Testing the FR I/BL Lac unifying model with HST observations

Hubble Space Telescope (HST) observations provide a novel way of testing unified models for FR I radio sources and BL Lac objects. The detection of extended dust discs in some radio galaxies provides information on their jet orientation. Given this, the strength of the compact nuclear sources of FR I and BL Lacs can be compared with model predictions. As a pilot project, we selected five radio galaxies which show extended nuclear discs in the HST images. The relative orientation of the projected radio-jets and of the extended nuclear discs indicates that they are not perpendicular, as the simplest geometrical model would suggest, but that they form an angle of ~ 20 - 40 degrees with the symmetry axis of the disc: a significant change of orientation occurs between the innermost AGN structure and the kpc-scale. Nevertheless, the discs appear to be useful indicators of the radio sources orientation since the angles formed by the disc axis and the jet with the line of sight differ by only ~ 10 - 20 degrees. At the center of each disc an unresolved nuclear source is present. We compared its luminosity with the optical core luminosity of BL Lacs selected for having similar host galaxy magnitude and extended radio luminosity. The BL Lac cores are between 2 E2 and 3 E5 times brighter than the corresponding radio galaxies ones. The FR I/BL Lac core luminosity ratio shows a suggestive correlation with the orientation of the radio galaxies with respect to the line of sight. The behavior of this ratio is quantitatively consistent with a scenario in which the emission in the FR I and BL Lac is dominated by the beamed radiation from a relativistic jet with Doppler factor ~ 5 - 10, thus supporting the basic features of the proposed unification schemes.Comment: 11 pages, 10 figures, submitted to MNRAS, revised versio

Brane Inflation, Solitons and Cosmological Solutions: I

In this paper we study various cosmological solutions for a D3/D7 system directly from M-theory with fluxes and M2-branes. In M-theory, these solutions exist only if we incorporate higher derivative corrections from the curvatures as well as G-fluxes. We take these corrections into account and study a number of toy cosmologies, including one with a novel background for the D3/D7 system whose supergravity solution can be completely determined. This new background preserves all the good properties of the original model and opens up avenues to investigate cosmological effects from wrapped branes and brane-antibrane annihilation, to name a few. We also discuss in some detail semilocal defects with higher global symmetries, for example exceptional ones, that could occur in a slightly different regime of our D3/D7 model. We show that the D3/D7 system does have the required ingredients to realise these configurations as non-topological solitons of the theory. These constructions also allow us to give a physical meaning to the existence of certain underlying homogeneous quaternionic Kahler manifolds.Comment: Harvmac, 115 pages, 9 .eps figures; v2: typos corrected, references added and the last section expanded; v3: Few minor typos corrected and references added. Final version to appear in JHE

Probing the presence of planets in transition discs' cavities via warps: the case of TW Hya

We are entering the era in which observations of protoplanetary discs properties can indirectly probe the presence of massive planets or low mass stellar companions interacting with the disc. In particular, the detection of warped discs can provide important clues to the properties of the star-disc system. In this paper we show how observations of warped discs can be used to infer the dynamical properties of the systems. We concentrate on circumbinary discs, where the mass of the secondary can be planetary. First, we provide some simple relations that link the amplitude of the warp in the linear regime to the parameters of the system. Secondly, we apply our method to the case of TW Hya, a transition disc for which a warp has been proposed based on spectroscopic observations. Assuming values for the disc and stellar parameters from observations, we conclude that, in order for a warp induced by a planetary companion to be detectable, the planet mass should be large ($M_{\rm p} \approx 10 - 14M_{\rm J}$) and the disc should be viscous ($\alpha \approx 0.15 - 0.25$). We also apply our model to LkCa 15 and T Cha, where a substellar companion has been detected within the central cavity of the transition discs.Comment: 12 pages, 4 figures, 2 tables. Accepted for publication in MNRA

