Showing papers by "Roger Blandford published in 2003"

The Cosmic Lens All-Sky Survey - I. Source selection and observations

Abstract: The Cosmic Lens All-Sky Survey (CLASS) is an international collaborative program which has obtained high-resolution radio images of over 10000 flat-spectrum radio sources in order to create the largest and best studied statistical sample of radioloud gravitationally lensed systems. With this survey, combined with detailed studies of the lenses found therein, constraints can be placed on the expansion rate, matter density, and dark energy (e.g. cosmological constant, quintessence) content of the Universe that are complementary to and independent of those obtained through other methods. CLASS is aimed at identifying lenses where multiple images are formed from compact flat-spectrum radio sources, which should be easily identifiable in the radio maps. Because CLASS is radio-based, dust obscuration in lensing galaxies is not a factor, and the relative insensitivity of the instrument to environmental conditions (e.g. weather, “seeing”) leads to nearly uniform sensitivity and resolution over the entire survey. In four observing “seasons” from 1994–1999, CLASS has observed 13783 radio sources with the VLA at 8.4 GHz in its largest “A” configuration (0. ′′ 2 resolution). When combined with the JVAS survey, the CLASS sample contains over 16,000 images. A complete sample of 11685 sources was observed, selected to have a flux density of at least 30 mJy in the GB6 catalogue at 4.85 GHz (spanning the declination range 0 ◦ 6 � 6 75 ◦ and |b| > 10 ◦ , excluding the galactic plane) and a spectral index � > 0.5 between the NVSS at 1.4 GHz and the GB6. A typical 30second CLASS snapshot reached an rms noise level of 0.4 mJy. So far, CLASS has found 16 new gravitational lens systems, and the JVAS/CLASS survey contains a total of 22 lenses. The follow-up of a small number of candidates using the VLA, MERLIN, the VLBA, and optical telescopes is still underway. In this paper, we present a summary of the CLASS observations, the JVAS/CLASS sample, and statistics on sub-samples of the survey. A companion paper presents the lens candidate selection and in a third paper the implications for cosmology are discussed. The source catalogues from the JVAS/CLASS project described in this paper are available from http://www.jb.man.ac.uk/research/gravlens/ .

The Cosmic Lens All-Sky Survey - II. Gravitational lens candidate selection and follow-up

Abstract: We report the final results of the search for gravitationally lensed flat-spectrum radio sources found in the combination of CLASS (Cosmic Lens All-Sky Survey) and JVAS (Jodrell Bank VLA Astrometric Survey). VLA (Very Large Array) observations of 16 503 sources have been made, resulting in the largest sample of arcsec-scale lens systems available. Contained within the 16 503 sources is a complete sample of 11 685 sources which have two-point spectral indices between 1.4 and 5 GHz flatter than −0.5, and 5-GHz flux densities 30 mJy. A subset of 8958 sources form a well-defined statistical sample suitable for analysis of the lens statistics. We describe the systematic process by which 149 candidate lensed sources were picked from the statistical sample on the basis of possessing multiple compact components in the 0.2-arcsec resolution VLA maps. Candidates were followed up with 0.05-arcsec resolution MERLIN and 0.003-arcsec VLBA observations at 5 GHz and rejected as lens systems if they failed well-defined surface brightness and/or morphological tests. To illustrate the candidate elimination process, we show examples of sources representative of particular morphologies that have been ruled out by the follow-up observations. 194 additional candidates, not in the well-defined sample, were also followed up. Maps for all the candidates can be found on the World Wide Web at http://www.jb.man.ac.uk/research/gravlens/index.html. We summarize the properties of each of the 22 gravitational lens systems in JVAS/CLASS. 12 are double-image systems, nine are four-image systems and one is a six-image system. 13 constitute a statistically well-defined sample giving a point-source lensing rate of 1:690 ± 190. The interpretation of the results in terms of the properties of the lensing galaxy population and cosmological parameters will be published elsewhere.

