1,195 research outputs found
Near-Field Radio Holography of Large Reflector Antennas
We summarise the mathematical foundation of the holographic method of
measuring the reflector profile of an antenna or radio telescope. In
particular, we treat the case, where the signal source is located at a finite
distance from the antenna under test, necessitating the inclusion of the
so-called Fresnel field terms in the radiation integrals. We assume a ``full
phase'' system with reference receiver to provide the reference phase. We
describe in some detail the hardware and software implementation of the system
used for the holographic measurement of the 12m ALMA prototype submillimeter
antennas. We include a description of the practicalities of a measurement and
surface setting. The results for both the VertexRSI and AEC
(Alcatel-EIE-Consortium) prototype ALMA antennas are presented.Comment: 14 pages, 14 figures, to appear in IEEE Antennas and Propagation
Magazine, Vol. 49, No. 5, October 2007. Version 2 includes nice mug-shots of
the author
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
Evidence for Proportionate Partition Between the Magnetic Field and Hot Gas in Turbulent Cassiopeia A
We present a deep X-ray observation of the young Galactic supernova remnant
Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range
(232 ks) image reveals low-surface-brightness X-ray structure, which appears
qualitatively similar to corresponding radio features. We consider the
correlation between the X-ray and radio morphologies and its physical
implications. After correcting for the inhomogeneous absorption across the
remnant, we performed a point by point (4" resolution) surface brightness
comparison between the X-ray and radio images. We find a strong (r = 0.75)
log-log correlation, implying an overall relationship of . This is
consistent with proportionate partition (and possibly equipartition) between
the local magnetic field and the hot gas --- implying that Cas A's plasma is
fully turbulent and continuously amplifying the magnetic field.Comment: 8 pages with embedded bitmapped figures, Accepted by ApJ Letters
5/1/9
Cicrumnuclear Supernova Remnants and HII Regions in NGC 253
Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm
images of the region surrounding the nuclear 200-pc (~15") starburst in NGC
253. Twenty-two discrete sources stronger than 0.4 mJy have been detected
within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be
associated with the galaxy. None of the radio sources coincides with a detected
X-ray binary, so they appear to be due to supernova remnants and H II regions.
The region outside the central starburst has a derived radio supernova rate of
<~0.1/yr, and may account for at least 20% of the recent star formation in NGC
253. Most of the newly identified sources have steep, nonthermal radio spectra,
but several relatively strong thermal sources also exist, containing the
equivalent of tens of O5 stars. These stars are spread over tens of parsecs,
and are embedded in regions having average ionized gas densities of
20-200/cm^3, much lower than in the most active nuclear star-forming regions in
NGC 253 or in the super star clusters seen in other galaxies. The strongest
region of thermal emission coincides with a highly reddened area seen at
near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for
Astronomical Journal, July 200
A Compact Extreme Scattering Event Cloud Towards AO 0235+164
We present observations of a rare, rapid, high amplitude Extreme Scattering
Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is
compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance
of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a
minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and
timescale of the ESE observed here, we suggest that at least one of the
transients reported by Bower et al. (2007) may be attributed to ESEs.Comment: 11 pages, 2 figure
Mapping the CMB II: the second flight of the QMAP experiment
We report the results from the second flight of QMAP, an experiment to map
the cosmic microwave background near the North Celestial Pole. We present maps
of the sky at 31 and 42 GHz as well as a measurement of the angular power
spectrum covering the l-range 40-200. Anisotropy is detected at about 20 sigma
and is in agreement with previous results at these angular scales. We also
report details of the data reduction and analysis techniques which were used
for both flights of QMAP.Comment: 4 pages, with 5 figures included. Submitted to ApJL. Window functions
and color figures are available at
http://pupgg.princeton.edu/~cmb/welcome.htm
Radio Continuum Evidence for Outflow and Absorption in the Seyfert 1 Galaxy Markarian 231
The VLBA and the VLA have been used to image the continuum radio emission
from Mrk 231, a Seyfert 1 galaxy and the brightest infrared galaxy in the local
universe. The smallest scales reveal a double source less than 2 pc in extent.
The components of this central source have minimum brightness temperatures of
10^9 to 10^{10} K, spectral turnovers between 2 and 10 GHz, and appear to
define the galaxy nucleus plus the inner regions of a jet. The components may
be free-free absorbed or synchtrotron self-absorbed.
On larger scales, the images confirm a previously known north-south triple
source extending 40 pc and elongated perpendicular to a 350-pc starburst disk.
Both lobes show evidence for free-free absorption near 2 GHz, probably due to
ionized gas with a density of 1-2 X 10^3 cm^{-3} in the innermost parts of the
starburst disk. The absorbing gas may be ionized by the active nucleus or by
local regions of enhanced star formation. The elongation of the 40-pc triple
differs by 65 deg from that of the 2-pc source. The different symmetry axes on
different scales imply strong curvature in the inner part of the radio jet.
The radio continuum from the 350-pc disk has a spectral index near -0.4 above
1.4 GHz and is plausibly energized by a massive burst of star formation. On VLA
scales, asymmetric and diffuse emission extends for more than 25 kpc. This
emission has a steep spectrum, linear polarization exceeding 50% at some
locations, and shares the symmetry axis of the 40-pc triple. The diffuse radio
source is probably generated by energy deposition from a slow-moving nuclear
jet, which conceivably could help energize the off-nuclear starburst as well.Comment: 34 pages, 7 Postscript figures, LaTeX file in AASTeX format, accepted
in ApJ, Vol. 516, May 1, 199
The 1.4 GHz light curve of GRB 970508
We report on Westerbork 1.4 GHz radio observations of the radio counterpart
to -ray burst GRB~970508, between 0.80 and 138 days after this event.
The 1.4 GHz light curve shows a transition from optically thick to thin
emission between 39 and 54 days after the event. We derive the slope of the
spectrum of injected electrons () in two
independent ways which yield values very close to . This is in agreement
with a relativistic dynamically near-adiabatic blast wave model whose emission
is dominated by synchrotron radiation and in which a significant fraction of
the electrons cool fast.Comment: Paper I. Accepted for publication in the Astrophysical Journal
Letter
- âŠ