1,469 research outputs found
Transverse motions in CSOs?
The measurement of proper motions in CSOs is a powerful tool to determine the
dynamical evolution of the newly born extragalactic radio sources. We observed
3 CSOs with the VLBA in 2004 and in 2006 to monitor changes in their structure
and measure the separation velocity of the hot spots. It is important to
increase the size of the samples of CSOs with measured expansion velocity to
test the existance of frustrated objects, and put stringent constraints on the
current models. We found for all the three objects observed a transverse motion
of the hotspots, and we suggest as the more likey explanation a precession in
the jet axis. This behaviour likely inhibits or at least slows down the radio
source growth because the head of the hotspot continuously hits new regions of
the ISM. Therefore these radio sources may represent an old population of
GPS/CSOs.Comment: 4 pages, 3 figures. Accepted for publication in Astronomische
Nachrichte
High Frequency Peakers: young radio sources or flaring blazars?
We present new, simultaneous, multifrequency observations of 45 out of the 55
candidate High Frequency Peakers (HFP) selected by Dallacasa et al. (2000),
carried out 3 to 4 years after a first set of observations. Our sub-sample
consists of 10 galaxies, 28 stellar objects (``quasars'') and 7 unidentified
sources. Both sets of observations are sensitive enough to allow the detection
of variability at the 10% level or lower. While galaxies do not show
significant variability, most quasars do. Seven of them no longer show the
convex spectrum which is the defining property of Gigahertz Peaked Spectrum
(GPS)/HFP sources and are interpreted as blazars caught by Dallacasa et al.
(2000) during a flare, when a highly self-absorbed component dominated the
emission. In general, the variability properties (amplitude, timescales,
correlation between peak luminosity and peak frequency of the flaring
component) of the quasar sub-sample resemble those of blazars. We thus conclude
that most HFP candidates identified with quasars may well be flaring blazars.Comment: 20 pages, 18 figures, accepted for publication in Astronomy &
Astrophysic
Physical properties in young radio sources. VLBA observations of high frequency peaking radio sources
Multifrequency Very Long Baseline Array (VLBA) observations were performed to
study the radio morphology and the synchrotron spectra of four high frequency
peaking radio sources. They are resolved in several compact components and the
radio emission is dominated by the hotspots/lobes. The core region is
unambiguously detected in J1335+5844 and J1735+5049. The spectra of the main
source components peak above 3 GHz. Assuming that the spectral peak is produced
by synchrotron self-absorption, we estimate the magnetic field directly from
observable quantities and in half of the components it agrees with the
equipartition field, while in the others the difference exceeds an order of
magnitude. By comparing the physical properties of the targets with those of
larger objects we found that the luminosity increases with the linear size for
sources smaller than a few kpc, while it decreases for larger objects. The
asymmetric sources J1335+5844 and J1735+5049 suggest that the ambient medium is
inhomogeneous and is able to influence the evolution of the radio emission even
during its first stages. The core luminosity increases with the linear size for
sources up to a few kpc, while it seems constant for larger sources suggesting
an evolution independent from the source total luminosity.Comment: 16 pages, 10 figures, accepted for publication in MNRA
High Frequency Peakers: The Faint Sample
We present a sample of sources with convex radio spectra peaking at
frequencies above a few GHz, known as "High Frequency Peakers" (HFPs). A
"bright" sample with a flux density limit of 300 mJy at 5 GHz has been
presented by Dallacasa et al. (2000). Here we present the "faint" sample with
flux density between 50 and 300 mJy at 5GHz, restricted to the area around the
North Galactic Cap, where the FIRST catalogue is available. The candidates have
been observed with the VLA at several frequencies ranging from 1.4 to 22 GHz,
in order to derive a simultaneous radio spectrum. The final list of confirmed
HFP sources consists of 61 objects.Comment: 3 pages. Accepted for publication in Astronomische Nachrichte
VLBA images of High Frequency Peakers
We propose a morphological classification based on the parsec scale structure