Kerr-Schild spacetimes with (A)dS background

General properties of Kerr-Schild spacetimes with (A)dS background in arbitrary dimension are studied. It is shown that the geodetic Kerr-Schild vector k is a multiple WAND of the spacetime. Einstein Kerr-Schild spacetimes with non-expanding k are shown to be of Weyl type N, while the expanding spacetimes are of type II or D. It is shown that this class of spacetimes obeys the optical constraint. This allows us to solve Sachs equation, determine r-dependence of boost weight zero components of the Weyl tensor and discuss curvature singularities.Comment: 17 pages, minor change

Linear and non-linear theory of a parametric instability of hydrodynamic warps in Keplerian discs

We consider the stability of warping modes in Keplerian discs. We find them to be parametrically unstable using two lines of attack, one based on three-mode couplings and the other on Floquet theory. We confirm the existence of the instability, and investigate its nonlinear development in three dimensions, via numerical experiment. The most rapidly growing non-axisymmetric disturbances are the most nearly axisymmetric (low m) ones. Finally, we offer a simple, somewhat speculative model for the interaction of the parametric instability with the warp. We apply this model to the masing disc in NGC 4258 and show that, provided the warp is not forced too strongly, parametric instability can fix the amplitude of the warp.Comment: 14 pages, 6 figures, revised version with appendix added, to be published in MNRA

Using Graph Properties to Speed-up GPU-based Graph Traversal: A Model-driven Approach

While it is well-known and acknowledged that the performance of graph algorithms is heavily dependent on the input data, there has been surprisingly little research to quantify and predict the impact the graph structure has on performance. Parallel graph algorithms, running on many-core systems such as GPUs, are no exception: most research has focused on how to efficiently implement and tune different graph operations on a specific GPU. However, the performance impact of the input graph has only been taken into account indirectly as a result of the graphs used to benchmark the system. In this work, we present a case study investigating how to use the properties of the input graph to improve the performance of the breadth-first search (BFS) graph traversal. To do so, we first study the performance variation of 15 different BFS implementations across 248 graphs. Using this performance data, we show that significant speed-up can be achieved by combining the best implementation for each level of the traversal. To make use of this data-dependent optimization, we must correctly predict the relative performance of algorithms per graph level, and enable dynamic switching to the optimal algorithm for each level at runtime. We use the collected performance data to train a binary decision tree, to enable high-accuracy predictions and fast switching. We demonstrate empirically that our decision tree is both fast enough to allow dynamic switching between implementations, without noticeable overhead, and accurate enough in its prediction to enable significant BFS speedup. We conclude that our model-driven approach (1) enables BFS to outperform state of the art GPU algorithms, and (2) can be adapted for other BFS variants, other algorithms, or more specific datasets

The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering Toward the Anticenter

We use radio-wave scattering data to constrain the distribution of ionized gas in the outer Galaxy. Like previous models, our model for the H II disk includes parameters for the radial scale length and scale height of the H II, but we allow the H II disk to warp and flare. Our model also includes the Perseus arm. We use a likelihood analysis on 11 extragalactic sources and 7 pulsars. Scattering in the Perseus arm is no more than 60% of the level contributed by spiral arms in the inner Galaxy, equivalent to a 1 GHz scattering diameter of 1.5 mas. Our analysis favors an unwarped, nonflaring disk with a 1 kpc scale height, though this may reflect the non-uniform and coarse coverage provided by the available data. The lack of a warp indicates that VLBI observations near 1 GHz with an orbiting station having baseline lengths of a few Earth diameters will not be affected by interstellar scattering at Galactic latitudes |b| ~ 15 degrees. The radial scale length is 15--20 kpc, but the data cannot distinguish between a gradual decrease in the electron density and a truncated distribution. We favor a truncated one, because we associate the scattering with massive star formation, which is also truncated near 20 kpc. The distribution of electron density turbulence decreases more rapidly with Galactocentric distance than does the hydrogen distribution. Alternate ionizing and turbulent agents---the intergalactic ionizing flux and satellite galaxies passing through the disk---do not contribute significantly to scattering. We cannot exclude the possibility that a largely ionized, but quiescent disk extends to >~ 100 kpc, similar to that for some Ly-alpha absorbers.Comment: 34 pages, LaTeX2e with AASTeX aaspp4 macro, 9 figures in 9 PostScript files, accepted for publication in Ap