Search

The high-frequency peaked BL Lac object PKS 2005-489 was the target of a multi-wavelength campaign with simultaneous observations in the TeV gamma-ray (H.E.S.S.), GeV gamma-ray (Fermi/LAT), X-ray (RXTE, Swift), UV (Swift) and optical (ATOM, Swift) bands. This campaign was carried out during a high flux state in the synchrotron regime. The flux in the optical and X-ray bands reached the level of the historical maxima. The hard GeV spectrum observed with Fermi/LAT connects well to the very high energy (VHE, E>100GeV) spectrum measured with H.E.S.S. with a peak energy between ~5 and 500 GeV. Compared to observations with contemporaneous coverage in the VHE and X-ray bands in 2004, the X-ray flux was ~50 times higher during the 2009 campaign while the TeV gamma-ray flux shows marginal variation over the years. The spectral energy distribution during this multi-wavelength campaign was fit by a one zone synchrotron self-Compton model with a well determined cutoff in X-rays. The parameters of a one zone SSC model are inconsistent with variability time scales. The variability behaviour over years with the large changes in synchrotron emission and small changes in the inverse Compton emission does not warrant an interpretation within a one-zone SSC model despite an apparently satisfying fit to the broadband data in 2009., Comment: published in A&A, 9pages, 4figures. Due to some style difficulties using the standard style file with the long author list, the abstract has been included manually. A version with the correct style can be found in the website http://www.lsw.uni-heidelberg.de/users/skaufman/publications/aa16170-10published.pdf

We measured separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Because the instrument does not have an onboard magnet, we distinguish the two species by exploiting the Earth's shadow, which is offset in opposite directions for opposite charges due to the Earth's magnetic field. We estimate and subtract the cosmic-ray proton background using two different methods that produce consistent results. We report the electron-only spectrum, the positron-only spectrum, and the positron fraction between 20 GeV and 200 GeV. We confirm that the fraction rises with energy in the 20-100 GeV range. The three new spectral points between 100 and 200 GeV are consistent with a fraction that is continuing to rise with energy., Comment: 5 figures, 1 table, revtex 4.1, updated to match PRL published version

Satellite galaxies of the Milky Way are among the most promising targets for dark matter searches in gamma rays. We present a search for dark matter consisting of weakly interacting massive particles, applying a joint likelihood analysis to 10 satellite galaxies with 24 months of data of the Fermi Large Area Telescope. No dark matter signal is detected. Including the uncertainty in the dark matter distribution, robust upper limits are placed on dark matter annihilation cross sections. The 95% confidence level upper limits range from about 1e-26 cm^3 s^-1 at 5 GeV to about 5e-23 cm^3 s^-1 at 1 TeV, depending on the dark matter annihilation final state. For the first time, using gamma rays, we are able to rule out models with the most generic cross section (~3e-26 cm^3 s^-1 for a purely s-wave cross section), without assuming additional boost factors., Comment: 6 pages, 2 figures; Contact authors: Johann Cohen-Tanugi, Jan Conrad, and Maja Llena Garde

We present a detailed statistical analysis of the correlation between radio and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi during its first year of operation, with the largest datasets ever used for this purpose. We use both archival interferometric 8.4 GHz data (from the VLA and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of 199 objects). Our unprecedentedly large sample permits us to assess with high accuracy the statistical significance of the correlation, using a surrogate-data method designed to simultaneously account for common-distance bias and the effect of a limited dynamical range in the observed quantities. We find that the statistical significance of a positive correlation between the cm radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the whole AGN sample, with a probability <1e-7 for the correlation appearing by chance. Using the OVRO data, we find that concurrent data improve the significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size allows us to study the dependence of correlation strength and significance on specific source types and gamma-ray energy band. We find that the correlation is very significant (chance probability <1e-7) for both FSRQs and BL Lacs separately; a dependence of the correlation strength on the considered gamma-ray energy band is also present, but additional data will be necessary to constrain its significance., Comment: Accepted for publications by ApJ. Contact authors: M. Giroletti, V. Pavlidou, A. Reimer

We investigate the Fermi LAT gamma-ray and 15 GHz VLBA radio properties of a joint gamma-ray- and radio-selected sample of AGNs obtained during the first 11 months of the Fermi mission (2008 Aug 4 - 2009 Jul 5). Our sample contains the brightest 173 AGNs in these bands above declination -30 deg. during this period, and thus probes the full range of gamma-ray loudness (gamma-ray to radio band luminosity ratio) in the bright blazar population. The latter quantity spans at least four orders of magnitude, reflecting a wide range of spectral energy distribution (SED) parameters in the bright blazar population. The BL Lac objects, however, display a linear correlation of increasing gamma-ray loudness with synchrotron SED peak frequency, suggesting a universal SED shape for objects of this class. The synchrotron self-Compton model is favored for the gamma-ray emission in these BL Lacs over external seed photon models, since the latter predict a dependence of Compton dominance on Doppler factor that would destroy any observed synchrotron SED peak - gamma-ray loudness correlation. The high-synchrotron peaked (HSP) BL Lac objects are distinguished by lower than average radio core brightness temperatures, and none display large radio modulation indices or high linear core polarization levels. No equivalent trends are seen for the flat-spectrum radio quasars (FSRQ) in our sample. Given the association of such properties with relativistic beaming, we suggest that the HSP BL Lacs have generally lower Doppler factors than the lower-synchrotron peaked BL Lacs or FSRQs in our sample., Comment: 22 pages, 11 figures, 4 tables; accepted for publication in the Astrophysical Journal

