3,217 research outputs found
GMRT observations of X-shaped radio sources
We present results from a study of X-shaped sources based on observations
using the Giant Metrewave Radio Telescope (GMRT). These observations were
motivated by our low frequency study of 3C 223.1 (Lal & Rao 2005), an X-shaped
radio source, which showed that the wings (or low-surface-brightness jets) have
flatter spectral indices than the active lobes (or high-surface-brightness
jets), a result not easily explained by most models. We have now obtained GMRT
data at 240 and 610 MHz for almost all the known X-shaped radio sources and
have studied the distribution of the spectral index across the sources. While
the radio morphologies of all the sources at 240 and 610 MHz show the
characteristic X-shape, the spectral characteristics of the X-shaped radio
sources, seem to fall into three categories, namely, sources in which (A) the
wings have flatter spectral indices than the active lobes, (B) the wings and
the active lobes have comparable spectral indices, and (C) the wings have
steeper spectral indices than the active lobes. We discuss the implications of
the new observational results on the various formation models that have been
proposed for X-shaped sources.Comment: The paper contains 12 figures and 3 tables, accepted for publication
in MNRAS Main Journal, please note, some figures are of lower qualit
The two-component giant radio halo in the galaxy cluster Abell 2142
We report on a spectral study at radio frequencies of the giant radio halo in
A2142 (z=0.0909), which we performed to explore its nature and origin. A2142 is
not a major merger and the presence of a giant radio halo is somewhat
surprising. We performed deep radio observations with the GMRT at 608 MHz, 322
MHz, and 234 MHz and with the VLA in the 1-2 GHz band. We obtained high-quality
images at all frequencies in a wide range of resolutions. The radio halo is
well detected at all frequencies and extends out to the most distant cold front
in A2142. We studied the spectral index in two regions: the central part of the
halo and a second region in the direction of the most distant south-eastern
cold front, selected to follow the bright part of the halo and X-ray emission.
We complemented our observations with a preliminary LOFAR image at 118 MHz and
with the re-analysis of archival VLA data at 1.4 GHz. The two components of the
radio halo show different observational properties. The central brightest part
has higher surface brightess and a spectrum whose steepness is similar to those
of the known radio halos, i.e. . The ridge, which fades into the larger scale emission, is broader in
size and has considerably lower surface brightess and a moderately steeper
spectrum, i.e. . We propose that
the brightest part of the radio halo is powered by the central sloshing in
A2142, similar to what has been suggested for mini-halos, or by secondary
electrons generated by hadronic collisions in the ICM. On the other hand, the
steeper ridge may probe particle re-acceleration by turbulence generated either
by stirring the gas and magnetic fields on a larger scale or by less energetic
mechanisms, such as continuous infall of galaxy groups or an off-axis merger.Comment: 18 pages, 10 figures, 4 tables - A&A, accepte
Junction of several weakly interacting quantum wires: a renormalization group study
We study the conductance of three or more semi-infinite wires which meet at a
junction. The electrons in the wires are taken to interact weakly with each
other through a short-range density-density interaction, and they encounter a
general scattering matrix at the junction. We derive the renormalization group
equations satisfied by the S-matrix, and we identify its fixed points and their
stabilities. The conductance between any pair of wires is then studied as a
function of physical parameters such as temperature. We discuss the possibility
of observing the effects of junctions in present day experiments, such as the
four-terminal conductance of a quantum wire and crossed quantum wires.Comment: RevTeX, 13 pages, including 4 eps figure
Complex diffuse radio emission in the merging PLANCK ESZ cluster Abell 3411
We present VLA radio and Chandra X-ray observations of the merging galaxy
cluster Abell 3411. For the cluster, we find an overall temperature of 6.4 keV
and an X-ray luminosity of 2.8 x 10^{44} erg s^{-1} between 0.5 and 2.0 keV.
The Chandra observation reveals the cluster to be undergoing a merger event.
The VLA observations show the presence of large-scale diffuse emission in the
central region of the cluster, which we classify as a 0.9 Mpc size radio halo.