Polarization of prompt GRB emission: evidence for electromagnetically-dominated outflow

Abstract: Observations by the {\RHESSI} satellite of large polarization of the prompt $\gamma$-ray emission from the Gamma Ray Burst GRB021206 \citep{coburn03} imply that the magnetic field coherence scale is larger than the size of the visible emitting region, $\sim R/\Gamma$, where $R$ is the radius of the flow, $\Gamma$ is the associated Lorentz factor. Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs \citep{lyutikov02}, in which large scale, dynamically dominant, magnetic fields are present in the outflow from the very beginning, provide a natural explanation of this large reported linear polarization. We derive Stokes parameters of synchrotron emission of a relativistically moving plasma with a given magnetic field configuration and calculate the pulse averaged polarization fraction of the emission from a relativistically expanding shell carrying global toroidal magnetic field. For viewing angles larger than $1/\Gamma$ the observed patch of the emitting shell has almost homogeneous magnetic field, producing a large fractional polarization ($56% $ for a power-law energy distribution of relativistic particles $dn/d\epsilon \propto \epsilon^{-3}$). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in uniform magnetic field due to relativistic kinematic effects.

Polarization of Prompt Gamma-Ray Burst Emission: Evidence for Electromagnetically Dominated Outflow

Abstract: Observations by the RHESSI satellite of the large polarization of the prompt γ-ray emission from γ-ray burst (GRB) 021206 imply that the magnetic field coherence scale is larger than the size of the visible emitting region, ~R/Γ, where R is the radius of the flow and Γ is the associated Lorentz factor. Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs, in which large-scale, dynamically dominant, magnetic fields are present in the outflow from the very beginning, provide a natural explanation of this large reported linear polarization. We derive the Stokes parameters of the synchrotron emission of a relativistically moving plasma with a given magnetic field configuration and calculate the pulse-averaged polarization fraction of the emission from a relativistically expanding shell carrying a global toroidal magnetic field. For viewing angles larger than 1/Γ, the observed patch of the emitting shell has an almost homogeneous magnetic field, producing a large fractional polarization (56% for a power-law energy distribution of relativistic particles, dn/d -3). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in a uniform magnetic field because of relativistic kinematic effects.

Locating the Launching Region of T Tauri Winds: The Case of DG Tau

Abstract: It is widely believed that T Tauri winds are driven magnetocentrifugally from accretion disks close to the central stars The exact launching conditions are uncertain We show that a general relation exists between the poloidal and toroidal velocity components of a magnetocentrifugal wind at large distances and the rotation rate of the launching surface, independent of the uncertain launching conditions We discuss the physical basis of this relation and verify it using a set of numerically-determined large-scale wind solutions Both velocity components are in principle measurable from spatially resolved spectra, as has been done for the extended low-velocity component (LVC) of the DG Tau wind by Bacciotti et al For this particular source, we infer that the spatially resolved LVC originates from a region on the disk extending from $\sim 03$ to $\sim 40\AU$ from the star, which is consistent with, and a refinement over, the previous rough estimate of Bacciotti et al

Locating the Launching Region of T Tauri Winds: The Case of DG Tauri

Abstract: It is widely believed that T Tauri winds are driven magnetocentrifugally from accretion disks close to the central stars. The exact launching conditions are uncertain. We show that a general relation exists between the poloidal and toroidal velocity components of a magnetocentrifugal wind at large distances and the rotation rate of the launching surface, independent of the uncertain launching conditions. We discuss the physical basis of this relation and verify it by using a set of numerically determined large-scale wind solutions. Both velocity components are in principle measurable from spatially resolved spectra, as has been done for the extended low-velocity component (LVC) of the DG Tauri wind by Bacciotti et al. For this particular source, we infer that the spatially resolved LVC originates from a region on the disk extending from ~0.3 to ~4.0 AU from the star, which is consistent with, and a refinement over, the rough estimate of Bacciotti et al.

