1,475 research outputs found
Critical Percolation Phase and Thermal BKT Transition in a Scale-Free Network with Short-Range and Long-Range Random Bonds
Percolation in a scale-free hierarchical network is solved exactly by
renormalization-group theory, in terms of the different probabilities of
short-range and long-range bonds. A phase of critical percolation, with
algebraic (Berezinskii-Kosterlitz-Thouless) geometric order, occurs in the
phase diagram, in addition to the ordinary (compact) percolating phase and the
non-percolating phase. It is found that no connection exists between, on the
one hand, the onset of this geometric BKT behavior and, on the other hand, the
onsets of the highly clustered small-world character of the network and of the
thermal BKT transition of the Ising model on this network. Nevertheless, both
geometric and thermal BKT behaviors have inverted characters, occurring where
disorder is expected, namely at low bond probability and high temperature,
respectively. This may be a general property of long-range networks.Comment: Added explanations and data. Published version. 4pages, 4 figure
An Attempt to Probe the Radio Jet Collimation Regions in NGC 4278, NGC 4374 (M84), and NGC 6166
NRAO Very Long Baseline Array (VLBA) observations of NGC 4278, NGC 4374
(M84), NGC 6166, and M87 (NGC 4486) have been made at 43 GHz in an effort to
image the jet collimation region. This is the first attempt to image the first
three sources at 43 GHz using Very Long Baseline Interferometry (VLBI)
techniques. These three sources were chosen because their estimated black hole
mass and distance implied a Schwarzschild radius with large angular size,
giving hope that the jet collimation regions could be studied. Phase
referencing was utilize for the three sources because of their expected low
flux densities. M87 was chosen as the calibrator for NGC 4374 because it
satisfied the phase referencing requirements: nearby to the source and
sufficiently strong. Having observed M87 for a long integration time, we have
detected its sub-parsec jet, allowing us to confirm previous high resolution
observations made by Junor, Biretta & Livio, who have indicated that a wide
opening angle was seen near the base of the jet. Phase referencing successfully
improved our image sensitivity, yielding detections and providing accurate
positions for NGC 4278, NGC 4374 and NGC 6166. These sources are point
dominated, but show suggestions of extended structure in the direction of the
large-scale jets. However, higher sensitivity will be required to study their
sub-parsec jet structure
X-ray Properties of Radio-Selected Dual Active Galactic Nuclei
Merger simulations predict that tidally induced gas inflows can trigger
kpc-scale dual active galactic nuclei (dAGN) in heavily obscured environments.
Previously with the Very Large Array, we have confirmed four dAGN with
redshifts between and projected separations between 4.3 and
9.2 kpc in the SDSS Stripe 82 field. Here, we present X-ray
observations that spatially resolve these dAGN and compare their
multi-wavelength properties to those of single AGN from the literature. We
detect X-ray emission from six of the individual merger components and obtain
upper limits for the remaining two. Combined with previous radio and optical
observations, we find that our dAGN have properties similar to nearby
low-luminosity AGN, and they agree well with the black hole fundamental plane
relation. There are three AGN-dominated X-ray sources, whose X-ray
hardness-ratio derived column densities show that two are unobscured and one is
obscured. The low obscured fraction suggests these dAGN are no more obscured
than single AGN, in contrast to the predictions from simulations. These three
sources show an apparent X-ray deficit compared to their mid-infrared continuum
and optical [OIII] line luminosities, suggesting higher levels of obscuration,
in tension with the hardness-ratio derived column densities. Enhanced
mid-infrared and [OIII] luminosities from star formation may explain this
deficit. There is ambiguity in the level of obscuration for the remaining five
components since their hardness ratios may be affected by non-nuclear X-ray
emissions, or are undetected altogether. They require further observations to
be fully characterized.Comment: 11 pages, 5 figures, Accepted for publication in the Astrophysical
Journa
Deep VLBI Imaging of Faint Radio Sources in the NOAO Bootes Field
We have conducted a deep, very long baseline interferometry (VLBI)
observation at 1.4 GHz of an area of sky located within the NOAO Bootes field,
using the NRAO VLBA and 100-m Green Bank Telescope. Applying wide-field VLBI
techniques, a total of 61 sources, selected from a WSRT image, were surveyed
simultaneously with a range of different sensitivities and resolutions. The
inner 0-2', of the field reached an unprecedented 1-sigma rms noise level ~ 9
uJy/beam and yielded 2 detections. A further 7 sources were detected in the
rest of the field. All of the sources have a brightness temperature in excess
of 10^5 K; suggesting they are AGN. Optical identifications are available for 8
of the 9 VLBI detections - only VLBI J142906.6095 remains unidentified (I >
25.6m). Two sources are not detected in K-band (K > 18.5m) suggesting that some
significant fraction of these compact radio sources may be located at z > 1.
The VLBI detection rate for sub-mJy radio sources is 8^{+4}_{-5} %. The VLBI
detection rate for mJy sources is higher, 29^{+11}_{-12}%. This trend is
expected if the radio emission associated with fainter sub-mJy and microJy
sources increasingly arises from extended regions of star formation. The 9 VLBI
detections pin-point the precise location of AGN or candidate AGN, and their
VLBI positions can help to anchor the NOAO Bootes field to the ICRF. The
simultaneous detection of several sub-mJy and mJy radio sources, in a single
observation, suggest that their combined response may be used to self-calibrate
wide-field VLBI data. Future VLBI observations of faint sub-mJy and microJy
radio sources can take full advantage of this "full-beam" calibration
technique.Comment: ApJ in press, 36 pages. Reduced abstract presented her
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