18,834 research outputs found
WMAP extragalactic point sources as potential Space VLBI calibrators
The point source list of the Wilkinson Microwave Anisotropy Probe (WMAP) is a
uniform, all-sky catalogue of bright sources with flux density measurements at
high (up to 94 GHz) radio frequencies. We investigated the five-year WMAP list
to compile a new catalogue of bright and compact extragalactic radio sources to
be potentially studied with Very Long Baseline Interferometry at millimeter
wavelengths (mm-VLBI) and Space VLBI (SVLBI). After comparing the WMAP data
with the existing mm-VLBI catalogues, we sorted out the yet unexplored sources.
Using the 41, 61 and 94 GHz WMAP flux densities, we calculated the spectral
indices. By collecting optical identifications, lower-frequency radio flux
densities and VLBI images from the literature, we created a list of objects
which have not been investigated with VLBI at 86 GHz before. With total flux
density at least 1 Jy and declination above -40 degree, we found 37 suitable
new targets. It is a nearly 25% addition to the known mm-VLBI sources. Such
objects are also potentially useful as phase-reference calibrators for the
future Japanese SVLBI mission ASTRO-G at its highest observing frequency (43
GHz). The phase-referencing capability of ASTRO-G would allow long integrations
and hence better sensitivity for observing faint target sources close to
suitable phase calibrators in the sky
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
Into the central 10 pc of the most distant known radio quasar. VLBI imaging observations of J1429+5447 at z=6.21
Context: There are about 60 quasars known at redshifts z>5.7 to date. Only
three of them are detected in the radio above 1 mJy flux density at 1.4 GHz
frequency. Among them, J1429+5447 (z=6.21) is the highest-redshift radio quasar
known at present. These rare, distant, and powerful objects provide important
insight into the activity of the supermassive black holes in the Universe at
early cosmological epochs, and on the physical conditions in their environment.
Aims: We studied the compact radio structure of J1429+5447 on the
milli-arcsecond (mas) angular scale, in order to compare the structural and
spectral properties with those of other two z~6 radio-loud quasars, J0836+0054
(z=5.77) and J1427+3312 (z=6.12). Methods: We performed Very Long Baseline
Interferometry (VLBI) imaging observations of J1429+5447 with the European VLBI
Network (EVN) at 1.6 GHz on 2010 June 8, and at 5 GHz on 2010 May 27. Results:
Based on its observed radio properties, the compact but somewhat resolved
structure on linear scales of <100 pc, and the steep spectrum, the quasar
J1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer the
question whether the compact steep-spectrum radio emission is a "universal"
feature of the most distant radio quasars, it is essential to study more, yet
to be discovered radio-loud active galactic nuclei at z>6.Comment: 4 pages, 2 figures, accepted for publication as a Letter to the
editor in Astronomy & Astrophyic
EVN detection of a compact radio source as a counterpart to Fermi J1418+3541
Fermi J1418+3541 is a suspected blazar recently detected as a flaring gamma-ray point source, identified with likely radio, optical and infrared counterparts within the Fermi LAT error circle. We detected the proposed radio counterpart of Fermi J1418+3541 with the European VLBI Network (EVN), in real-time e-VLBI mode at 5 GHz on 2013 Jan 16 (project code RSF07). The source is dominated by a compact radio core, practically unresolved on intercontinental baselines from Europe to South Africa
Four hot DOGs eaten up with the EVN
Hot dust-obscured galaxies (hot DOGs) are a rare class of hyperluminous
infrared galaxies recently identified with the Wide-field Infrared Survey
Explorer (WISE) satellite. The majority of the ~1000-member all-sky population
should be at high redshifts (z~2-3), at the peak of star formation in the
history of the Universe. This class most likely represents a short phase during
galaxy merging and evolution, a transition from starburst- to AGN-dominated
phases. For the first time, we observed four hot DOGs with known mJy-level
radio emission using the European VLBI Network (EVN) at 1.7 GHz, in a hope to
find compact radio features characteristic to AGN activity. All four target
sources are detected at ~15-30 mas angular resolution, confirming the presence
of an active nucleus. The sources are spatially resolved, i.e. the flux density
of the VLBI-detected components is smaller than the total flux density,
suggesting that a fraction of the radio emission originates from larger-scale
(partly starburst-related) activity. Here we show the preliminary results of
our e-EVN observations made in 2014 February, and discuss WISE J1814+3412, an
object with kpc-scale symmetric radio structure, in more detail.Comment: 6 pages, 1 figure; appears in the proceedings of the 12th European
VLBI Network Symposium and Users Meeting (7-10 October 2014, Cagliari,
Italy), eds. A. Tarchi, M. Giroletti & L. Feretti. JREF Proceedings of
