9,208 research outputs found
Correlation of inflation-produced magnetic fields with scalar fluctuations
If the conformal invariance of electromagnetism is broken during inflation,
then primordial magnetic fields may be produced. If this symmetry breaking is
generated by the coupling between electromagnetism and a scalar field---e.g.
the inflaton, curvaton, or the Ricci scalar---then these magnetic fields may be
correlated with primordial density perturbations, opening a new window to the
study of non-gaussianity in cosmology. In order to illustrate, we couple
electromagnetism to an auxiliary scalar field in a de Sitter background. We
calculate the power spectra for scalar-field perturbations and magnetic fields,
showing how a scale-free magnetic field spectrum with rms amplitude of ~nG at
Mpc scales may be achieved. We explore the Fourier-space dependence of the
cross-correlation between the scalar field and magnetic fields, showing that
the dimensionless amplitude, measured in units of the power spectra, can grow
as large as ~500 H_I/M, where H_I is the inflationary Hubble constant and M is
the effective mass scale of the coupling.Comment: 11 pages, 3 pdf figure
Radio-loudness in black hole transients: evidence for an inclination effect
Accreting stellar-mass black holes appear to populate two branches in a
radio:X-ray luminosity plane. We have investigated the X-ray variability
properties of a large number of black hole low-mass X-ray binaries, with the
aim of unveiling the physical reasons underlying the radio-loud/radio-quiet
nature of these sources, in the context of the known accretion-ejection
connection. A reconsideration of the available radio and X-ray data from a
sample of black hole X-ray binaries confirms that being radio-quiet is the more
normal mode of behaviour for black hole binaries. In the light of this we chose
to test, once more, the hypothesis that radio loudness could be a consequence
of the inclination of the X-ray binary. We compared the slope of the
`hard-line' (an approximately linear correlation between X-ray count rate and
rms variability, visible in the hard states of active black holes), the orbital
inclination, and the radio-nature of the sources of our sample. We found that
high-inclination objects show steeper hard-lines than low-inclination objects,
and tend to display a radio-quiet nature (with the only exception of V404 Cyg),
as opposed to low-inclination objects, which appear to be radio-loud(er). While
in need of further confirmation, our results suggest that - contrary to what
has been believed for years - the radio-loud/quiet nature of black-hole low
mass X-ray binaries might be an inclination effect, rather than an intrinsic
source property. This would solve an important issue in the context of the
inflow-outflow connection, thus providing significant constraints to the models
for the launch of hard-state compact jets.Comment: 16 pages, 6 figures, accepted for pubblication on MNRA
Mining web data for competency management
We present CORDER (COmmunity Relation Discovery by named Entity Recognition) an un-supervised machine learning algorithm that exploits named entity recognition and co-occurrence data to associate individuals in an organization with their expertise and associates. We
discuss the problems associated with evaluating
unsupervised learners and report our initial evaluation
experiments
Gravitational lensing and dynamics in SL2S\,J02140-0535: Probing the mass out to large radius
We aim to probe the mass of SL2S\,J02140-0535, a galaxy group at = 0.44
from the Strong Lensing Legacy Survey (SL2S). We combine strong lensing
modeling and dynamical constraints. The strong lensing analysis is based on
multi-band HST/ACS observations exhibiting strong lensing features that we have
followed-up spectroscopically with VLT/FORS2. To constrain the scale radius of
an NFW mass profile that cannot be constrained by strong lensing, we propose a
new method by taking advantage of the large-scale dynamical information
provided by VLT/FORS2 and KECK/LRIS spectroscopy of group members. In constrast
to other authors, we show that the observed lensing features in
SL2S\,J02140-0535 belong to different background sources: one at = 1.7
0.1 produces three images, while the other at = 1.023 0.001 has
only a single image. Our unimodal NFW mass model reproduces these images very
well. It is characterized by a concentration parameter = 6.0
0.6, which is slightly greater than the value expected from CDM
simulations for a mass of M 1 10 M_{\sun}.
The spectroscopic analysis of group members also reveals a unimodal structure
that exhibits no evidence of merging. We compare our dynamic mass estimate with
an independent weak-lensing based mass estimate finding that both are
consistent. Our combined lensing and dynamical analysis of SL2S\,J02140-0535
demonstrates the importance of spectroscopic information in reliably
identifying the lensing features. Our findings argue that the system is a
relaxed, massive galaxy group where mass is traced by light. This work shows a
potentially useful method for constraining large-scale properties inaccessible
to strong lensing, such as the scale radius of the NFW profile.Comment: Accepted for publication in A&
The value function of an asymptotic exit-time optimal control problem
We consider a class of exit--time control problems for nonlinear systems with
a nonnegative vanishing Lagrangian. In general, the associated PDE may have
multiple solutions, and known regularity and stability properties do not hold.
In this paper we obtain such properties and a uniqueness result under some
explicit sufficient conditions. We briefly investigate also the infinite
horizon problem
SARCS strong lensing galaxy groups: I - optical, weak lensing, and scaling laws
We present the weak lensing and optical analysis of the SL2S-ARCS (SARCS)
sample of strong lens candidates. The sample is based on the Strong Lensing
Legacy Survey (SL2S), a systematic search of strong lensing systems in the
photometric Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The SARCS
sample focuses on arc-like features and is designed to contain mostly galaxy
groups. We briefly present the weak lensing methodology that we use to estimate
the mass of the SARCS objects. Among 126 candidates, we obtain a weak lensing
detection for 89 objects with velocity dispersions of the Singular Isothermal
Sphere mass model ranging from 350 to 1000 km/s with an average value of
600km/s, corresponding to a rich galaxy group (or poor cluster). From the
galaxies belonging to the bright end of the group's red sequence (M_i<-21), we
derive the optical properties of the SARCS candidates. We obtain typical
richnesses of N=5-15 galaxies and optical luminosities of L=0.5-1.5e+12 Lsol
(within a radius of 0.5 Mpc). We use these galaxies to compute luminosity
density maps, from which a morphological classification reveals that a large
fraction of the sample are groups with a complex light distribution, either
elliptical or multimodal, suggesting that these objects are dynamically young
structures. We finally combine the lensing and optical analyses to draw a
sample of 80 most secure group candidates, i.e. weak lensing detection and
over-density at the lens position in the luminosity map, to remove false
detections and galaxy-scale systems from the initial sample. We use this
reduced sample to probe the optical scaling relations in combination with a
sample of massive galaxy clusters. We detect the expected correlations over the
probed range in mass with a typical scatter of 25% in the SIS velocity
dispersion at a given richness or luminosity, making these scaling laws
interesting mass proxie
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