The Hubble Constant from the Gravitational Lens B1608+656

Abstract: We present a refined gravitational lens model of the four-image lens system B1608+656 based on new and improved observational constraints: (1) the three independent time delays and flux ratios from Very Large Array observations, (2) the radio-image positions from Very Large Baseline Array observations, (3) the shape of the deconvolved Einstein ring from optical and infrared Hubble Space Telescope images, (4) the extinction-corrected lens-galaxy centroids and structural parameters, and (5) a stellar velocity dispersion, σap = 247 ± 35 km s-1, of the primary lens galaxy (G1), obtained from an echelle spectrum taken with the Keck II Telescope. The lens-mass model consists of two elliptical mass distributions with power-law density profiles and an external shear, totaling 22 free parameters, including the density slopes that are the key parameters for determining the value of H0 from lens time delays. This has required the development of a new lens code that is highly optimized for speed. The minimum-χ2 model reproduces all observations very well, including the stellar velocity dispersion and the shape of the Einstein ring. A combined gravitational lens and stellar dynamical analysis leads to a value of the Hubble constant of H0 = 75 km s- 1 Mpc -1 (68% CL; Ωm = 0.3, ΩΛ = 0.7). The nonlinear error analysis includes correlations between all free parameters, in particular the density slopes of G1 and G2, yielding an accurate determination of the random error on H0. The lens galaxy G1 is ~5 times more massive than the secondary lens galaxy (G2) and has a mass density slope of γ = 2.03 ± 0.03 (68% CL) for ρ ∝ r, very close to isothermal (γ' = 2). After extinction correction, G1 exhibits a smooth surface brightness distribution with an R1/4 profile and no apparent evidence for tidal disruption by interactions with G2. Given the scope of the observational constraints and the gravitational lens models, as well as the careful corrections to the data, we believe this value of H0 to be little affected by known systematic errors (5%).

Two Dimensional Adiabatic Flows onto a Black Hole: I. Fluid Accretion

Abstract: When gas accretes onto a black hole, at a rate either much less than or much greater than the Eddington rate, it is likely to do so in an "adiabatic" or radiatively inefficient manner. Under fluid (as opposed to MHD) conditions, the disk should become convective and evolve toward a state of marginal instability. The resulting disk structure is "gyrentropic," with convection proceeding along common surfaces of constant angular momentum, Bernoulli function and entropy, called "gyrentropes." We present a family of two-dimensional, self-similar models which describes the time-averaged disk structure. We then suppose that there is a self-similar, Newtonian torque and that the Prandtl number is large. This torque drives inflow and meridional circulation and the resulting flow is computed. Convective transport will become ineffectual near the disk surface. It is conjectured that this will lead to a large increase of entropy across a "thermal front" which we identify as the effective disk surface and the base of an outflow. The conservation of mass, momentum and energy across this thermal front permits a matching of the disk models to self-similar outflow solutions. We then demonstrate that self-similar disk solutions can be matched smoothly onto relativistic flows at small radius and thin disks at large radius. This model of adiabatic accretion is contrasted with some alternative models that have been discussed recently. The disk models developed in this paper should be useful for interpreting numerical, fluid dynamical simulations. Related principles to those described here may govern the behaviour of astrophysically relevant, magnetohydrodynamic disk models.

Gamma ray bursts as electromagnetic outflows

Abstract: (Abridged) We interpret gamma ray bursts as relativistic, electromagnetic explosions. Specifically, we propose that they are created when a rotating, relativistic, stellar-mass progenitor loses much of its rotational energy in the form of a Poynting flux during an active period lasting $\sim 100$ s. Initially, a non-spherically symmetric, electromagnetically-dominated bubble expands non-relativistically inside the star, most rapidly along the rotational axis of the progenitor. After the bubble breaks out from the stellar surface and most of the electron-positron pairs annihilate, the bubble expansion becomes highly relativistic. After the end of the source activity most of the electromagnetic energy is concentrated in a thin shell inside the contact discontinuity between the ejecta and the shocked circumstellar material. This electromagnetic shell pushes a relativistic blast wave into the circumstellar medium. Current-driven instabilities develop in this shell at a radius $\sim3\times10^{16}$ cm and lead to dissipation of magnetic field and acceleration of pairs which are responsible for the $\gamma$-ray burst. At larger radii, the energy contained in the electromagnetic shell is mostly transferred to the preceding blast wave. Particles accelerated at the forward shock may combine with electromagnetic field from the electromagnetic shell to produce the afterglow emission.