The high-frequency-peaked BL Lacertae object RGB J0710+591 was observed in the very high-energy (VHE; E > 100 GeV) wave band by the VERITAS array of atmospheric Cherenkov telescopes. The observations, taken between 2008 December and 2009 March and totaling 22.1 hr, yield the discovery of VHE gamma rays from the source. RGB J0710+591 is detected at a statistical significance of 5.5 standard deviations (5.5{\sigma}) above the background, corresponding to an integral flux of (3.9 +/- 0.8) x 10-12 cm-2 s-1 (3% of the Crab Nebula's flux) above 300 GeV. The observed spectrum can be fit by a power law from 0.31 to 4.6 TeV with a photon spectral index of 2.69 +/- 0.26stat +/- 0.20sys. These data are complemented by contemporaneous multiwavelength data from the Fermi Large Area Telescope, the Swift X-ray Telescope, the Swift Ultra-Violet and Optical Telescope, and the Michigan-Dartmouth-MIT observatory. Modeling the broadband spectral energy distribution (SED) with an equilibrium synchrotron self-Compton model yields a good statistical fit to the data. The addition of an external-Compton component to the model does not improve the fit nor brings the system closer to equipartition. The combined Fermi and VERITAS data constrain the properties of the high-energy emission component of the source over 4 orders of magnitude and give measurements of the rising and falling sections of the SED.

The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. We use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilating into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly affected by the underlying distribution of dark matter, and by using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe's transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersymmetry models) annihilating predominantly into quarks. For the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals., Comment: 29 pages, 7 figures, to be published in JCAP. Corresponding authors: Jan Conrad, Michael Gustafsson, Alexander Sellerholm, Gabrijela Zaharijas

(Abridged) We have conducted a detailed investigation of the broad-band spectral properties of the \gamma-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi gamma-ray spectra with Swift, radio, infra-red, optical and other hard X-ray/gamma-ray data, collected within three months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous Spectral Energy Distributions (SED) for 48 LBAS blazars.The SED of these gamma-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual Log $\nu $ - Log $\nu$ F$_\nu$ representation, the typical broad-band spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SEDs to characterize the peak intensity of both the low and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broad-band colors (i.e. the radio to optical and optical to X-ray spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency $\nu_p^S$ is positioned between 10$^{12.5}$ and 10$^{14.5}$ Hz in broad-lined FSRQs and between $10^{13}$ and $10^{17}$ Hz in featureless BL Lacertae objects.We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron - inverse Compton scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton (SSC) models cannot explain most of our SEDs, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. (...), Comment: 85 pages, 38 figures, submitted to ApJ

Strong gravitational lens systems with measured time delays between the multiple images provide a method for measuring the "time-delay distance" to the lens, and thus the Hubble constant. We present a Bayesian analysis of the strong gravitational lens system B1608+656, incorporating (i) new, deep Hubble Space Telescope (HST) observations, (ii) a new velocity dispersion measurement of 260+/-15 km/s for the primary lens galaxy, and (iii) an updated study of the lens' environment. When modeling the stellar dynamics of the primary lens galaxy, the lensing effect, and the environment of the lens, we explicitly include the total mass distribution profile logarithmic slope gamma' and the external convergence kappa_ext; we marginalize over these parameters, assigning well-motivated priors for them, and so turn the major systematic errors into statistical ones. The HST images provide one such prior, constraining the lens mass density profile logarithmic slope to be gamma'=2.08+/-0.03; a combination of numerical simulations and photometric observations of the B1608+656 field provides an estimate of the prior for kappa_ext: 0.10 +0.08/-0.05. This latter distribution dominates the final uncertainty on H_0. Compared with previous work on this system, the new data provide an increase in precision of more than a factor of two. In combination with the WMAP 5-year data set, we find that the B1608+656 data set constrains the curvature parameter to be -0.031 < Omega_k < 0.009 (95% CL), a level of precision comparable to that afforded by the current Type Ia SNe sample. Asserting a flat spatial geometry, we find that, in combination with WMAP, H_0 = 69.7 +4.9/-5.0 km/s/Mpc and w=-0.94 +0.17/-0.19 (68% CL), suggesting that the observations of B1608+656 constrain w as tightly as do the current Baryon Acoustic Oscillation data. (abridged), Comment: 24 pages, 8 figures, revisions based on referee's comments, accepted for publication in ApJ

Strong gravitational lensing is a powerful technique for probing galaxy mass distributions and for measuring cosmological parameters. We present a pixelated approach to modeling simultaneously the lens potential and source intensity of strong gravitational lens systems with extended source-intensity distributions. For systems with sources of sufficient extent such that the separate lensed images are connected by intensity measurements, the accuracy in the reconstructed potential is solely limited by the quality of the data. We apply this potential reconstruction technique to deep HST observations of B1608+656, a four-image gravitational lens system formed by a pair of interacting lens galaxies. We present a comprehensive Bayesian analysis of the system that takes into account the extended source-intensity distribution, dust extinction, and the interacting lens galaxies. Our approach allows us to compare various models of the components of the lens system, which include the point-spread function (PSF), dust, lens galaxy light, source-intensity distribution, and lens potential. Using optimal combinations of the PSF, dust, and lens galaxy light models, we successfully reconstruct both the lens potential and the extended source-intensity distribution of B1608+656. The resulting reconstruction can be used as the basis of a measurement of the Hubble constant. We use our reconstruction of the gravitational potential to study the relative distribution of mass and light in the lensing galaxies. We find that the mass-to-light ratio for the primary lens galaxy is (2.0+/-0.2)h M_{\sun} L_{B,\sun}^{-1} within the Einstein radius 3.9 h^{-1} kpc, in agreement with what is found for noninteracting lens galaxies at the same scales. (Abridged), Comment: 25 pages, 11 figures, added references and revised text to match published version in ApJ

