11,294 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
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.
Multi-frequency investigation of the parsec- and kilo-parsec-scale radio structures in high-redshift quasar PKS 1402+044
We investigate the frequency-dependent radio properties of the jet of the
luminous high-redshift (z = 3.2) radio quasar PKS 1402+044 (J1405+0415) by
means of radio interferometric observations. The observational data were
obtained with the VLBI Space Observatory Programme (VSOP) at 1.6 and 5 GHz,
supplemented by other multi-frequency observations with the Very Long Baseline
Array (VLBA; 2.3, 8.4, and 15 GHz) and the Very Large Array (VLA; 1.4, 5, 15,
and 43 GHz). The observations span a period of 7 years. We find that the
luminous high-redshift quasar PKS 1402+044 has a pronounced "core-jet"
morphology from the parsec to the kilo-parsec scales. The jet shows a steeper
spectral index and lower brightness temperature with increasing distance from
the jet core. The variation of brightness temperature agrees well with the
shock-in-jet model. Assuming that the jet is collimated by the ambient magnetic
field, we estimate the mass of the central object as ~10^9 M_sun. The upper
limit of the jet proper motion of PKS 1402+044 is 0.03 mas/yr (~3c) in the
east-west direction.Comment: 9 pages, 6 figures
The radio structure of 3C 316, a galaxy with double-peaked narrow optical emission lines
The galaxy 3C\,316 is the brightest in the radio band among the
optically-selected candidates exhibiting double-peaked narrow optical emission
lines. Observations with the Very Large Array (VLA), Multi-Element Remotely
Linked Interferometer Network (e-MERLIN), and the European VLBI Network (EVN)
at 5\,GHz have been used to study the radio structure of the source in order to
determine the nature of the nuclear components and to determine the presence of
radio cores. The e-MERLIN image of 3C 316 reveals a collimated coherent
east-west emission structure with a total extent of about 3 kpc. The EVN image
shows seven discrete compact knots on an S-shaped line. However, none of these
knots could be unambiguously identified as an AGN core. The observations
suggest that the majority of the radio structure belongs to a powerful radio
AGN, whose physical size and radio spectrum classify it as a compact
steep-spectrum source. Given the complex radio structure with radio blobs and
knots, the possibility of a kpc-separation dual AGN cannot be excluded if the
secondary is either a naked core or radio quiet.Comment: 12 pages, 3 figures, 2 tables. Accepted for publication in the MNRA
Four dual AGN candidates observed with the VLBA
According to hierarchical structure formation models, merging galaxies are
expected to be seen in different stages of their coalescence. However,
currently there are no straightforward observational methods neither to select
nor to confirm a large number of dual active galactic nuclei (AGN) candidates.
Most attempts involve the better understanding of double-peaked narrow emission
line sources, to distinguish the objects where the emission lines originate
from narrow-line kinematics or jet-driven outflows from those which might
harbour dual AGN. We observed four such candidate sources with the Very Long
Baseline Array (VLBA) at 1.5 GHz with 10 milli-arcsecond angular
resolution where spectral profiles of AGN optical emission suggested the
existence of dual AGN. In SDSS J210449.13-000919.1 and SDSS J23044.82-093345.3,
the radio structures are aligned with the optical emission features, thus the
double-peaked emission lines might be the results of jet-driven outflows. In
the third detected source SDSS J115523.74+150756.9, the radio structure is less
extended and oriented nearly perpendicular to the position angle derived from
optical spectroscopy. The fourth source remained undetected with the VLBA but
it has been imaged with the Very Large Array at arcsec resolution a few months
before our observations, suggesting the existence of extended radio structure.