Magnetocentrifugal Launching of Jets from Accretion Disks. II. Inner Disk-driven Winds

Abstract: We numerically follow the time evolution of axisymmetric outflows magnetocentrifugally driven from the inner portion of accretion disks from their launching surface to large observable distances. Special attention is paid to the collimation of part of the outflow into a dense, narrow jet around the rotation axis after a steady state has been reached. For parameters typical of T Tauri stars, we define a fiducial jet as that outlined by the contour of constant density at 104 cm-3. We find that the jet, so defined, appears nearly cylindrical well above the disk, in agreement with previous asymptotic analyses. Closer to the equatorial plane, the density contour can either bulge outward or pinch inward, depending on the conditions at the launching surface, particularly the mass flux distribution. We find that even though a dense, jetlike feature is always formed around the axis, there is no guarantee that the high-density axial jet would dominate the more tenuous, wide-angle part of the wind. Specifically, on the 100 AU scale, resolvable by the Hubble Space Telescope and ground-based adaptive optics for nearby T Tauri winds, the fraction of the wind mass flux enclosed by the fiducial jet can vary substantially, again depending on the launching conditions. We show two examples in which the fraction is ~20% and ~45%. These dependences may provide a way to constrain the conditions at the launching surface, which are poorly known at present.

Covariant magnetoionic theory — I. Ray propagation

Abstract: Accretion on to compact objects plays a central role in high-energy astrophysics. In these environments, both general relativistic and plasma effects may have a significant impact upon the propagation of photons. We present a fully general relativistic magnetoionic theory, capable of tracing rays in the geometrical optics approximation through a magnetized plasma in the vicinity of a compact object. We consider both the cold and warm, ion and pair plasmas. When plasma effects become large the two plasma eigenmodes follow different ray trajectories, resulting in a large observable polarization. This has implications for accreting systems ranging from pulsars and X-ray binaries to active galactic nuclei.

Magneto-Centrifugal Launching of Jets from Accretion Disks. II: Inner Disk-Driven Winds

Abstract: We follow numerically the time evolution of axisymmetric outflows driven magneto-centrifugally from the inner portion of accretion disks, from their launching surface to large, observable distances. Special attention is paid to the collimation of part of the outflow into a dense, narrow jet around the rotation axis, after a steady state has been reached. For parameters typical of T Tauri stars, we define a fiducial ``jet'' as outlined by the contour of constant density at 10^4 cm^{-3}. We find that the jet, so defined, appears nearly cylindrical well above the disk, in agreement with previous asymptotic analyses. Closer to the equatorial plane, the density contour can either bulge outwards or pinch inwards, depending on the conditions at the launching surface, particularly the mass flux distribution. We find that even though a dense, jet-like feature is always formed around the axis, there is no guarantee that the high-density axial jet would dominate the more tenuous, wide-angle part of the wind. Specifically, on the 100 AU scale, resolvable by HST and ground-based adaptive optics for nearby T Tauri winds, the fraction of the wind mass flux enclosed by the fiducial jet can vary substantially, again depending on the launching conditions. We show two examples in which the fraction is ~20% and ~45%. These dependences may provide a way to constrain the conditions at the launching surface, which are poorly known at present.