We examine the radio properties of EGRET-detected blazars observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly an order of magnitude below the MOJAVE survey and most other samples that have been used to study the properties of EGRET blazars. At lower flux levels, radio flux density does not directly correlate with gamma-ray flux density. We do find that the EGRET-detected blazars tend to have higher brightness temperatures, greater core fractions, and possibly larger than average jet opening angles. A weak correlation is also found with jet length and with polarization. All of the well-established trends can be explained by systematically larger Doppler factors in the gamma-ray loud blazars, consistent with the measurements of higher apparent velocities found in monitoring programs carried out at radio frequencies above 10 GHz., Comment: 20 pages, 7 figures, accepted to ApJ

The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5 degrees wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for discovery of new objects. Supernova imaging observations are being acquired between 1 September and 30 November of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation., Comment: Submitted to The Astronomical Journal (24 pages, 10 figures)

The existence of flux-ratio anomalies between fold and cusp images in galaxy-scale strong-lens systems has led to an interpretation based on the presence of a high mass-fraction of cold-dark-matter (CDM) substructures around galaxies, as predicted by numerical N-body simulations. The flux-ratio anomaly is particularly evident in the radio-loud quadruple gravitational lens system CLASS B2045+265. In this paper, new high-resolution radio, optical, and infrared imaging of B2045+265 is presented which sheds more light on this anomaly and its possible causes. First, deep Very Long Baseline Array observations show very compact images, possibly with a hint of a jet, but with no evidence for differential scattering or scatter broadening. Second, optical and infrared observations with the Hubble Space Telescope and through Adaptive-Optics imaging with the W. M. Keck Telescope, show a previously undiscovered object -- interpreted as a (tidally disrupted) dwarf satellite based on its colours and slight extension -- between the main lens galaxy and the three anomalous flux-ratio images. Third, colour variations in the early-type lens galaxy indicate recent star-formation, possibly the result of secondary infall of gas-rich satellites. A population of such galaxies around the lens system could explain the previously discovered strong [O II] emission. However, spiral structure and/or normal star formation in the lens galaxy cannot be excluded. In light of these new data, we propose a lens model for the system, including the observed dwarf satellite, which reproduces all positional and flux-ratio constraints, without the need for additional CDM substructure. [abridged], Comment: 10 pages, 6 figures, accepted for publication in MNRAS

We report the serendipitous discovery of two strong gravitational lens candidates (ACS J160919+6532 and ACS J160910+6532) in deep images obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, each less than 40 arcsec from the previously known gravitational lens system CLASS B1608+656. The redshifts of both lens galaxies have been measured with Keck and Gemini: one is a member of a small galaxy group at z~0.63, which also includes the lensing galaxy in the B1608+656 system, and the second is a member of a foreground group at z~0.43. By measuring the effective radii and surface brightnesses of the two lens galaxies, we infer their velocity dispersions based on the passively evolving Fundamental Plane (FP) relation. Elliptical isothermal lens mass models are able to explain their image configurations within the lens hypothesis, with a velocity dispersion compatible with that estimated from the FP for a reasonable source-redshift range. Based on the large number of massive early-type galaxies in the field and the number-density of faint blue galaxies, the presence of two additional lens systems around CLASS B1608+656 is not unlikely in hindsight. Gravitational lens galaxies are predominantly early-type galaxies, which are clustered, and the lensed quasar host galaxies are also clustered. Therefore, obtaining deep high-resolution images of the fields around known strong lens systems is an excellent method of enhancing the probability of finding additional strong gravitational lens systems., Comment: Submitted to ApJ. 8 pages, 6 figures

Strong gravitational lens systems with extended sources are of special interest because they provide additional constraints on the models of the lens systems. To use a gravitational lens system for measuring the Hubble constant, one would need to determine the lens potential and the source intensity distribution simultaneously. A linear inversion method to reconstruct a pixellated source brightness distribution of a given lens potential model was introduced by Warren & Dye. In the inversion process, a regularisation on the source intensity is often needed to ensure a successful inversion with a faithful resulting source. In this paper, we use Bayesian analysis to determine the optimal regularisation constant (strength of regularisation) of a given form of regularisation and to objectively choose the optimal form of regularisation given a selection of regularisations. We consider and compare quantitatively three different forms of regularisation previously described in the literature for source inversions in gravitational lensing: zeroth-order, gradient and curvature. We use simulated data with the exact lens potential to demonstrate the method. We find that the preferred form of regularisation depends on the nature of the source distribution., Comment: 18 pages, 10 figures; Revisions based on referee's comments after initial submission to MNRAS

The key to using a strong gravitational lens system to measure the Hubble constant is to obtain an accurate model of the lens potential. In this paper, we investigate the properties of gravitational lens B1608+656, a quadruply-imaged lens system with an extended source intensity distribution. Our analysis is valid for generic quadruply-lensed systems. Limit curves and isophotal separatrices are defined for such systems, and we show that the isophotal separatrices must intersect at the critical curves and the satellite isophotes must be tangent to the limit curves. The current model of B1608+656 (Koopmans et al. 2003) satisfies these criteria for some, but not all, of the isophotal separatrices within the observational uncertainty. We study a non-parametric method of potential reconstruction proposed by Blandford, Surpi & Kundic (2001) and demonstrate that although the method works in principle and elucidates image formation, the initial potential only converges to the true model when it is within ~ 1 percent of the true model., Comment: 12 pages, 12 figures. Minor revisions based on referee's comments after initial submission to MNRAS