In addition, a complex region of diffuse, polarized emission is found in the
southeastern outskirts of the cluster, along the projected merger axis of the
system. We classify this region of diffuse emission as a radio relic. The total
extent of this radio relic is 1.9 Mpc. For the combined emission in the cluster
region, we find a radio spectral index of -1.0 \pm 0.1 between 74 MHz and 1.4
GHz. The morphology of the radio relic is peculiar, as the relic is broken up
into five fragments. This suggests that the shock responsible for the relic has
been broken up due to interaction with a large-scale galaxy filament connected
to the cluster or other substructures in the ICM. Alternatively, the complex
morphology reflects the presence of electrons in fossil radio bubbles that are
re-accelerated by a shock.Comment: Accepted for publication in ApJ, 8 pages, 5 figure
Metal chaperones prevent zinc-mediated cognitive decline
© 2014 Elsevier Inc. Zinc transporter-3 (ZnT3) protein is responsible for loading zinc into presynaptic vesicles and consequently controls the availability of zinc at the glutamatergic synapse. ZnT3 has been shown to decline with age and in Alzheimer's disease (AD) and is crucially involved in learning and memory. In this study, we utilised whole animal behavioural analyses in the ZnT3 KO mouse line, together with electrophysiological analysis of long-term potentiation in brain slices from ZnT3 KO mice, to show that metal chaperones (clioquinol, 30 mg/kg/day for 6 weeks) can prevent the age-dependent cognitive phenotype that characterises these animals. This likely occurs as a result of a homeostatic restoration of synaptic protein expression, as clioquinol significantly restored levels of various pre- and postsynaptic proteins that are critical for normal cognition, including PSD-95; AMPAR and NMDAR2b. We hypothesised that this clioquinol-mediated restoration of synaptic health resulted from a selective increase in synaptic zinc content within the hippocampus. While we demonstrated a small regional increase in hippocampal zinc content using synchrotron x-ray fluorescence microscopy, further sub-region analyses are required to determine whether this effect is seen in other regions of the hippocampal formation that are more closely linked to the synaptic plasticity effects observed in this study. These data support our recent report on the use of a different metal chaperone (PBT2) to prevent normal age-related cognitive decline and demonstrate that metal chaperones are efficacious in preventing the zinc-mediated cognitive decline that characterises ageing and disease
NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901
AX J1745.6-2901 is a high-inclination (eclipsing) transient neutron star (NS)
Low Mass X-ray Binary (LMXB) showcasing intense ionised Fe K absorption. We
present here the analysis of 11 XMM-Newton and 15 NuSTAR new data-sets
(obtained between 2013-2016), therefore tripling the number of observations of
AX J1745.6-2901 in outburst. Thanks to simultaneous XMM-Newton and NuSTAR
spectra, we greatly improve on the fitting of the X-ray continuum. During the
soft state the emission can be described by a disk black body (
keV and inner disc radius km), plus hot ( keV)
black body radiation with a small emitting radius ( km)
likely associated with the boundary layer or NS surface, plus a faint
Comptonisation component. Imprinted on the spectra are clear absorption
features created by both neutral and ionised matter. Additionally, positive
residuals suggestive of an emission Fe K disc line and consistent with
relativistic ionised reflection are present during the soft state, while such
residuals are not significant during the hard state. The hard state spectra are
characterised by a hard () power law, showing no evidence
for a high energy cut off ( keV) and implying a small optical
depth (). The new observations confirm the previously witnessed trend
of exhibiting strong Fe K absorption in the soft state, that significantly
weakens during the hard state. Optical (GROND) and radio (GMRT) observations
suggest for AX J1745.6-2901 a standard broad band SED as typically observed in
accreting neutron stars.Comment: Accepted for publication in MNRA
Transport through quasi-ballistic quantum wires: the role of contacts
We model one-dimensional transport through each open channel of a quantum
wire by a Luttinger liquid with three different interaction parameters for the
leads, the contact regions and the wire, and with two barriers at the contacts.
We show that this model explains several features of recent experiments, such
as the flat conductance plateaux observed even at finite temperatures and for
different lengths, and universal conductance corrections in different channels.
We discuss the possibility of seeing resonance-like structures of a fully open
channel at very low temperatures.Comment: revtex, 5 pages, 1 eps figure; clarifications added in light of new
experiment
Radio-continuum spectra of ram pressure stripped galaxies in the Coma Cluster
We used the nearby Coma Cluster as a laboratory in order to probe the
impact of ram pressure on star formation as well as to constrain the
characteristic timescales and velocities for the stripping of the non-thermal
ISM. We used high-resolution (),
multi-frequency () radio continuum
imaging of the Coma Cluster to resolve the low-frequency radio spectrum across
the discs and tails of 25 ram pressure stripped galaxies. With resolved
spectral index maps across these galaxy discs, we constrained the impact of ram
pressure perturbations on galaxy star formation. We measured multi-frequency
flux-density profiles along each of the ram pressure stripped tails in our
sample. We then fit the resulting radio continuum spectra with a simple
synchrotron aging model. We showed that ram pressure stripped tails
in Coma have steep () spectral indices. The
discs of galaxies undergoing ram pressure stripping have integrated spectral
indices within the expected range for shock acceleration from supernovae (), though there is a tail towards flatter values.
In a resolved sense, there are gradients in spectral index across the discs of
ram pressure stripped galaxies in Coma. These gradients are aligned with the
direction of the observed radio tails, with the flattest spectral indices being
found on the `leading half'. From best-fit break frequencies we estimated the
projected plasma velocities along the tail to be on the order of hundreds of
kilometers per second, with the precise magnitude depending on the assumed
magnetic field strength.Comment: 18 pages, 10 figures, 2 appendices, accepted for publication in A&
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