Extrinsic radio variability of JVAS/CLASS gravitational lenses

Abstract: We present flux ratio curves of the fold and cusp (i.e., close multiple) images of six Jodrell Bank VLA Astrometric Survey and Cosmic Lens All-Sky Survey (JVAS/CLASS) gravitational lens systems. The data were obtained over a period of 8.5 months in 2001 with the Multi Element Radio- Linked Interferometer Network ( MERLIN) at 5 GHz with 50 mas resolution, as part of a MERLIN Key Project. Even though the time delays between the fold and cusp images are small (less than or similar to1 day) compared to the timescale of intrinsic source variability, all six lens systems show evidence that suggests the presence of extrinsic variability. In particular, the cusp images of B2045+ 265 - regarded as the strongest case of the violation of the cusp relation (i.e., the sum of the magnifications of the three cusp images add to zero) - show extrinsic variations in their flux ratios up to similar to40% peak to peak on timescales of several months. Its low Galactic latitude of b approximate to - 10degrees and a line of sight toward the Cygnus superbubble region suggest that Galactic scintillation is the most likely cause. The cusp images of B1422+ 231 at b approximate to +69degrees do not show strong extrinsic variability. Galactic scintillation can therefore cause significant scatter in the cusp and fold relations of some radio lens systems ( up to 10% rms), even though these relations remain violated when averaged over a less than or similar to 1 yr time baseline.

The Redshift of the Lensed Object in the Einstein Ring B0218+357

Abstract: We present a secure redshift of z = 0944 ± 0002 for the lensed object in the Einstein ring gravitational lens B0218+357 based on five broad emission lines, in good agreement with our preliminary value announced several years ago based solely on the detection of a single emission line

A variational formalism for tidal excitation: non-rotating, homentropic stars

Abstract: We present a variational formalism for describing the dynamical evolution of an oscillating star with a point-mass companion in the linear, non-relativistic regime. This includes both the excitation of normal modes and the back-reaction of the modes on the orbit. The general formalism for arbitrary fluid configurations is presented, and then specialized to a homentropic potential flow. Our formalism explicitly identifies and conserves both energy and angular momentum. We also consider corrections to the orbit up to 7/2 post-Newtonian order.

B0850+054: a new gravitational lens system from CLASS

Abstract: We report the discovery of a new gravitational lens system from the CLASS survey. Radio observations with the VLA, WSRT and MERLIN show that the radio source B0850+054 is composed of two compact components with identical spectra, a separation of 0.7 arcsec and a flux density ratio of 6 : 1. VLBA observations at 5 GHz reveal structures that are consistent with the gravitational lens hypothesis. The brighter of the two images is resolved into a linear string of at least six subcomponents, whilst the weaker image is radially stretched towards the lens galaxy. UKIRT K -band imaging detects an 18.7-mag extended object, but the resolution of the observations is not sufficient to resolve the lensed images and the lens galaxy. Mass modelling has not been possible with the present data and the acquisition of high-resolution optical data is a priority for this system.

The Gravitational Lens B1608+656. I. V-, I-, and H-Band Hubble Space Telescope Imaging

Abstract: We present a multiwavelength analysis of high-resolution observations of the quadruple lens B1608+656 from the Hubble Space Telescope archive, acquired with WFPC2 through filters F606W (V band) and F814W (I band) and with NIC1 in filter F160W (H band). In the three bands, the observations show extended emission from the four images of the source in a ringlike configuration that surrounds the two resolved, ensing galaxies. B1608+656 was discovered as a double-lobed radio source and later identified as a poststarburst galaxy in the optical. Based on photometry and optical spectroscopy we estimate that the stellar population of the source has an age of ~500 Myr. This provides a model for the spectrum of the source that extends over spectral regions where no observations are available and is used to generate Tiny Tim point-spread functions (PSFs) for the filters. Deconvolutions performed with the Lucy-Richardson method are presented, and the limitations of these restorations are discussed. V-I and I-H color maps show evidence of extinction by dust associated with one of the lensing galaxies, a late-type galaxy presumably disrupted after its close encounter with the other lens, an elliptical galaxy. The extinction affects the two lens galaxies and two of the four multiple images. The diagnostic of wavelength-dependent effects in the images shows that corrections for contamination with light from the lenses, extinction, and PSF convolution need to be applied before using the extended structure in the images as a constraint on lens models. We will present the restoration of the images in a subsequent paper.