We report the discovery of a new gravitational lens system from the CLASS survey, CLASS B0631+519. VLA, MERLIN and VLBA observations show a doubly-imaged radio core, a doubly-imaged lobe and a second lobe that is probably quadruply-imaged. The maximum image separation is 1.16 arcseconds. The VLBA resolves the most magnified image of the flat-spectrum radio core into a number of sub-components spread across approximately 20 milli-arcseconds. Optical and near-infrared imaging with the ACS and NICMOS cameras on the HST show that there are two galaxies along the line of sight to the lensed source, as was previously discovered by optical spectroscopy. The nearer galaxy at z=0.0896 is a small blue irregular, while the more distant galaxy at z=0.6196 is an elliptical type and appears to contribute most of the lensing effect. The host galaxy of the lensed source is visible in NICMOS imaging as a set of arcs that form an almost complete Einstein ring. Mass modelling using non-parametric techniques can reproduce the ring and indicates that the irregular galaxy has a (localised) effect on the flux density distribution in the Einstein ring at the 5-10% level., Comment: 15 pages, 13 figures, accepted for publication in MNRAS

We present observations of CLASS B2108+213, the widest separation gravitational lens system discovered by the Cosmic Lens All-Sky Survey. Radio imaging using the VLA at 8.46 GHz and MERLIN at 5 GHz shows two compact components separated by 4.56 arcsec with a faint third component in between which we believe is emission from a lensing galaxy. 5-GHz VLBA observations reveal milliarcsecond-scale structure in the two lensed images that is consistent with gravitational lensing. Optical emission from the two lensed images and two lensing galaxies within the Einstein radius is detected in Hubble Space Telescope imaging. Furthermore, an optical gravitational arc, associated with the strongest lensed component, has been detected. Surrounding the system are a number of faint galaxies which may help explain the wide image separation. A plausible mass distribution model for CLASS B2108+213 is also presented., Comment: 9 pages, 8 figures, accepted for publication in MNRAS

Our fundamental lack of understanding of the acceleration of the Universe suggests that we consider a kinematic description. The simplest formalism involves the third derivative of the scale factor through a jerk parameter. A new approach is presented for describing the results of astronomical observations in terms of the contemporary jerk parameter and this is related to the equation of state approach. Simple perturbative expansions about $\Lambda$CDM are given., Comment: 10 pages, 6 figures, to appear in Observing Dark Energy (NOAO/Tucson proceedings)

We present the optical spectra of four newly discovered gravitational lenses from the Cosmic Lens All-Sky Survey (CLASS). These observations were carried out using the Low Resolution Imaging Spectrograph on the W. M. Keck-I Telescope as part of a program to study galaxy-scale gravitational lenses. From our spectra we found the redshift of the background source in CLASS B0128+437 (z_s=3.1240+-0.0042) and the lensing galaxy redshifts in CLASS B0445+123 (z_l=0.5583+-0.0003) and CLASS B0850+054 (z_l=0.5883+-0.0006). Intriguingly, we also discovered that CLASS B0631+519 may have two lensing galaxies (z_l,1=0.0896+-0.0001, z_l,2=0.6196+-0.0004). We also found a single unidentified emission line from the lensing galaxy in CLASS B0128+437 and the lensed source in CLASS B0850+054. We find the lensing galaxies in CLASS B0445+123 and CLASS B0631+519 (l,2) to be early-type galaxies with Einstein Radii of 2.8-3.0 h^{-1} kpc. The deflector in CLASS B0850+054 is a late-type galaxy with an Einstein Radius of 1.6 h^{-1} kpc., Comment: 9 pages, 5 figures, accepted for publication in MNRAS

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), Comment: 17 pages, 6 figures, 5 tables; revised version with correct fig.6 and clarified text based on referee report; conclusions unchanged

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., Comment: To appear in "High Energy Processes and Phenomena in Astrophysics, IAU Symposium 214", X. Li, Z. Wang, V. Trimble (eds)

We present flux-ratio curves of the fold and cusp (i.e. close multiple) images of six 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 (<~1 day) compared to the time-scale 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 ~40 percent peak-to-peak on time scales of several months. Its low Galactic latitude of b=-10 degree 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=+69 degree 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 percent rms), even though these relations remain violated when averaged over a <~1 year time baseline., Comment: 17pag, 2 figs, 2 tables; version as accepted for publication in ApJ

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 radio-loud 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 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 at 0.2 arcsecond resolution. When combined with the JVAS survey, the CLASS sample contains over 16,000 images. A complete sample of 11685 flat-spectrum sources was observed, selected from GB6 catalogue at 4.85 GHz and the NVSS at 1.4 GHz. So far, CLASS has found 16 new gravitational lens systems, and the JVAS/CLASS survey contains a total of 22 lenses. (Abridged), Comment: 14 pages, 9 figures, accepted for publication in MNRAS

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 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 having two-point spectral indices between 1.4 and 5 GHz flatter than -0.5 and 5 GHz flux densities $\geq$30 mJy. A subset of 8,958 sources form a well-defined statistical sample suitable for analysis of the lens statistics. We describe the systematic process by which 149 candidate lensed sources were picked from the statistical sample on the basis of possessing multiple compact components in the 0.2 arcsec-resolution VLA maps. Candidates were followed up with 0.05 arcsec resolution MERLIN and 0.003 arcsec VLBA observations at 5 GHz and rejected as lens systems if they failed well-defined surface brightness and/or morphological tests. Maps for all the candidates can be found on the World Wide Web at http://www.jb.man.ac.uk/research/gravlens/index.html We summarize the properties of each of the 22 gravitational lens systems in JVAS/CLASS. Twelve are double-image systems, nine are four-image systems and one is a six-image system. Thirteen constitute a statistically well-defined sample giving a point-source lensing rate of 1:690$\pm$190. The interpretation of the results in terms of the properties of the lensing galaxy population and cosmological parameters will be published elsewhere. (Abridged), Comment: v2: web link in the abstract fixed; some corrections made in section 6.1

We report the discovery of a new gravitational lens system from the CLASS survey. Radio observations with the VLA, the WSRT and MERLIN show that the radio source B0850+054 is comprised 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 sub-components 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., Comment: 5 pages, 4 figures, accepted for publication in MNRAS

