison of the flux ratio anomalies between the X-ray and optical bands allows us to conclude that the optical emission regions of the lensed quasars are much larger than expected from basic disk models (by factors of ~10-100). We also conclude that the lensing galaxies, as a group, contain a substantial amount of dark matter in the vicinity of the Einstein radius (i.e., at distances of ~5--10 kpc from the galaxy centers).

Elena Rasia

Observing galaxy cluster simulations with an X-ray telescope.

Abstract:In the near future thousands of galaxy clusters will be available for cosmological studies. The theory usually refers to their total mass as key ingredient to obtain the cosmological parameters. Analyzing recent high-resolution hydrosimulations, I studied the bias affecting the mass estimate obtained with X-ray analysis and the tricky problem of temperature definition in simulations. For this purpose we built the software package "X-MAS" (X-ray MAp Simulator), devoted to create mock Chandra X-ray observations of simulated clusters. Studying the dynamical distribution of both gas and dark matter, we showed that the models typically used to derive the mass fail in reproducing the true mass of the system. This discrepancy, seen in theoretical analysis, is confirmed in the X-ray observed clusters, for which the X-ray mass is always an underestimate. The difference can be solved considering the contribution of the gas motion and the gap between the true temperature and the spectroscopic one, which is biased towards lower values due to the use of a single temperature model to fit a spectrum of a multi-temperature plasma. To describe properly the nature of this last quantity in simulated clusters we propose a new formulae called "spectroscopic-like temperature". With this definition the amplitude of the power spectrum derived from the M-T relation is influenced. The future will be to create a mock X-ray catalogue of simulated clusters processed through X-MAS to study further the spectral analysis and to understand deeply the spectral interpretation, analyzing the temperature and metalicity measurement; to explore the effects of merging and orientation on sample selection; to investigate the scaling relations between observable and the total mass of the systems; to extend the study on the accuracy of the X-ray mass estimates.

Abstract:It is likely that intracluster stellar populations are partially responsible for the metal enrichment of the intracluster medium since their resulting supernovae will inject metals directly into the ICM. In order to investigate the relative role that host versus hostless supernovae play in ICM metal enrichment, we have undertaken a survey of ~50 X-ray selected galaxy clusters (0.1 < z < 0.2) using the 1 degree imager on the 2.3m Bok telescope. We have three principal goals: 1) determine the mean fraction of intracluster star light by observing the relative fraction of host versus hostless SN-Ia, 2) determine the overall SNe-Ia rate in our cluster sample sufficiently well to place clear constraints on the SN-Ia 'delay time', which is the time between the formation of a stellar system and the eventual explosion of some of its members as SN-Ia, and 3) combine these two measurements to determine the contribution of intracluster SNe to the global chemical enrichment of clusters. Here we present the initial results of the survey and future plans.

Abstract:Hyper-accreting disks accrete at a super-Eddington rate of a fraction to a few solar masses per second. The resulting density and temperature conditions lead to a degenerate state for electrons and a nuclei-free, neutron rich baryon composition within ~100 Schwarzschild radii. This has led to suspicions that the composition of outflows powered by those disks would have a neutron excess with important consequences for their dynamics. However, this issue can be quantitatively addressed only if the disk vertical structure is computed. In this talk I present the first calculation of the vertical structure of hyper-accretion disks. I solve consistently the hydrodynamics, the thermodynamics and the chemical composition. The energy balance considers neutrino emission as the main cooling mechanism. Since optically thin and thick regions co-exist at a given radius, the neutrino flux must be tracked solving the radiative transfer equations. The results allow us to quantitatively discuss the jet and wind nuclear composition.

Abstract:We present the results of a joint Chandra and Suzaku spectroscopic study of the Arches cluster, the most active young massive star-forming site in the Galactic center region. Chandra studies revealed that the cluster is accompanied by hard X-ray emission extending by ~5pc (Yusef-Zadeh et al. 2001; Law & Yusef-Zadeh 2003; Wang et al. 2006). Many interpretations regarding this curious emission have been proposed, including colliding-wind plasma, reflected X-ray emission, and cold ISM ionized by accelerated particles. We obtained a high S/N spectrum of the Arches X-rays by Suzaku as well as spatially resolved spectra by Chandra. We found that the spectrum consists of a thermal plasma, a hard power-law, and two Gaussian line (FeII Ka&b) components. We found that the extended emission is of a non-thermal origin, which is responsible both for the power-law and the Gaussian line emission. We discuss two ideas to explain both components: (1) X-ray photoionization that produces fluorescence lines and the Thomson continuum and (2) non-thermal electron impact ionization of Fe and bremsstrahlung continuum. But whichever scenario is adopted, the photon or particle flux from the Arches cluster is too low to account for the observed properties (Tsujimoto et al. 2006 PASJ, in press.

Numerical Studies of GRB Afterglows.

Abstract:Afterglows of gamma-ray bursts are believed to arise from external shocks. The modeling of GRB afterglows is usually carried out by analytical approaches based upon simplification of the dynamics of relativistic jets. However, the evolution of GRB jets is a multi-dimensional problem which can be solved only by direct numerical hydrodynamical simulations. We have performed high-resolution simulations of relativistic GRB jets propagating through interstellar medium using the RAM code. We have calculated multi-wavelength lightcurves based on the hydro simulations and the standard external shock model. In this talk, I will present the results of our calculations. In particular, I will discuss the jet break in the lightcurves, and the use of late time afterglows to estimate the energetics of GRBs.

Last modified: 08/24/11