In none of the four sources did we detect two radio-emitting cores, a
convincing signature of duality.Comment: 35 pages, 3 figures, 2 tables, accepted for publication in Ap
VLBI observation of the newly discovered z=5.18 quasar SDSS J0131-0321
Few high-redshift, radio-loud quasars are known to date. The extremely
luminous, radio-bright quasar, SDSS J013127.34-032100.1 was recently discovered
at a redshift of . We observed the source with high resolution very
long baseline interferometry (VLBI) at 1.7 GHz with the European VLBI Network
(EVN) and found a single compact radio component. We estimated a lower limit to
the brightness temperature of the detected radio component, T_B~10^{11} K.
Additionaly, when compared to archival radio data, the source showed
significant flux density variation. These two findings are indicative of the
blazar nature of the source.Comment: 5 pages, 2 figures. Accepted for publication in MNRAS Letter
Minimizing Unsatisfaction in Colourful Neighbourhoods
Colouring sparse graphs under various restrictions is a theoretical problem
of significant practical relevance. Here we consider the problem of maximizing
the number of different colours available at the nodes and their
neighbourhoods, given a predetermined number of colours. In the analytical
framework of a tree approximation, carried out at both zero and finite
temperatures, solutions obtained by population dynamics give rise to estimates
of the threshold connectivity for the incomplete to complete transition, which
are consistent with those of existing algorithms. The nature of the transition
as well as the validity of the tree approximation are investigated.Comment: 28 pages, 12 figures, substantially revised with additional
explanatio
Room temperature and low-field resonant enhancement of spin Seebeck effect in partially compensated magnets
Resonant enhancement of spin Seebeck effect (SSE) due to phonons was recently
discovered in Y3Fe5O12 (YIG). This effect is explained by hybridization between
the magnon and phonon dispersions. However, this effect was observed at low
temperatures and high magnetic fields, limiting the scope for applications.
Here we report observation of phonon-resonant enhancement of SSE at room
temperature and low magnetic field. We observed in Lu2BiFe4GaO12 and
enhancement 700 % greater than that in a YIG film and at very low magnetic
fields around 10-1 T, almost one order of magnitude lower than that of YIG. The
result can be explained by the change in the magnon dispersion induced by
magnetic compensation due to the presence of non-magnetic ion substitutions.
Our study provides a way to tune the magnon response in a crystal by chemical
doping with potential applications for spintronic devices.Comment: 17 pages, 4 figure
Interface roughening with nonlinear surface tension
Using stability arguments, this Brief Report suggests that a term that
enhances the surface tension in the presence of large height fluctuations
should be included in the Kardar-Parisi-Zhang equation. A one-loop
renormalization group analysis then shows for interface dimensions larger than
an unstable strong-coupling fixed point that enters the system
from infinity. The relevance of these results to the roughening transition is
discussed.Comment: 4 pages RevTeX, 1 figur
J1110+4817 -- a compact symmetric object candidate revisited
Compact symmetric objects (CSOs) are radio-emitting active galactic nuclei
(AGNs) typically with a double-lobed radio structure confined to within 1 kpc.
CSOs represent the earliest evolutionary phase of jetted AGNs. Some of them may
eventually evolve into large-scale extended double sources, while others stall
within the host galaxy and die out, depending on the longevity of nuclear
activity, the jet power, and parameters of the surrounding galactic
environment. Studying CSOs is a useful tool for understanding the evolution of
the galaxies and the interactions between the jets and the medium of the host
galaxy. Based on milliarcsec-resolution imaging observations using very long
baseline interferometry (VLBI), it is not always straightforward to distinguish
between a compact double-lobed or a core-jet structure. The quasar J1110+4817
was considered a CSO candidate in the literature earlier, but because of the
lack of clear evidence, it could not be securely classified as a CSO. Here we
present a comprehensive analysis of archival multi-frequency VLBI observations
combined with accurate Gaia optical astrometric information. Lower-frequency
VLBI images reveal an extended radio feature nearly perpendicular to the main
structural axis of the source, apparently emanating from the brighter northern
feature, that is rare among the known CSOs. While the presence of a binary AGN
system cannot be fully excluded, the most plausible explanation is that
J1110+4817 is a CSO.Comment: 8 pages, 4 figures; accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
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