A new two-image gravitational lens system has been discovered as a result of the Cosmic Lens All-Sky Survey (CLASS). Radio observations with the VLA, MERLIN and the VLBA at increasingly higher resolutions all show two components with a flux density ratio of ~7:1 and a separation of 1.34". Both components are compact and have the same spectral index. Followup 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 WHT show ~1.7" 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., Comment: 6 pages, 5 figures, accepted by MNRAS

We derive constraints on cosmological parameters and the properties of the lensing galaxies from gravitational lens statistics based on the final Cosmic Lens All Sky Survey (CLASS) data. For a flat universe with a classical cosmological constant, we find that the present matter fraction of the critical density is $\Omega_{\rm m}=0.31^{+0.27}_{-0.14}$ (68%) $^{+0.12}_{-0.10}$ (systematic). For a flat universe with a constant equation of state for dark energy $w = p_x({pressure})/\rho_x({energy density})$, we find $w < -0.55^{+0.18}_{-0.11}$ (68%)., Comment: To appear 7 October 2002 issue (Vol. 89, issue 15) of Phys. Rev. Lett

The extreme hypothesis that essentially all types of ultrarelativistic outflow -- specifically AGN jets, pulsar wind nebulae and GRB --are electromagnetic, rather than gas dynamical, phenomena is considered. Electromagnetic flows are naturally anisotropic and self-collimating so as to produce jet-like features. The relativistic force-free description of these flows, which is simpler than the relativistic MHD description, is explained. It is suggested that the magnetic field associated with AGN jets and GRB is quite extensively distributed in latitude, without necessarily increasing by much the total power. It is also proposed that the observed emission from these sources traces out regions of high current density where global instabilities drive a turbulence spectrum that is ultimately responsible for the particle acceleration and the synchrotron, inverse Compton and synchro-Compton emission. The direct extraction of spin energy from a black hole is re-examined and an electromagnetic model of GRB explosions is developed. It is also suggested that some GRB ``lighthouses'' be identified with accretion-induced collapse of a neutron star to form a black hole in a binary system., Comment: 25 pages, 2 figures. To appear in "Lighthouses of the Universe" Proc. Symposium held in Garching, Germany (Aug 6-9 2001) ed. M. Gilfanov, R. Sunyaev et al. Berlin:Springer. Revised version, correcting minor errors

A summary is presented of a symposium held to review recent progress in X-ray astronomy and to discuss prospects for the next generation of X-ray missions. The summary is organized around ten general, astrophysical questions that may provide scientific drivers for the design of new facilities., Comment: 6 pages. To appear in "New Century of X-ray Astronomy" Proc. Symposium held in Yokohama, Japan (March 6-8 2001) ed. H. Kunieda & H. Inoue San Francisco:ASP

Two related topics are discussed. 1. Accretion onto black holes at low and high (though not very high) rates is believed to proceed adiabatically ({\em ie} non-radiatively). It is argued that the liberated energy is carried off by an outflow, probably involving almost all of the gas that is supplied. Two dimensional, fluid, accretion disks, in which mass, angular momentum and energy are transported to the disk surface, are summarized. It is conjectured that relatively minor changes are needed to describe magnetised disks. By contrast, the disk surface physics is argued to dictate the character of the outflow. 2. Ultrarelativistic jets appear to be produced by active galactic nuclei (AGN), pulsars and $\gamma$-ray bursts (GRB). In all three cases, it is argued that the power is generated electromagnetically by a magnetic rotator, (in a DC not AC form), and transported in this manner to the emission site. A model of a relativistically expanding electromagnetic shell is described and used to provide a simple model of a GRB in which the $\gamma$-rays are produced by unstable electrical currents flowing along the rotation axis. The shell drives a relativistic blast wave into the surrounding medium with a speed that varies with latitude and whose afterglow emission may exhibit achromatic breaks. Similar processes may be at work in non-relativistic plerions like the Crab Nebula and, possibly, AGN jets. The observational implications of these two classes of model and the prospects for performing instructive, numerical experiments to elucidate them further are briefly outlined., Comment: 18 pages, 1 figure. To appear in "Current High-Energy Emission around Black Holes" Proc. 2nd KIAS Astrophysics Workshop held in Seoul, Korea (Sep 3-7 2001) ed. C.-H. Lee Singapore:World Scientific

We present a multi-wavelength analysis of high-resolution observations of the quadruple lens B1608+656 from the HST 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 ring-like configuration that surrounds the two, resolved, lensing galaxies. B1608+656 was discovered as a double-lobed radio source, and later identified as a post-starburst galaxy in the optical. Based on photometry and optical spectroscopy we estimate that the stellar population of the source has an age of $\sim$ 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 PSFs for the filters. Deconvolutions performed with the Lucy-Richardson method are presented, and the limitations of these restorations is 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., Comment: 29 pages. Submitted to ApJ

A summary critique is presented of the topics discussed at a workshop entitled ``Probing the Physics of Active Galactic Nuclei by Multi-wavelength Monitoring'' held at Goddard Space Flight Center 21-23 June 2000. Particular emphasis is placed on the larger astronomical context of research on active galactic nuclei. An important goal is to use multi-wavelength monitoring to produce ``engineering drawings'' of how gas flows in the vicinity of a black hole, as well is in the associated accretion disk inflows and jet outflows. Some of the clear choices that have to be made before formulating a model of a particular class of source are outlined. Future observational possibilities are briefly summarized., Comment: 20 pages

There is strong observational evidence that AGN, Galactic X-ray transients and (probably) $\gamma$-ray bursts are associated with black holes, and that these sources are able to form collimated, ultrarelativistic outflows. There is much interest in trying to understand how these prime movers are able to release energy from accreting gas and their own spin energy. Electromagnetic field plays a large role in many of the mechanisms under active consideration. In this article, several of the many possible ``metabolic pathways'' through which mass, angular momentum and energy can flow around and away from black hole magnetospheres are discussed. Particular importance is attached to the interactions between the inflowing disk, the outflowing wind, the black hole and the jet. Some important unresolved questions are identified and it is argued that large scale numerical computation will almost certainly be necessary to address them., Comment: 21 pages Progress of Theoretical Physics Supplement in press

