961 research outputs found
On kinetic energy stabilized superconductivity in cuprates
The possibility of kinetic energy driven superconductivity in cuprates as was
recently found in the model is discussed. We argue that the violation of
the virial theorem implied by this result is serious and means that the
description of superconductivity within the model is pathological.Comment: 3 pages, v2 includes additional reference
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
Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548
The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor
the radio continuum counterpart to the optical broad line region (BLR) in the
Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained
at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited
to observations spanning 10-60 days and 3-4 yr, and acquired at matched
resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA
observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is
photometrically variable at 8.4 GHz by % and % between VLA
observations separated by 41 days and 4.1 yr, respectively. The 41-day changes
are milder (%) at 4.9 GHz and exhibit an inverted spectrum (, ) from 4.9 to 8.4 GHz. The nucleus
is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between
VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc
between VLBA observations separated by 3.1 yr. Such photometric variability and
astrometric stability is consistent with a black hole being the ultimate energy
source for the BLR, but is problematic for star cluster models that treat the
BLR as a compact supernova remnant and, for NGC 5548, require a new supernova
event every 1.7 yr within an effective radius 42 mas = 22 pc. A deep
image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data
from quiescent VLA observations. This image shows faint bipolar lobes
straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These
synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from
the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March
10, volume 53
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
Interferometric Phase Calibration Sources in the Declination Range 0deg to -30deg
We present a catalog of 321 compact radio sources in the declination range
0deg > delta > -30deg. The positions of these sources have been measured with a
two-dimensional rms accuracy of 35 milliarcseconds using the NRAO Very Large
Array. Each source has a peak flux density >50 mJy at 8.4 GHz. We intend for
this catalog to be used mainly for selection of phase calibration sources for
radio interferometers, although compact radio sources have other scientific
uses.Comment: 9 pages. To appear in ApJS. Catalog (Table 3) is abbreviated in
printed version. Complete catalog available at
ftp://ftp.aoc.nrao.edu/pub/staff/jwrobel/WPW2003_ApJS.tx
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