207 research outputs found
Polarimetry of Compact Symmetric Objects
We present multi-frequency VLBA observations of two polarized Compact
Symmetric Objects (CSOs), J0000+4054 and J1826+1831, and a polarized CSO
candidate, J1915+6548. Using the wavelength-squared dependence of Faraday
rotation, we obtained rotation measures (RMs) of -180 \pm 10 rad m^-2 and 1540
\pm 7 rad m^-2 for the latter two sources. These are lower than what is
expected of CSOs (several 1000 rad m^-2) and, depending on the path length of
the Faraday screens, require magnetic fields from 0.03 to 6 \mu G. These CSOs
may be more heavily affected by Doppler boosting than their unpolarized
counterparts, suggesting that a jet-axis orientation more inclined towards the
line of sight is necessary to detect any polarization. This allows for low RMs
if the polarized components are oriented away from the depolarizing
circumnuclear torus. These observations also add a fourth epoch to the proper
motion studies of J0000+4054 and J1826+1831, constraining their kinematic age
estimates to >610 yrs and 2600 \pm 490 yrs, respectively. The morphology,
spectrum, and component motions of J1915+6548 are discussed in light of its new
classification as a CSO candidate, and its angle to the line of sight (~50\deg)
is determined from relativistic beaming arguments.Comment: 29 pages, including 9 figures; Accepted by Astrophysical Journal, 16
Feb 0
Dating COINS: Kinematic Ages for Compact Symmetric Objects
We present multi-epoch VLBA observations of Compact Symmetric Objects (CSOs)
from the COINS sample (CSOs Observed In the Northern Sky). These observations
allow us to make estimates of, or place limits on, the kinematic ages for those
sources with well-identified hot spots. This study significantly increases the
number of CSOs with well-determined ages or limits. The age distribution is
found to be sharply peaked under 500 years, suggesting that many CSOs die
young, or are episodic in nature, and very few survive to evolve into FR II
sources like Cygnus A. Jet components are found to have higher velocities than
hot spots which is consistent with their movement down cleared channels. We
also report on the first detections of significant polarization in two CSOs,
J0000+4054 (2.1%) and J1826+1831 (8.8%). In both cases the polarized emission
is found in jet components on the stronger side of the center of activity.Comment: 34 pages including 7 figures, Accepted to ApJ on Dec 7, 200
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
A survey of Low Luminosity Compact sources and its implication for evolution of radio-loud AGNs. I. Radio data
We present a new sample of Compact Steep Spectrum (CSS) sources with radio
luminosity below 10^26 W/Hz at 1.4 GHz called the low luminosity compact (LLC)
objects. The sources have been selected from FIRST survey and observed with
MERLIN at L-band and C-band. The main criterion used for selection was
luminosity of the objects and approximately one third of the CSS sources from
the new sample have a value of radio luminosity comparable to FRIs. About 80%
of the sources have been resolved and about 30% of them have weak extended
emission and disturbed structures when compared with the observations of higher
luminosity CSS sources. We studied correlation between radio power and linear
size, and redshift with a larger sample that included also published samples of
compact objects and large scale FRIIs and FRIs. The low luminosity compact
objects occupy the space in radio power versus linear size diagram below the
main evolutionary path of radio objects. We suggest that many of them might be
short-lived objects, and their radio emission may be disrupted several times
before becoming FRIIs. We conclude that there exists a large population of
short-lived low luminosity compact objects unexplored so far and part of them
can be precursors of large scale FRIs.Comment: 19 pages, 5 figures, 3 tables, MNRAS in pres
New prediction of extragalactic GeV gamma-ray emission from radio lobes of young AGN jets
We present a new prediction of GeV -ray emission from radio lobes of
young AGN jets. In the previous work of Kino et al. (2007), MeV -ray
bremsstrahlung emission was predicted from young cocoons/radio-lobes in the
regime of no coolings. In this study, we include cooling effects of
bremsstrahlung emission and adiabatic loss. With the initial conditions
determined by observed young radio lobes, we solve a set of equations
describing the expanding lobe evolution. Then we find that the lobes initially
have electron temperature of GeV, and they cool down to MeV by the
adiabatic loss. Correspondingly, the lobes initially yield bright
bremsstrahlung luminosity in GeV range and they fade out. We estimate
these -ray emissions and show that nearby young radio lobes could be
detected with Fermi Gamma-ray Space Telescope.Comment: 5 pages, 3 figures, MNRAS Letters, accepte
Low Power Compact Radio Galaxies at High Angular Resolution
We present sub-arcsecond resolution multi-frequency (8 and 22 GHz)
VLA images of five low power compact (LPC) radio sources, and phase
referenced VLBA images at 1.6 GHz of their nuclear regions. At the VLA
resolution we resolve the structure and identify component positions and flux
densities. The phase referenced VLBA data at 1.6 GHz reveals flat-spectrum,
compact cores (down to a few milliJansky) in four of the five sources. The
absolute astrometry provided by the phase referencing allows us to identify the
center of activity on the VLA images. Moreover, these data reveal rich
structures, including two-sided jets and secondary components. On the basis of
the arcsecond scale structures and of the nuclear properties, we rule out the
presence of strong relativistic effects in our LPCs, which must be
intrinsically small (deprojected linear sizes <~ 10 kpc). Fits of continuous
injection models reveal break frequencies in the GHz domain, and ages in the
range 10^5-10^7 yrs. In LPCs, the outermost edge may be advancing more slowly
than in more powerful sources or could even be stationary; some LPCs might also
have ceased their activity. In general, the properties of LPCs can be related
to a number of reasons, including, but not limited to: youth, frustration, low
kinematic power jets, and short-lived activity in the radio.Comment: 15 pages, 9 .eps figures, accepted by A&
Are young radio sources in equipartition?