Observations over the past decade have verified, beyond reasonable doubt, that most galactic nuclei contain massive black holes. Hole masses are being measured and firm evidence for spin is being sought. Attention is now returning to the study of how gas flows around black holes and how energy is released both from the accreting gas and from the hole itself in the form of radiation, relativistic jets and non-relativistic, hydromagnetic winds. Some of the different possibilities currently under investigation are briefly reviewed and some recent clues from radio scintillation, polarization and X-ray observations are discussed. It is argued that observations of persistent circular polarisation in Sgr A$^\ast$ support the presence of an ordered disk magnetic field. It is also conjectured that adiabatic, sub- and super-critical accretion flows are demand-limited, not supply-driven and are associated with large mass outflow as appears to be the case in Sgr A$^\ast$. This principle may have to be modified when a massive black hole forms in a protogalaxy and this modification may account for the proposed hole mass-bulge velocity dispersion relation. The final stages of this process may release sufficient wind energy from the nucleus to prevent disk formation., Comment: 8 pages IAU Symposium 205 ``Galaxies and their constituents at the highest angular resolutions'' ed. R. Schilizzi (San Francisco:ASP) in press

There are now several types of relativistic flows in astrophysical settings. The foremost examples are jets and disks orbiting spinning black holes, pulsar winds and gamma ray bursts. As discussed at this meeting, these flows exhibit unusual kinematic and dynamical properties, that distinguish them from non-relativistic flows. It is possible that all collimated outflows are essentially hydromagnetic or electromagnetic. Future study of relativistic flows will rely heavily on numerical experiments. Model relativistic flows provide a basis for carrying out secondary studies of the underlying plasma physics, particle acceleration, magnetic field amplification and the emission and transfer of radiation, particularly at shock fronts. Some current opportunities in observation, phenomenology and theory are briefly suggested., Comment: 11 pages. Similarities and Universality in Relativistic Flows ed. M. Georganopoulos, A. Guthmann, K. Manolakou & A. Markowith (Berlin:Springer) in press

A summary is presented of a meeting on Particles and Fields in Radio Galaxies held in Oxford in August 2000. Recent detailed studies of radio maps and the first X-ray images from Chandra X-ray Observatory, together with new capabilities in simulating hydromagnetic flows, are transforming our understanding of how jets are formed by accretion disks orbiting massive black holes, how these jets dissipate and how they inflate the giant radio-emitting lobes by which they were first identified. As they can be imaged in such detail, extragalactic radio sources remain central to the study of jet outflows in general although there is a strong convergence with corresponding studies of Galactic superluminal sources, gamma ray bursts and young stellar objects., Comment: 12 pages Proc. Oxford Radio Galaxy Workshop ed. R. Laing & K. Blundell (San Francisco: PASP) in press

In this speculative, millenial essay, I try to anticipate what sort of novel gravitational optics investigations might be observed, after it becomes possible to map and monitor roughly a trillion sources (of which a billion may be usefully variable) comprehensively throughout electromagnetic and other spectra over the whole sky. Existing techniques suffice to produce three dimensional maps of the dark matter distribution of the accessible universe, to explore black hole spacetimes and to magnify images of the first luminous sources, terrestrial planets and compact objects., Comment: 4 pages PASP Millenium Essay in press

We analyse the kinematics of the entire spectroscopic sample of 99 recently discovered high proper-motion white dwarfs by Oppenheimer et al. using a maximum-likelihood analysis, and discuss the claim that the high-velocity white dwarfs are members of a halo population with a local density at least ten times greater than traditionally assumed. We argue that the observations, as reported, are consistent with the presence of an almost undetected thin disc plus a thick disc, with densities as conventionally assumed. In addition, there is a kinematically distinct, flattened, halo population at the more than 99% confidence level. Surprisingly, the thick disc and halo populations are indistinguishable in terms of luminosity, color and apparent age (1-10 Gyr). Adopting a bimodal, Schwarzschild model for the local velocity ellipsoid, with the ratios sigma_U:sigma_V:sigma_W=1:2/3:1/2, we infer radial velocity dispersions of sigma_U=62(+8/-10) km/s and 150(+80/-40) km/s (90% C.L.) for the local thick disc and halo populations, respectively. The thick disc result agrees with the empirical relation between asymmetric drift and radial velocity dispersion, inferred from local stellar populations. The local thick-disc plus halo density of white dwarfs is n^{td+h}=(1.9+-0.5)x10^-3 pc^-3 (90% C.L.), of which n^{h}=1.1(+2.1/-0.7)x10^-4 pc^-3 (90% C.L.) belongs to the halo, a density about five times higher than previously thought. (Abridged), Comment: 19 pages, 11 figures; submitted to MNRAS

A single-screen model of the gravitational lens system 2016+112 is proposed, that explains recent Hubble Space Telescope} (HST) infrared (NICMOS-F160W) observations and new high-resolution European VLBI Network (EVN) 5-GHz radio observations, presented in this paper. In particular, we find that a massive `dark' structure at the lens position, previously suggested by X-ray, optical and spectroscopic observations of the field around 2016+112, is not necessarily required to accommodate the strong lensing constraints. A massive structure to the north-west of the lens system, suggested from a weak-lensing analysis of the field, is included in the model. The lensed source is an X-ray bright active galaxy at z=3.273 with a central bright optical continuum core and strong narrow emission lines, suggestive of a type-II quasar. The EVN 5-GHz radio maps show a radio-jet structure with at least two compact subcomponents. We propose that the diamond caustic crosses the counter-jet of the radio source, so that part of the counter-jet, host galaxy and narrow-line emission regions are quadruply imaged. The remainder of the radio source, including the core, is doubly imaged. Our lens model predicts a very high magnification (mu~300) at the bightness peaks of the inner two radio components of complex C. If the jet exhibits relativistic velocities on micro-arsecond scales, it might result in apparent hyperluminal motion. However, the lack of strong radio variability and the peaked radio spectrum imply that these motions need not be present in the source. Our model furthermore implies that the optical spectrum of C' can only show features of the AGN and its host galaxy., Comment: 10 pages, 3 figures, accepted by MNRAS; minor changes

