1,640 research outputs found
The VLA Low-frequency Sky Survey
The Very Large Array (VLA) Low-frequency Sky Survey (VLSS) has imaged 95% of
the 3*pi sr of sky north of declination = -30 degrees at a frequency of 74 MHz
(4 meter wavelength). The resolution is 80" (FWHM) throughout, and the typical
RMS noise level is ~0.1 Jy/beam. The typical point-source detection limit is
0.7 Jy/beam and so far nearly 70,000 sources have been catalogued. This survey
used the 74 MHz system added to the VLA in 1998. It required new imaging
algorithms to remove the large ionospheric distortions at this very low
frequency throughout the entire ~11.9 degree field of view. This paper
describes the observation and data reduction methods used for the VLSS and
presents the survey images and source catalog. All of the calibrated images and
the source catalog are available online (http://lwa.nrl.navy.mil/VLSS) for use
by the astronomical community.Comment: 53 pages, including 3 tables and 15 figures. Has been accepted for
publication in the Astronomical Journa
GraphSE: An Encrypted Graph Database for Privacy-Preserving Social Search
In this paper, we propose GraphSE, an encrypted graph database for online
social network services to address massive data breaches. GraphSE preserves
the functionality of social search, a key enabler for quality social network
services, where social search queries are conducted on a large-scale social
graph and meanwhile perform set and computational operations on user-generated
contents. To enable efficient privacy-preserving social search, GraphSE
provides an encrypted structural data model to facilitate parallel and
encrypted graph data access. It is also designed to decompose complex social
search queries into atomic operations and realise them via interchangeable
protocols in a fast and scalable manner. We build GraphSE with various
queries supported in the Facebook graph search engine and implement a
full-fledged prototype. Extensive evaluations on Azure Cloud demonstrate that
GraphSE is practical for querying a social graph with a million of users.Comment: This is the full version of our AsiaCCS paper "GraphSE: An
Encrypted Graph Database for Privacy-Preserving Social Search". It includes
the security proof of the proposed scheme. If you want to cite our work,
please cite the conference version of i
Diffuse radio sources in the cluster of galaxies Abell 548b
We report extensive VLA and ATCA observations of the two diffuse radio
sources in the cluster of galaxies Abell 548b, which confirm their
classification as relics. The two relics (named A and B) show similar flux
density, extent, shape, polarization and spectral index and are located at
projected distances of about 430 and 500 kpc from the cluster center, on the
same side of the cluster's X-ray peak. On the basis of spectral indices of
discrete radio sources embedded within the diffuse features, we have attempted
to distinguish emission peaks of the diffuse sources from unrelated sources. We
have found that both relics, in particular the B-relic, show possible fine
structure, when observed at high resolution. Another diffuse source (named C)
is detected close in projection to the cluster center. High-resolution images
show that it contains two discrete radio sources and a diffuse component, which
might be a candidate for a small relic source. The nature and properties of the
diffuse radio sources are discussed. We conclude that they are likely related
to the merger activity in the cluster.Comment: Accepted for publication in MNRAS. 11 pages, 7 figures. Some figures
are degraded to reduce their size. A version with high resolution images is
available at http://www.ira.inaf.it/~lferetti/OUTGOING/papA548b.ps.g
The extraordinary radio galaxy MRC B1221-423: probing deeper at radio and optical wavelengths
We present optical spectra and high-resolution multi-wavelength radio
observations of the compact steep-spectrum radio source MRC B1221-423
(z=0.1706). MRC B1221-423 is a very young (~10^5 yr), powerful radio source
which is undergoing a tidal interaction with a companion galaxy. We find strong
evidence of interaction between the AGN and its environment. The radio
morphology is highly distorted, showing a dramatic interaction between the
radio jet and the host galaxy, with the jet being turned almost back on itself.
H I observations show strong absorption against the nucleus at an infall
velocity of ~250 km/s compared to the stellar velocity, as well as a second,
broader component which may represent gas falling into the nucleus. Optical
spectra show that star formation is taking place across the whole system. Broad
optical emission lines in the nucleus show evidence of outflow. Our
observations confirm that MRC B1221-423 is a young radio source in a gas-rich
nuclear environment, and that there was a time delay of a few x 100 Myr between
the onset of star formation and the triggering of the AGN.Comment: 15 pages, 13 figures, to appear in MNRA
Chandra Observations of the Disruption of the Cool Core in Abell 133
We present the analysis of a Chandra observation of the galaxy cluster Abell
133, which has a cooling flow core, a central radio source, and a diffuse,
filamentary radio source which has been classified as a radio relic.
The X-ray image shows that the core has a complex structure. The most
prominent feature is a "tongue" of emission which extends from the central cD
galaxy to the northwest and partly overlaps the radio relic. One possibility is
that this tongue is produced by Kelvin-Helmholtz (KH) instabilities through the
interaction between the cold gas around the cD galaxy and hot intracluster
medium. We estimate the critical velocity and time scale for the KH instability
to be effective for the cold core around the cD galaxy. We find that the KH
instability can disrupt the cold core if the relative velocity is >~400 km
s^-1. We compare the results with those of clusters in which sharp, undisrupted
cold fronts have been observed; in these clusters, the low temperature gas in
their central regions has a more regular distribution. In contrast to Abell
133, these cluster cores have longer timescales for the disruption of the core
by the KH instability when they are normalized to the timescale of the cD
galaxy motion. Thus, the other cores are less vulnerable to KH instability.