X-ray astronomy in the new Millenium. A Summary

Abstract: Recent X-ray observations have had a major impact on topics ranging from protostars to cosmology. They have also drawn attention to important and general physical processes that currently limit our understanding of thermal and nonthermal X-ray sources. These include unmeasured atomic astrophysics data (wavelengths, oscillator strengths etc.), basic hydromagnetic processes (e.g. shock structure, reconnection), plasma processes (such as electron-ion equipartition and heat conduction) and radiative transfer (in disks and accretion columns). Progress on these problems will probably come from integrative studies that draw upon observations, throughout the electromagnetic spectrum, of different classes of source. X-ray observations are also giving a new perspective on astronomical subjects, like the nature of galactic nuclei and the evolution of stellar populations. They are contributing to answering central cosmological questions including the measurement of the matter content of the universe, understanding its overall luminosity density, describing its chemical evolution and locating the first luminous objects. X-ray astronomy has a healthy future with several international space missions under construction and in development.

Frontiers of X-ray astronomy

Abstract: Preface 1. Forty years on from Aerobee 150: a personal perspective K. Pounds 2. X-ray spectroscopy of astrophysical plasmas S. M. Kahn, E. Behar, A. Kinkhabwala and D. W. Savin 3. X-rays from stars M. Gudel 4. X-ray observations of accreting white-dwarf systems M. Cropper, G. Ramsay, C. Hellier, K. Mukai, C. Mauche and D. Pandel 5. Accretion flows in X-ray binaries C. Done 6. Recent X-ray observations of supernova remnants C. R. Canizares 7. Luminous X-ray sources in spiral and star-forming galaxies M. Ward 8. Cosmological constraints from Chandra observations of galaxy clusters S. W. Allen 9. Clusters of galaxies: a cosmological probe R. Mushotzky 10. Obscured active galactic nuclei: the hidden side of the X-ray Universe G. Matt 11. The Chandra Deep Field-North Survey and the cosmic X-ray background W. N. Brandt, D. M. Alexander, F. E. Bauer and A. E. Hornschemeier 12. Hunting the first black holes G. Hasinger 13. X-ray astronomy in the new millennium: a summary R. D. Blandford.

CLASS B0445+123: a new two-image gravitational lens system

Abstract: A new two-image gravitational lens system has been discovered as a result of the Cosmic Lens All-Sky Survey. Radio observations with the Very Large Array (VLA), the MultiElement Radio Linked Interferometer Network and the Very Long Baseline Array at increasingly higher resolutions all show two components with a flux density ratio of similar to7 : 1 and a separation of 1.34 arcsec. Both components are compact and have the same spectral index. Follow-up observations made with the VLA at 8.4 GHz show evidence of a feature to the south-east of the brighter component and a corresponding extension of the weaker component to the north-west. Optical observations with the William Herschel Telescope show similar to1.7-arcsec extended emission aligned in approximately the same direction as the separation between the radio components with an R -band magnitude of 21.8 +/- 0.4.

Proposed Next Generation GRB Mission: EXIST

Abstract: A next generation Gamma Ray Burst (GRB) mission to follow the upcoming Swift mission is described. The proposed Energetic X‐ray Imaging Survey Telescope, EXIST, would yield the limiting (practical) GRB trigger sensitivity, broad‐band spectral and temporal response, and spatial resolution over a wide field. It would provide high resolution spectra and locations for GRBs detected at GeV energies with GLAST. Together with the next generation missions Constellation‐X, NGST and LISA and optical‐survey (LSST) telescopes, EXIST would enable GRBs to be used as probes of the early universe and the first generation of stars. EXIST alone would give ∼10–50″ positions (long or short GRBs), approximate redshifts from lags, and constrain physics of jets, orphan afterglows, neutrinos and SGRs.