The knowledge of physical conditions in young radio sources is important for
defining the framework of models describing radio source evolution. We
investigate whether young radio sources are in equipartition (i.e. minimum
energy) conditions by comparing the equipartition magnetic fields of 5 High
Frequency Peakers (HFP) with values directly inferred from the spectral peak
assumed to be produced by synchrotron self absorption. Multi-frequency VLBA
observations of 5 HFPs were carried out in both the optically thick and thin
part of the spectrum to determine the spectral shape and angular size of the
components for which individual radio spectra were obtained. We find that the
magnetic fields measured using observations agree well with those obtained by
assuming equipartition, which implies that these sources are in minimum energy
condition and the turnover in their spectra is due probably to SSA. In two
source components, we found that the peak of the spectrum is caused by
absorption of a thermal plasma instead of being due to SSA. The magnetic fields
found in the various components range from 10 to 100 mG. In the presence of
such high magnetic fields, electron populations with rather low emit
in the GHz-regime. In one source, we detect low-surface brightness extended
emission at low frequency located ~30 mas (~50 pc) from the main source. This
feature may be related to either an earlier episode of radio activity or a
discontinuous start of the radio activity (sputtering). By comparing our data
with previous VLBA observations, we estimate the hotspot advance speed to be in
the range 0.1-0.7c and kinematic ages of a few hundred years.Comment: 12 pages, 7 figures; accepted for publication in A&
Serum aspirin esterase is strongly associated with glucose and lipids in healthy subjects: different association patterns in subjects with type 2 diabetes mellitus
<p>Abstract</p> <p>Background</p> <p>Aspirin esterase (AE) activity can account for part of aspirin pharmacokinetics in the circulation, possibly being associated with the impairment of aspirin effectiveness as an inhibitor of platelet aggregation.</p> <p>Aims</p> <p>The study was aimed at investigating the correlations of serum AE activity with cholinesterase (ChE) and metabolic variables in healthy subjects in comparison to subjects with type 2 diabetes mellitus (T2DM).</p> <p>Methods</p> <p>In cardiovascular disease-free T2DM subjects and healthy controls, the AE activity levels and/or the correlation patterns between AE and the other variables were analyzed.</p> <p>Results</p> <p>Neither AE nor ChE activities were higher in the subjects with T2DM. Serum AE activity strongly correlated with ChE as well as glucose/lipids variables such as total cholesterol and triglyceride in healthy subjects, while the correlations between AE and glucose/lipids variables were not present in T2DM subjects.</p> <p>Conclusions</p> <p>These data may reflect the pathophysiological changes between healthy and T2DM subjects. Our data may thus provide the basis for future studies to unravel the mechanisms.</p
Constraining the spectral age of very asymmetric CSOs
We constrain the spectral ages for two very asymmetric Compact Symmetric
Objects (CSO) from the B3-VLA-CSS sample, and we investigate the role of the
ambient medium potentially able to influence the individual source evolution.
Multi-frequency VLBA observations have been carried out to study the
distribution of the break frequency of the spectra across different regions of
each source. From the analysis of synchrotron spectra and assuming an
equipartition magnetic field, we find radiative ages of about 2x10^3 and 10^4
years for B0147+400 and B0840+424, respectively. The derived individual
hot-spot advance speed is in the range between 0.03c and 0.3c, in agreement
with kinematic studies carried out on other CSOs. The very asymmetric
morphology found in both sources is likely related to an inhomogeneous ambient
medium in which the sources are growing, rather than to different intrinsic
hot-spot pressures on the two sides.Comment: 8 pages, 1 figure; accepted for pubblication in A&
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