of fifty-one High Frequency Peakers (HFPs) from the ``bright'' HFP sample. VLBA
images at two adjacent frequencies (chosen among 8.4, 15.3, 22.2 and 43.2 GHz)
have been used to investigate the morphological properties of the HFPs in the
optically thin part of their spectrum. We confirm that there is quite a clear
distinction between the pc-scale radio structure of galaxies and quasars: the
78% of the galaxies show a ``Double/Triple'' morphology, typical of Compact
Symmetric Objects (CSOs), while the 87% of the quasars are characterised by
Core-Jet or unresolved structure. This suggests that most HFP candidates
identified with quasars are likely blazar objects in which a flaring
self-absorbed component at the jet base was outshining the remainder of the
source at the time of the selection based on the spectral shape. Among the
sources classified as CSOs or candidates it is possible to find extremely young
radio sources with ages of about 100 years or even less.Comment: 21 pages, 8 figures; accepted for pubblication in A&A. Paper version
with full resolution images is available at
http://www.ira.inaf.it/~ddallaca/orienti.p
Proper motion and apparent contraction in J0650+6001
We present a multi-epoch and multi-frequency VLBI study of the compact radio
source J0650+6001. In VLBI images the source is resolved into three components.
The central component shows a flat spectrum, suggesting the presence of the
core, while the two outer regions, with a steeper spectral index, display a
highly asymmetric flux density. The time baseline of the observations
considered to derive the source expansion covers about 15 years. During this
time interval, the distance between the two outer components has increased by
0.28+/-0.13 mas, that corresponds to an apparent separation velocity of
0.39c+/-0.18c and a kinematic age of 360+/-170 years. On the other hand, a
multi-epoch monitoring of the separation between the central and the southern
components points out an apparent contraction of about 0.29+/-0.02 mas,
corresponding to an apparent contraction velocity of 0.37c+/-0.02c. Assuming
that the radio structure is intrinsically symmetric, the high flux density
ratio between the outer components can be explained in terms of Doppler beaming
effects where the mildly relativistic jets are separating with an intrinsic
velocity of 0.43c+/-0.04c at an angle between 12 and 28 degrees to the line of
sight. In this context, the apparent contraction may be interpreted as a knot
in the jet that is moving towards the southern component with an intrinsic
velocity of 0.66c+/-0.03c, and its flux density is boosted by a Doppler factor
of 2.0.Comment: 7 pages, 5 pages. Accepted for publication in MNRA
Weak CSS Sources from FIRST Survey
We report early results of an observational campaign targeted on a sample of
compact steep spectrum sources selected from the FIRST survey which are
significantly weaker than those investigated before. The selection criteria and
procedure are given in detail. We present here an assortment of MERLIN and VLBI
observations and make some general comments based on the morphologies of the
sources presented.Comment: A contribution to The Third Workshop on GHz-Peaked Spectrum and
Compact Steep Spectrum Radio Sources, Kerastari, Greece, May 28-31, 2002.
Refereed and accepted by Publications of the Astronomical Society of
Australia. Final version copyedited by PASA Edito
A giant radio halo in the massive and merging cluster Abell 1351
We report on the detection of diffuse radio emission in the X-ray luminous
and massive galaxy cluster A1351 (z=0.322) using archival Very Large Array data
at 1.4 GHz. Given its central location, morphology, and Mpc-scale extent, we
classify the diffuse source as a giant radio halo. X-ray and weak lensing
studies show A1351 to be a system undergoing a major merger. The halo is
associated with the most massive substructure. The presence of this source is
explained assuming that merger-driven turbulence may re-accelerate high-energy
particles in the intracluster medium and generate diffuse radio emission on the
cluster scale. The position of A1351 in the logP - logL plane
is consistent with that of all other radio-halo clusters known to date,
supporting a causal connection between the unrelaxed dynamical state of massive
() clusters and the presence of giant radio halos.Comment: 4 pages, 3 figures, proof corrections include
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