We examine the claim by Oppenheimer et al. (2001) that the local halo density of white dwarfs is an order of magnitude higher than previously thought. As it stands, the observational data support the presence of a kinematically distinct population of halo white dwarfs at the >99% confidence level. A maximum-likelihood analysis gives a radial velocity dispersion of sigma^h_U=150(+80/-40) km/s and an asymmetric drift of v_a^h=176(+102/-80) km/s, for a Schwarzschild velocity distribution function with sigma_U:sigma_V:sigma_W=1:2/3:1/2. Halo white dwarfs have a local number density of 1.1(+2.1/-0.7)x10^-4 pc-3, which amounts to 0.8(+1.6/-0.5) per cent of the nominal local dark-matter halo density and is 5.0(+9.5/-3.2) times higher and thus only marginally in agreement with previous estimates (all errors indicate the 90% C.L.). We discuss several direct consequences of this white-dwarf population (e.g. microlensing) and postulate a potential mechanism to eject young white dwarfs from the disc to the halo, through the orbital instabilities in triple or multiple stellar systems., Comment: 5 pages, to appear in the proceedings of the Yale Cosmology Workshop "The Shapes of Galaxies and their Halos" (ed. Priya Natarajan); revised numerical results, using a corrected likelihood function (thanks to David Graff and Andy Gould); general conclusions remain similar

We report the discovery of a new two-image gravitational lens system from the Cosmic Lens All-Sky Survey, CLASS B2319+051. Radio imaging with the Very Large Array (VLA) and Multi-Element Radio-Linked Interferometer Network (MERLIN) shows two compact components with a flux density ratio of 5:1, separated by 1.36 arcsec. Observations with the Very Long Baseline Array (VLBA) resolve each of the radio components into a pair of parity-reversed subcomponents. Hubble Space Telescope (HST) observations with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) show a bright elliptical galaxy (G1) coincident with the radio position, and a second irregular galaxy (G2) 3.4 arcsec to the northwest. Previous spectroscopic studies have indicated that these galaxies are at different redshifts: z(G1) = 0.624, z(G2) = 0.588. Infrared counterparts to the lensed radio components are not detected in the NICMOS image, and the source redshift has not yet been determined. Preliminary mass modeling based on the VLBA subcomponent data indicates that the lensing potential includes a strong external shear contribution. A VLA monitoring program is currently being undertaken to measure the differential time delay., Comment: 16 pages, 7 figs, several typos corrected, AJ in press (August 2001)

Strongly correlated non-intrinsic variability between 5 and 8.5 GHz has been observed in one of the lensed images of the gravitational lens B1600+434. These non-intrinsic (i.e. `external') variations are interpreted as `radio-microlensing' of relativistic micro-arsec-scale jet components in the source at a redshift of z=1.59 by massive compact objects in the halo of the edge-on disk lens galaxy at z=0.41. We shortly summarize these observations and discuss several new observational and theoretical programs to investigate this new phenomenon in more detail., Comment: 2 pages, LateX, 1 PostScript figures included, to appear in the proceedings of IAU Symposium 201 `New Cosmological Data and the Values of the Fundamental Parameters' (A. Lasenby and A. Wilkinson, eds.)

High resolution MERLIN observations of a newly-discovered four-image gravitational lens system, B0128+437, are presented. The system was found after a careful re-analysis of the entire CLASS dataset. The MERLIN observations resolve four components in a characteristic quadruple-image configuration; the maximum image separation is 542 mas and the total flux density is 48 mJy at 5 GHz. A best-fit lens model with a singular isothermal ellipsoid results in large errors in the image positions. A significantly improved fit is obtained after the addition of a shear component, suggesting that the lensing system is more complex and may consist of multiple deflectors. The integrated radio spectrum of the background source indicates that it is a GigaHertz-Peaked Spectrum (GPS) source. It may therefore be possible to resolve structure within the radio images with deep VLBI observations and thus better constrain the lensing mass distribution., Comment: Accepted for publication in MNRAS. 4 pages, 3 included PostScript figures

Mid-infrared observations of the central source of NGC 1068 have been obtained with a spatial resolution in the deconvolved image of 0.1" (~ 7pc). The central source is extended by ~1" in the north-south direction but appears unresolved in the east-west direction over most of its length. About two-thirds of its flux can be ascribed to a core structure which is itself elongated north-south and does not show a distinct unresolved compact source. The source is strongly asymmetric, extending significantly further to the north than to the south. The morphology of the mid-infrared emission appears similar to that of the radio jet, and has features which correlate with the images in [O III]. Its 12.5-24.5 micron color temperature ranges from 215 to 260 K and does not decrease smoothly with distance from the core. Silicate absorption is strongest in the core and to the south and is small in the north. The core, apparently containing two-thirds of the bolometric luminosity of the inner 4" diameter area, may be explained by a thick, dusty torus near the central AGN viewed at an angle of ~65 deg to its plane. There are, however, detailed difficulties with existing models, especially the narrow east-west width of the thin extended mid-infrared "tongue" to the north of the core. We interpret the tongue as re-processed visual and ultraviolet radiation that is strongly beamed and that originates in the AGN., Comment: 42 pages, 2 tables, 9 figures; Accepted for publication in AJ

