6,575 research outputs found
The giant radio galaxy 8C0821+695 and its environment
We present new VLA and Effelsberg observations of the radio galaxy
8C0821+695. We have obtained detailed images in total intensity and
polarization of this 2 Mpc sized giant. The magnetic field has a configuration
predominantly parallel to the source main axis. We observe Faraday rotation at
low frequencies, most probably produced by an ionized medium external to the
radio source. The spectral index distribution is that typical of FR II radio
galaxies, with spectral indices gradually steepening from the source extremes
towards the core. Modeling the spectrum in the lobes using standard synchrotron
loss models yields the spectral age of the source and the mean velocity of the
jet-head with respect to the lobe material. The existence of a possible
backflow in the lobe is considered to relate spectral with dynamical
determinations of the age and the velocity with respect to the external medium.
Through a very simple model, we obtain a physical characterization of the jets
and the external medium in which the radio galaxy expands. The results in
8C0821+695 are consistent with a relativistic jet nourishing the lobes which
expand in a hot, low density halo. We infer a deceleration of the source
expansion velocity which we explain through a progressive increase in the
hot-spot size.Comment: 11 pages; 8 figures; accepted in A&
The Broad Line Radio Galaxy J2114+820
In the frame of the study of a new sample of large angular size radio
galaxies selected from the NRAO VLA Sky Survey, we have made radio observations
of J2114+820, a low power radio galaxy with an angular size of 6'. Its radio
structure basically consists of a prominent core, a jet directed in north-west
direction and two extended S-shaped lobes. We have also observed the optical
counterpart of J2114+820, a bright elliptical galaxy with a strong unresolved
central component. The optical spectrum shows broad emission lines. This fact,
together with its low radio power and FR-I type morphology, renders J2114+820 a
non-trivial object from the point of view of the current unification schemes of
radio loud active galactic nuclei.Comment: 6 pages, 5 figures. To appear in the proceedings of EVN/JIVE
Symposium No. 4, New Astronomy Reviews (eds. Garrett et al.
Lipodystrophy syndrome in HIV-infected children on HAART
Lipodystrophy syndrome (LD) is common in HIV-infected children, particularly those taking didanosine, stavudine or zidovudine. Lipo-atrophy in particular causes major stigmatisation and interferes with adherence. In addition, LD may have significant long-term health consequences, particularly cardiovascular. Since the stigmatising fat distribution changes of LD are largely permanent, the focus of management remains on early detection and arresting progression. Practical guidelines for surveillance and avoidance of LD in routine clinical practice are presented. The diagnosis of LD is described and therapeutic options are reviewed. The most important therapeutic intervention is to switch the most likely offending antiretroviral to a non-LD-inducing agent as soon as LD is recognised. Typically, when lipo-atrophy or lipohypertrophy is diagnosed the thymidine nucleoside reverse transcriptase inhibitor (NRTI) is switched to a non-thymidine agent such as abacavir (or tenofovir in adults).Where dyslipidaemia is predominant, a dietician review is helpful, and the clinician may consider switching to a protease inhibitor-sparing regimen or to atazanavir
B2 1144+35: A Giant Low Power Radio Galaxy with Superluminal Motion
We report on centimeter VLA and VLBI observations of the giant, low power
radio galaxy 1144+35. These observations are sensitive to structures on scales
from less than 1 parsec to greater than 1 megaparsec. Diffuse steep spectrum
lobes on the megaparsec scale are consistent with an age of 10
years. On the parsec scale, a complex jet component is seen to move away from
the center of activity with an apparent velocity 2.7 h c. It shows
a central spine -- shear layer morphology. A faint parsec scale counterjet is
detected and an intrinsic jet velocity of 0.95 c and angle to the line of sight
of 25 are derived, consistent with an intrinsically symmetric ejection.
The central spine in the parsec scale jet is expected to move at a higher
velocity and a Lorentz factor 15 has been estimated near the
core.The age of this inner VLBI structure is 300 years. Assuming a
constant angle to the line-of-sight, the jet velocity is found to decrease from
0.95 c at 20 mas (32 pc on the plane of the sky) to 0.02 c at 15 arcsec (24 kpc
on the plane of the sky). These findings lend credence to the claim that (1)
even the jets of low power radio galaxies start out relativistic; and (2) these
jets are decelerated to subrelativistic velocities by the time they reach
kiloparsec scales.Comment: 21 pages, 16 separated figures. A version with figures and table in
the text is available at: ftp://terra.bo.cnr.it/papers/journals - it is a ps
gzipped file, named giovannini_apr99.gz (792kb) - ApJ in pres
Radio Continuum Emission from the Magnetar SGR J1745-2900: Interaction with Gas Orbiting Sgr A*
We present radio continuum light curves of the magnetar SGR J17452900 and
Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations
between 2012 and 2014. During this period, a powerful X-ray outburst from SGR
J17452900 occurred on 2013-04-24. Enhanced radio emission is delayed with
respect to the X-ray peak by about seven months. In addition, the flux density
of the emission from the magnetar fluctuates by a factor of 2 to 4 at
frequencies between 21 and 41 GHz and its spectral index varies erratically.
Here we argue that the excess fluctuating emission from the magnetar arises
from the interaction of a shock generated from the X-ray outburst with the
orbiting ionized gas at the Galactic center. In this picture, variable
synchrotron emission is produced by ram pressure variations due to
inhomogeneities in the dense ionized medium of the Sgr A West bar. The pulsar
with its high transverse velocity is moving through a highly blue-shifted
ionized medium. This implies that the magnetar is at a projected distance of
pc from Sgr A* and that the orbiting ionized gas is partially or
largely responsible for a large rotation measure detected toward the magnetar.
Despite the variability of Sgr A* expected to be induced by the passage of the
G2 cloud, monitoring data shows a constant flux density and spectral index
during this periodComment: 12 pages, 3 figures, ApJL (in press
Surface roughness and thermal conductivity of semiconductor nanowires: going below the Casimir limit
By explicitly considering surface roughness at the atomic level, we
quantitatively show that the thermal conductivity of Si nanowires can be lower
than Casimir's classical limit. However, this violation only occurs for deep
surface degradation. For shallow surface roughness, the Casimir formula is
shown to yield a good approximation to the phonon mean free paths and
conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact
treatment of roughness scattering is in stark contrast with a previously
proposed perturbative approach, which is found to overpredict scattering rates
by an order of magnitude. The obtained results suggest that a complete
theoretical understanding of some previously published experimental results is
still lacking.Comment: 11 pages, 4 figure
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