Science, PoS(EVN 2014)003,
http://pos.sissa.it/archive/conferences/230/003/EVN%202014_003.pd
Limiting stable currents in bounded electron and ion streams
The classical static analysis of the infinite planar diode has been extended to include the effects of finite transverse beam size. Simple expressions have been found for the increase in maximum stable current density over that of an infinite stream for finite cylindrical and strip streams flowing between plates of infinite diodes. The results are also given in terms of stream perveance. The effect of a nonuniform distribution of current across the stream is shown to be relatively small. Experimental values of maximum stable current agree with those obtained from the analysis. A further extension of the static analysis has been made to include the effects of additional conducting plane boundaries parallel to the stream motion. For length-to-width ratios L/D less than 0.25 the tube is adequately described by the results for the infinite planar diode and for L/D greater than 4, the infinitely-long drift tube theory suffices. At intermediate values of L/D, the maximum amount of current that can be stably passed through the tube is greater than that predicted by either asymptotic theory
Four hot DOGs in the microwave
Hot dust-obscured galaxies (hot DOGs) are a rare class of hyperluminous
infrared galaxies identified with the Wide-field Infrared Survey Explorer
(WISE) satellite. The majority of them is at high redshifts (z~2-3), at the
peak epoch of star formation in the Universe. Infrared, optical, radio, and
X-ray data suggest that hot DOGs contain heavily obscured, extremely luminous
active galactic nuclei (AGN). This class may represent a short phase in the
life of the galaxies, signifying the transition from starburst- to
AGN-dominated phases. Hot DOGs are typically radio-quiet, but some of them show
mJy-level emission in the radio (microwave) band. We observed four hot DOGs
using the technique of very long baseline interferometry (VLBI). The 1.7-GHz
observations with the European VLBI Network (EVN) revealed weak radio features
in all sources. The radio is free from dust obscuration and, at such high
redshifts, VLBI is sensitive only to compact structures that are characteristic
of AGN activity. In two cases (WISE J0757+5113, WISE J1603+2745), the flux
density of the VLBI-detected components is much smaller than the total flux
density, suggesting that ~70-90 per cent of the radio emission, while still
dominated by AGN, originates from angular scales larger than probed by the EVN.
The source WISE J1146+4129 appears a candidate compact symmetric object, and
WISE J1814+3412 shows a 5.1-kpc double structure, reminiscent of hot spots in a
medium-sized symmetric object. Our observations support that AGN residing in
hot DOGs may be genuine young radio sources where starburst and AGN activities
coexist.Comment: 8 pages, 4 tables, 1 figure; accepted for publication in the Monthly
Notices of the Royal Astronomical Societ
Two in one? A possible dual radio-emitting nucleus in the quasar SDSS J1425+3231
The radio-emitting quasar SDSS J1425+3231 (z=0.478) was recently found to
have double-peaked narrow [O III] optical emission lines. Based on the analysis
of the optical spectrum, Peng et al. (2011) suggested that this object harbours
a dual active galactic nucleus (AGN) system, with two supermassive black holes
(SMBHs) separated on the kpc scale. SMBH pairs should be ubiquitous according
to hierarchical galaxy formation scenarios in which the host galaxies and their
central black holes grow together via interactions and eventual mergers. Yet
the number of presently-confirmed dual SMBHs on kpc or smaller scales remains
small. A possible way to obtain direct observational evidence for duality is to
conduct high-resolution radio interferometric measurements, provided that both
AGN are in an evolutionary phase when some activity is going on in the radio.
We used the technique of Very Long Baseline Interferometry (VLBI) to image SDSS
J1425+3231. Observations made with the European VLBI Network (EVN) at 1.7 GHz
and 5 GHz frequencies in 2011 revealed compact radio emission at sub-mJy flux
density levels from two components with a projected linear separation of
\sim2.6 kpc. These two components support the possibility of a dual AGN system.
The weaker component remained undetected at 5 GHz, due to its steep radio
spectrum. Further study will be necessary to securely rule out a jet--shock
interpretation of the less dominant compact radio source. Assuming the dual AGN
interpretation, we discuss black hole masses, luminosities, and accretion rates
of the two components, using available X-ray, optical, and radio data. While
high-resolution radio interferometric imaging is not an efficient technique to
search blindly for dual AGN, it is an invaluable tool to confirm the existence
of selected candidates.Comment: 7 pages, 2 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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
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