We present the discovery of CLASS B0739+366, a new gravitational lens system from the Cosmic Lens All-Sky Survey. Radio imaging of the source with the Very Large Array (VLA) shows two compact components separated by $0\farcs54$, with a flux density ratio of $\sim$ 6:1. High-resolution follow-up observations using the Very Long Baseline Array (VLBA) at 1.7 GHz detect weak, parity-reversed jet emission from each of the radio components. Hubble Space Telescope NICMOS F160W observations detect infrared counterparts to the lensed images, as well as an extended object between them which we identify as the lensing galaxy. Redshifts for the galaxy and lensed source have not yet been obtained. For typical lens and source redshifts of $z=0.5$ and $z=1.5$, respectively, preliminary mass modeling predicts a time delay of $\sim7h^{-1}$ days in a flat $\Omega_{M}=1.0$ universe. The small predicted time delay and weak radio components will make CLASS B0739+366 a challenging target for Hubble constant determination., Comment: 15 pages, including 6 figures, to be published in AJ

We present radio, optical, near-infrared and spectroscopic observations of the source B0827+525. We consider this source as the best candidate from the Cosmic Lens All-Sky Survey (CLASS) for a `dark lens' system or binary radio-loud quasar. The system consists of two radio components with somewhat different spectral indices, separated by 2.815 arcsec. VLBA observations show that each component has substructure on a scale of a few mas. A deep K-band exposure with the W.M.Keck-II Telescope reveals emission near both radio components. The K-band emission of the weaker radio component appears extended, whereas the emission from the brighter radio component is consistent with a point source. Hubble Space Telescope F160W-band observations with the NICMOS instrument confirms this. A redshift of 2.064 is found for the brighter component, using the LRIS instrument on the W.M.Keck-II Telescope. The probability that B0827+525 consists of two unrelated compact flat-spectrum radio sources is ~3%, although the presence of similar substructure in both component might reduce this. We discuss two scenarios to explain this system: (i) CLASS B0827+525 is a `dark lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525 has met all criteria that thus far have in 100% of the cases confirmed a source as an indisputable gravitational lens system. Despite this, no lens galaxy has been detected with m_F160W<=23 mag. Hence, we might have found the first binary radio-loud quasar. At this moment, however, we feel that the `dark lens' hypothesis cannot yet be fully excluded., Comment: 9 pages, 6 figures; Accepted for publication in Astronomy & Astrophysics; Full-res. images 1 and 3 can be obtained from L.V.E.K

Observations of jets from quasars and other types of accreting black hole are briefly summarized. The importance of beaming and $\gamma$-ray observations for understanding the origin of these jets is emphasised. It is argued that both the power source and the collimation are likely to be magnetic in origin, although the details remain controversial. Ultrarelativistic jets may be formed by the spinning hole and collimated by a hydromagnetic disc wind. Progress in understanding jets has been handicapped by our inadequate knowledge of how magnetic field really behaves under cosmic conditions. Fortunately, significant insights are coming from solar observations, numerical simulation and laboratory plasma experiments. Some possible, evolutionary ramifications are briefly discussed and it is suggested that it is the mass of the black hole relative to that of the galaxy which determines the eventual galaxy morphology., Comment: Latex. 17pages Proc Discusison Meeting on Magnetic Activity in Stars, Discs and Quasars. Ed. D. Lynden-Bell, E. R. Priest and N. O. Weiss. To appear in Phil. Trans. Roy. Soc. A

Cosmic explosions are observed in many astrophysical environments. They range in scale from hydromagnetic instabilities in the terrestrial magnetotail and solar ``nanoflares'' to cosmological gamma ray bursts, supernovae and the protracted intervals of nuclear activity that produce the giant quasars and radio galaxies. There are many parallels in the analyses of the explosion sites that are highlighted at this workshop, specifically stellar coronae, accretion disks, supernovae and compact objects. In this introductory talk, some general issues are discussed and some more specific questions relating to the individual sites are raised., Comment: To appear in Cosmic Explosions: Proc. 10th Maryland Conference on Astrophysics. Ed. S. Holt and W. Zhang AIP

We present new results from a continuing Keck program to study gravitational lens systems. We have obtained redshifts for three lens systems, SBS 0909+532, HST 1411+5211, and CLASS B2319+051. For all of these systems, either the source or lens redshift (or both) has been previously unidentified. We find (z_l, z_s) = (0.830, 1.377) for SBS 0909+532; (z_l, z_s) = (0.465, 2.811) for HST 1411+5211, although the source redshift is still tentative; and (z_l1, z_l2) = (0.624, 0.588) for the two lensing galaxies in CLASS B2319+051. The background radio source in B2319+051 has not been detected optically; its redshift is, therefore, still unknown. We find that the spectral features of the central lensing galaxy in all three systems are typical of an early-type galaxy. The observed image splittings in SBS 0909+532 and HST 1411+5211 imply that the masses within the Einstein ring radii of the lensing galaxies are 1.4 x 10^{11} and 2.0 x 10^{11} h^{-1} M_sun, respectively. The resulting B band mass-to-light ratio for HST 1411+5211 is 41.3 +/- 1.2 h (M/L)_sun, a factor of 5 times higher than the average early-type lensing galaxy. This large mass-to-light is almost certainly the result of the additional mass contribution from the cluster CL 3C295 at z = 0.46. For the lensing galaxy in SBS 0909+532, we measure (M/L)_B = 4^{+11}_{-3} h (M/L)_sun where the large errors are the result of significant uncertainty in the galaxy luminosity. While we cannot measure directly the mass-to-light ratio of the lensing galaxy in B2319+051, we estimate that (M/L)_B is between 3-7 h (M/L)_sun., Comment: Accepted for publication in Astronomical Journal. 21 pages, including 7 figures