965 research outputs found
BL Lac evolution revisited
BL Lac objects are an elusive and rare class of active galactic nuclei. For
years their evolutionary behavior has appeared inconsistent with the trend
observed in the population of AGN at large. The so-called ``negative''
evolution implies that BL Lacs were either less or fainter in the past. This
effect is stronger for BL Lacs selected in X-ray surveys. We have investigated
if one of the selection criteria, namely the flat-radio spectrum (imposed on
the Radio-selected but not on the X-ray-selected samples), might explain the
different evolutionary trend.Comment: Proceedings of "Multiwavelength AGN Surveys", Cozumel, Dec 200
Radio spectra of a sample of X-ray selected BL Lacs
We present simultaneous multifrequency radio observations for a complete
subsample of 26 XBLs from the Einstein Extended Medium-Sensitivity Survey,
obtained with the Very Large Array (VLA). Spectra are computed using fluxes at
20, 6 and 3.6 cm. Unlike many radio selected samples, the EMSS did not impose
any criterion on the radio spectrum to identify BL Lac objects. It is therefore
possible to investigate the intrinsic radio spectral slope distribution and to
determine the effect produced by this selection criterion. We find that 15% of
the observed objects do not meet the flat-spectrum criterion imposed on some
other BL Lac samples. A dataset that includes non-simultaneous data (that are
also taken with different VLA configurations) shows an even higher percentage
of steep spectrum sources. This effect can be ascribed to a larger fraction of
extended flux detected with the more compact VLA configuration.Possible biases
introduced by the flat--radio-spectrum criterion in the radio-selected BL Lac
samples cannot explain the discrepancies observed in the evolutionary
properties of Radio and X-ray selected samples of BL Lacs.Comment: 9 pages, 5 figures, to be published in Astronomy and Astrophysic
Broadening the Scope of Nanopublications
In this paper, we present an approach for extending the existing concept of
nanopublications --- tiny entities of scientific results in RDF representation
--- to broaden their application range. The proposed extension uses English
sentences to represent informal and underspecified scientific claims. These
sentences follow a syntactic and semantic scheme that we call AIDA (Atomic,
Independent, Declarative, Absolute), which provides a uniform and succinct
representation of scientific assertions. Such AIDA nanopublications are
compatible with the existing nanopublication concept and enjoy most of its
advantages such as information sharing, interlinking of scientific findings,
and detailed attribution, while being more flexible and applicable to a much
wider range of scientific results. We show that users are able to create AIDA
sentences for given scientific results quickly and at high quality, and that it
is feasible to automatically extract and interlink AIDA nanopublications from
existing unstructured data sources. To demonstrate our approach, a web-based
interface is introduced, which also exemplifies the use of nanopublications for
non-scientific content, including meta-nanopublications that describe other
nanopublications.Comment: To appear in the Proceedings of the 10th Extended Semantic Web
Conference (ESWC 2013
X-Ray Spectral Variability of PKS 2005-489 During the Spectacular November 1998 Flare
We report on monitoring of the BL Lac object PKS 2005-489 by the Rossi X-ray
Timing Explorer (RXTE) in October-December 1998. During these months, the
source underwent a spectacular flare; at its peak on November 10, its 2-10 keV
flux was , over 30 times
brighter than in quiescence. During the rising phase, the X-ray spectrum of PKS
2005-489 hardened considerably, reaching near maximum. During the declining phase, the X-ray spectrum
steepened rapidly, reaching , then became somewhat harder
towards the end of December (). While such behavior has been
seen before, the simplicity, magnitude and duration of this flare allowed us to
study it in great detail. We argue that this flare was caused by either the
injection of particles into the jet or {\it in situ} particle acceleration, and
that the spectral steepening which followed the flare maximum was the result of
synchrotron cooling. Contrary to other recently observed blazar flares (e.g.,
Mkn 501, 3C 279, PKS 2155-304), our results do not imply a major shift in the
location of the synchrotron peak during this flare.Comment: ApJ Letters in press, 6 pages, 2 figures Corrected reference
"Dark Matter" in Accretion Disks
Using Spitzer Space Telescope photometric observations of the eclipsing,
interacting binary WZ Sge, we have discovered that the accretion disk is far
more complex than previously believed. Our 4.5 and 8 micron time series
observations reveal that the well known gaseous accretion disk is surrounded by
an asymmetric disk of dusty material with a radius approximately 15 times
larger than the gaseous disk. This dust ring contains only a small amount of
mass and is completely invisible at optical and near-IR wavelengths, hence
consisting of "dark matter". We have produced a model dust ring using 1 micron
spherical particles with a density of 3 g/cm and with a temperature profile
ranging from 700-1500K. Our discovery about the accretion disk structure and
the presence of a larger, outer dust ring have great relevance for accretion
disks in general, including those in other interacting binary systems, pre-main
sequence stars, and active galaxies.Comment: 34 pages, 8 figures (3 in color). Accepted to Ap
Variational Geometric Approach To Generalized Differential And Conjugate Calculi In Convex Analysis
This paper develops a geometric approach of variational analysis for the case of convex objects considered in locally convex topological spaces and also in Banach space settings. Besides deriving in this way new results of convex calculus, we present an overview of some known achievements with their uniïŹed and simpliïŹed proofs based on the developed geometric variational schemes. Key words. Convex and variational analysis, Fenchel conjugates, normals and subgradients, coderivatives, convex calculus, optimal value functions
- âŠ