Another possible origin of the tongue is that it is gas which has been uplifted
by a buoyant bubble of nonthermal plasma that we identify with the observed
radio relic. From the position of the bubble and the radio estimate of the age
of the relic source, we estimate avelocity of ~700 km s^-1 for the bubble. The
structure of the bubble and this velocity are consistent with numerical models
for such buoyant bubbles. (abridged)Comment: 38 pages, 15 figures, accepted for publication in Ap
Four Extreme Relic Radio Sources in Clusters of Galaxies
(Abridged) We describe the results of the highest-resolution radio
observations yet made of four relic radio sources in the Abell clusters A13,
A85, A133 and A4038. Our VLA images at 1.4 GHz with 4" resolution show a
remarkable variety of fine structure in the form of spectacular arcs, wisps,
plumes and loops. Their integrated radio flux densities fall very rapidly with
frequency, with power-law slopes between 2.1 and 4.4 near 1.4 GHz The relics
possess linear polarization levels ranging between 2.3 % (A133) and 35 % (A85);
the higher polarization fractions imply a highly ordered magnetic field in the
fine structure and low differential Faraday rotation in the intervening cluster
gas. The optical identification of host galaxies remains problematic. In A85,
A133 and A4038 the travel times for the brightest cluster galaxies are
significantly longer than the modeled ages of the relics and nearby bright
ellipticals provide a better match. Excess X-ray emission in the 0.5 keV-to-2
keV band was found near the relics in A85 and A133. The surface brightness was
too high to be attributed to the inverse-Compton mechanism alone. We found
excellent fits to the broad-band radio spectra using the anisotropic (KGKP)
model of spectral ageing, and we have extended the model to include diffusion
of particles between regions of different field strength (the Murgia-JP, or
MJP, model). The steep radio spectra imply ages for the relics of ~ 10^8 yr, at
the start of which period their radio luminosities would have been ~ 10^25 W/Hz
at 1.4 GHz.Comment: 43 pages, 13 figures, AJ, Sep 2001 (accepted
The evolution of extragalactic radio sources
A model for the evolution of low-luminosity radio galaxies is presented. In
the model, the lobes inflated by low-power jets are assumed to expand in near
pressure-balance against the external medium. Both cases of constant external
pressure and decreasing external pressure are considered. Evolution of an
individual source is described by the power-size track. The source appears as
its lobe is inflated and radio luminosity increases to above the detection
level; the source then moves along the track and eventually disappears as its
luminosity drops below the detection limit. The power-size tracks are
calculated including the combined energy losses due to synchrotron radiation,
adiabatic expansion, and inverse Compton scattering. It is shown that in
general, the constant-pressure model predicts an excess number of luminous,
small-size sources while underpredicting large-size sources in the power-size
diagram. The predicted spectra are steep for most sources, which is
inconsistent with observations. By comparison, the pressure-limiting model fits
observations better. In this model, low-luminosity sources undergo substantial
expansion losses in the initial phase and as a result, it predicts fewer
luminous, small-size sources. The resultant spectra are flat for most sources
except for the oldest ones, which seems consistent with observations. The
power-size tracks, in contrast to that of high-luminosity radio galaxies, are
characterized by a slow increase in luminosity for most of the source's life,
followed by a rapid decline when the synchrotron or inverse Compton scattering
losses set in.Comment: 13 pages, 8 figures, 2 tables, accepted for publication in Ap
VLA multifrequency observations of RS CVn binaries
We present multiepoch Very Large Array (VLA) observations at 1.4 GHz, 4.9
GHz, 8.5 GHz and 14.9 GHz for a sample of eight RS CVn binary systems. Circular
polarization measurements of these systems are also reported. Most of the
fluxes observed are consistent with incoherent emission from mildly
relativistic electrons. Several systems show an increase of the degree of
circular polarization with increasing frequency in the optically thin regime,
in conflict with predictions by gyrosynchrotron models. We observed a reversal
in the sense of circular polarization with increasing frequency in three
non-eclipsing systems: EI Eri, DM Uma and HD 8358. We find clear evidence for
coherent plasma emission at 1.4 GHz in the quiescent spectrum of HD 8358 during
the helicity reversal. The degrees of polarization of the other two systems
could also be accounted for by a coherent emission process. The observations of
ER Vul revealed two U-shaped flux spectra at the highest frequencies. The
U-shape of the spectra may be accounted for by an optically thin
gyrosynchrotron source for the low frequency part whereas the high frequency
part is dominated by a thermal emission component.Comment: 12 pages, 8 figures, LaTeX, uses aa.cls. Accepted for publication in
A&
Radio morphology and spectral analysis of cD galaxies in rich and poor galaxy clusters
We present a radio morphological study and spectral analysis for a sample of
13 cD galaxies in rich and poor clusters of galaxies.} Our study is based on
new high sensitivity Giant Metrewave Radio Telescope (GMRT) observations at
1.28 GHz, 610 MHz and 235 MHz, and on archival data. From a statistical sample
of cluster cD galaxies we selected those sources with little information
available in the literature and promising for the detection of aged radio
emission. Beyond the high sensitivity images for all 13 radio galaxies, we
present also a detailed spectral analysis for 7 of them. We found a variety of
morphologies and linear sizes, as typical for radio galaxies in the radio power
range sampled here (low to intermediate power radio galaxies). The spectral
analysis shows that 10/13 radio galaxies have steep radio spectrum, with
spectral index . In general, the radiative ages and growth
velocities are consistent with previous findings that the evolution of radio
galaxies at the cluster centres is affected by the dense external medium (i.e.
low growth velocities and old ages. We suggest that the dominant galaxies in A
2622 and MKW 03s are dying radio sources, which at present are not fed by
nuclear activity. On the other hand, the spectacular source at the centre of A
2372 might be a very interesting example of restarted radio galaxy. For this
source we estimated a life cycle of the order of 10 yr.Comment: Accepted by A&A, 25 pages, 28 figures, 6 tables and appendix Full
version including high quality images available at
http://www.ira.inaf.it/~tventuri/pap/Venturi.pd
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