The Hubble Constant from the Gravitational Lens B1608+656

Abstract: We present a refined gravitational lens model of the four-image lens system B1608+656 based on new and improved observational constraints: (i) the three independent time-delays and flux-ratios from VLA observations, (ii) the radio-image positions from VLBA observations, (iii) the shape of the deconvolved Einstein Ring from optical and infrared HST images, (iv) the extinction-corrected lens-galaxy centroids and structural parameters, and (v) a stellar velocity dispersion, sigma_ap=247+-35 km/s, of the primary lens galaxy (G1), obtained from an echelle spectrum taken with the Keck--II telescope. The lens mass model consists of two elliptical mass distributions with power-law density profiles and an external shear, totaling 22 free parameters, including the density slopes which are the key parameters to determine the value of H_0 from lens time delays. This has required the development of a new lens code that is highly optimized for speed. The minimum-chi^2 model reproduces all observations very well, including the stellar velocity dispersion and the shape of the Einstein Ring. A combined gravitational-lens and stellar dynamical analysis leads to a value of the Hubble Constant of H_0=75(+7/-6) km/s/Mpc (68 percent CL; Omega_m=0.3, Omega_Lambda=0.7. The non-linear error analysis includes correlations between all free parameters, in particular the density slopes of G1 and G2, yielding an accurate determination of the random error on H_0. The lens galaxy G1 is ~5 times more massive than the secondary lens galaxy (G2), and has a mass density slope of gamma_G1=2.03(+0.14/-0.14) +- 0.03 (68 percent CL) for rho~r^-gamma', very close to isothermal (gamma'=2). (Abridged)

General Relativistic Magnetoionic Theory

Abstract: We have developed methods for tracing rays and performing radiative transfer through a magneto active plasma in a general relativistic environment. The two electromagnetic plasma modes propagate differently due to a combination of dispersive and gravitational effects. We have found that, when given an appropriate environment surrounding the central black hole, it is indeed possible to generate a significant degree of circular polarisation without an appreciable amount of linear polarisation due to these effects alone.

The Phenomena of High Energy Astrophysics

Abstract: A brief summary of some highlights in the study of high energy astrophysical sources over the past decade is presented. It is argued that the great progress that has been made derives largely from the application of new technology to observation throughout all of the electromagnetic and other spectra and that, on this basis, the next decade should be even more exciting. However, it is imperative to observe cosmic sources throughout these spectra in order to obtain a full understanding of their properties. In addition, it is necessary to learn the universal laws that govern the macroscopic and the microscopic behavior of cosmic plasma over a great range of physical conditions by combining observations of different classes of source. These two injunctions are illustrated by discussions of cosmology, hot gas, supernova remnants and explosions, neutron stars, black holes and ultrarelativistic outflows. New interpreations of the acceleration of Galactic cosmic rays, the cooling of hot gas in rich clusters and the nature of ultrarelativistic outflows are outlined. The new frontiers of VHE $\gamma$-ray astronomy, low frequency radio astronomy, neutrino astronomy, UHE cosmic ray physics and gravitational wave astronomy are especially promising.

The Phenomena of High Energy Astrophysics

Abstract: A brief summary of some highlights in the study of high energy astrophysical sources over the past decade is presented. It is argued that the great progress that has been made derives largely from the application of new technology to observation throughout all of the electromagnetic and other spectra and that, on this basis, the next decade should be even more exciting. However, it is imperative to observe cosmic sources throughout these spectra in order to obtain a full understanding of their properties. In addition, it is necessary to learn the universal laws that govern the macroscopic and the microscopic behavior of cosmic plasma over a great range of physical conditions by combining observations of different classes of source. These two injunctions are illustrated by discussions of cosmology, hot gas, supernova remnants and explosions, neutron stars, black holes and ultrarelativistic outflows. New interpreations of the acceleration of Galactic cosmic rays, the cooling of hot gas in rich clusters and the nature of ultrarelativistic outflows are outlined. The new frontiers of VHE γ-ray astronomy, low frequency radio astronomy, neutrino astronomy, UHE cosmic ray physics and gravitational wave astronomy are especially promising.

Electromagnetic Models of Compact Radio Sources and Circular Polarisation

Abstract: An electromagnetic model of relativistic jets is outlined. It is suggested that these jets be interpreted as current flows and that they owe their persistence and collimation to the pinching action of the magnetic field. Such structures are unstable and it is suggested that the nonlinear development of these instabilities involves the formation of an electromagnetic turbulence spectrum. This turbulence may be responsible for the acceleration of relativistic electrons and positrons. It may also provide the electromagnetic field in which these electrons and positrons radiate. Some mechanisms through which circular polarisation may be created in this environment are outlined.

