12,166 research outputs found
Identification of Potential Weak Target Radio Quasars for ASTRO-G In-Beam Phase-Referencing
We apply an efficient selection method to identify potential weak Very Long
Baseline Interferometry (VLBI) target quasars simply using optical (SDSS) and
low-resolution radio (FIRST) catalogue data. Our search is restricted to within
12" from known compact radio sources that are detectable as phase-reference
calibrators for ASTRO-G at 8.4 GHz frequency. These calibrators have estimated
correlated flux density >20 mJy on the longest ground-space VLBI baselines. The
search radius corresponds to the primary beam size of the ASTRO-G antenna. We
show that ~20 quasars with at least mJy-level expected flux density can be
pre-selected as potential in-beam phase-reference targets for ASTRO-G at 8.4
GHz frequency. Most of them have never been imaged with VLBI. The sample of
these dominantly weak sources offers a good opportunity to study their radio
structures with unprecedented angular resolution provided by Space VLBI. The
method of in-beam phase-referencing is independent from the ability of the
orbiting radio telescope to do rapid position-switching manoeuvres between the
calibrators and the nearby reference sources, and less sensitive to the
satellite orbit determination uncertainties.Comment: 5 pages, accepted for the Publ. Astron. Soc. Japan (Vol. 61, No. 1,
Feb 2009
Exact results for the Kardar--Parisi--Zhang equation with spatially correlated noise
We investigate the Kardar--Parisi--Zhang (KPZ) equation in spatial
dimensions with Gaussian spatially long--range correlated noise ---
characterized by its second moment --- by means of dynamic field theory and the
renormalization group. Using a stochastic Cole--Hopf transformation we derive
{\em exact} exponents and scaling functions for the roughening transition and
the smooth phase above the lower critical dimension . Below
the lower critical dimension, there is a line marking the stability
boundary between the short-range and long-range noise fixed points. For , the general structure of the renormalization-group equations
fixes the values of the dynamic and roughness exponents exactly, whereas above
, one has to rely on some perturbational techniques. We discuss the
location of this stability boundary in light of the exact results
derived in this paper, and from results known in the literature. In particular,
we conjecture that there might be two qualitatively different strong-coupling
phases above and below the lower critical dimension, respectively.Comment: 21 pages, 15 figure
Dynamic Light Scattering from Semidilute Actin Solutions: A Study of Hydrodynamic Screening, Filament Bending Stiffness and the Effect of Tropomyosin/Troponin-Binding
Quasi-elastic light scattering (QELS) is applied to investigate the effect of
the tropomyosin/troponin complex (Tm/Tn) on the stiffness of actin filaments.
The importance of hydrodynamic screening in semidilute solutions is
demonstrated. A new concentration dependent expression for the dynamic
structure factor of semiflexible polymers in semidilute solutions
is used to analyze the experimental QELS data. A concentration independent
value for the bending modulus is thus obtained. It increases by 50\%
as a consequence of Tm/Tn binding in a 7:1:1 molar ratio of actin/Tm/Tn. In
addition a new expression for the initial slope of the dynamic structure factor
of a semiflexible polymer is used to determine the effective hydrodynamic
diameter of the actin filament. Our results confirm the general relevance of
the concept of (intrinsic) semiflexibility to polymer dynamics.Comment: 9 pages, RevTeX, 9 figures, all uuencoded gzipe
VLBI search for the radio counterpart of HESS J1943+213
HESS J1943+213, a TeV point source close to the Galactic plane recently
discovered by the H.E.S.S. collaboration, was proposed to be an extreme BL
Lacertae object, though a pulsar wind nebula (PWN) nature could not be
completely discarded. To investigate its nature, we performed high-resolution
radio observations with the European Very Long Baseline Interferometry Network
(EVN) and reanalyzed archival continuum and H {\sc i} data. The EVN
observations revealed a compact radio counterpart of the TeV source. The low
brightness temperature and the resolved nature of the radio source are
indications against the beamed BL Lacertae hypothesis. The radio/X-ray source
appears immersed in a 1\arcmin elliptical feature suggesting a possible
galactic origin (PWN nature) for the HESS source. We found that HESS\,J1943+213
is located in the interior of a \sim1\degr diameter H {\sc i} feature, and
explored the possibility of they being physically related.Comment: Significantly revised and extended. Accepted for publication in ApJ
(ApJ, 762, 63). (4 figures.
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