2- and 3-D simulations of magnetocentrifugal disk-winds: Acceleration and stability

Abstract: Protostellar jets and winds are probably driven magnetocentrifugally from the surface of accretion disks close to the central stellar objects. The exact launching conditions on the disk, such as the distributions of magnetic flux and mass ejection rate, are poorly known. They could be constrained from observations at large distances, provided that a robust model is available to link the observable properties of the jets and winds at the large distances to the conditions at the base of the flow. We describe a set of 2D axisymmetric simulations that are able to follow the acceleration and propagation of the wind from the disk surface to arbitrarily large distances. After a typical 2D flow reaches the steady state, we impose on it nonaxisymmetric perturbations and follow numerically its 3D evolution. We find that the wind reverts quickly to its initial axisymmetric state, with no indication of rapid growth of instabilities leading to flow disruption. Our calculations strengthen the case for the magnetocentrifugal jet and wind launching.

Covariant Magnetoionic Theory I: Ray Propagation

Abstract: Accretion onto compact objects plays a central role in high energy astrophysics. In these environments, both general relativistic and plasma effects may have significant impacts upon the propagation of photons. We present a full general relativistic magnetoionic theory, capable of tracing rays in the geometric optics approximation through a magnetised plasma in the vicinity of a compact object. We consider both the cold and warm, ion and pair plasmas. When plasma effects become large the two plasma eignemodes follow different ray trajectories resulting in a large observable polarisation. This has implications for accreting systems ranging from pulsars and X-ray binaries to AGN.

Covariant Magnetoionic Theory II: Radiative Transfer

Abstract: Accretion onto compact objects plays a central role in high energy astrophysics. In these environments, both general relativistic and plasma effects may have significant impacts upon the spectral and polarimetric properties of the accretion flow. In paper I we presented a fully general relativistic magnetoionic theory, capable of tracing rays in the geometric optics approximation through a magnetised plasma in the vicinity of a compact object. In this paper we discuss how to perform polarised radiative transfer along these rays. In addition we apply the formalism to a barotropic thick disk model, appropriate for low luminosity active galactic nuclei. We find that it is possible to generate large fractional polarisations over the innermost portions of the accretion flow, even when the emission mechanism is unpolarised. This has implications for accreting systems ranging from pulsars and X-ray binaries to AGN.

The Supernova / Acceleration Probe (SNAP) Mission

Abstract: The Supernova / Acceleration Probe (SNAP) is a proposed space-based experiment designed to measure the expansion history of the Universe, motivated by the discovery that the expansion is accelerating. It will study both the dark energy and the dark matter, through mapping the distanceredshift relation of Type Ia supernovae and through a wide-area weak gravitational lensing survey. A 2-m three mirror anastigmat wide-field telescope feeds a focal plane consisting of a 0.7 squaredegree imager tiled with equal areas of optical CCD’s and near infrared sensors, and a highefficiency low-resolution integral field spectrograph. The instrumentation suite provides simultaneous discovery and light-curve measurements for many supernovae with the capability to target individual objects for detailed spectral characterization. The SNAP mission can obtain highsignal-to-noise calibrated light-curves and spectra for over 2000 Type Ia supernovae at redshifts between z = 0.1 and 1.7. The resulting data set can not only determine the amount of dark energy with high precision, but test the nature of the dark energy by examining its equation of state. In particular, dark energy due to a cosmological constant or various classes of dynamical scalar fields can be differentiated, by measuring the dark energy’s equation of state density-to-pressure ratio to an accuracy of ± 0.05, and its time evolution to w' = dw/dz to ± 0.2. Although the survey strategy is tailored for supernova and weak gravitational lensing observations, the large survey area, depth, spatial resolution, time-sampling, and infrared extent of the resulting data will support a broad range of auxiliary science programs.