3,656 research outputs found
Radio Variability of Sagittarius A* - A 106 Day Cycle
We report the presence of a 106-day cycle in the radio variability of Sgr A*
based on an analysis of data observed with the Very Large Array (VLA) over the
past 20 years. The pulsed signal is most clearly seen at 1.3 cm with a ratio of
cycle frequency to frequency width f/Delta_f= 2.2+/-0.3. The periodic signal is
also clearly observed at 2 cm. At 3.6 cm the detection of a periodic signal is
marginal. No significant periodicity is detected at both 6 and 20 cm. Since the
sampling function is irregular we performed a number of tests to insure that
the observed periodicity is not the result of noise. Similar results were found
for a maximum entropy method and periodogram with CLEAN method. The probability
of false detection for several different noise distributions is less than 5%
based on Monte Carlo tests. The radio properties of the pulsed component at 1.3
cm are spectral index alpha ~ 1.0+/- 0.1 (for S nu^alpha), amplitude Delta
S=0.42 +/- 0.04 Jy and characteristic time scale Delta t_FWHM ~ 25 +/- 5 days.
The lack of VLBI detection of a secondary component suggests that the
variability occurs within Sgr A* on a scale of ~5 AU, suggesting an instability
of the accretion disk.Comment: 14 Pages, 3 figures. ApJ Lett 2000 accepte
VLA Detection of the Ionized Stellar Winds Arising from Massive Stars in the Galactic Center Arches Cluster
The Galactic center Arches stellar cluster, detected and studied until now
only in the near-infrared, is comprised of at least one hundred massive (M>20
Msun) stars. Here we report the detection at centimeter wavelengths of radio
continuum emission from eight radio sources associated with the cluster. Seven
of these radio sources have rising spectral indices between 4.9 and 8.5 GHz and
coincide spatially with the brightest stars in the cluster, as determine from
JHK photometry and Brackett alpha and Brackett Gamma spectroscopy. Our results
confirm the presence of powerful ionized winds in these stars. The eighth radio
source has a nonthermal spectrum and its nature is yet unclear, but it could be
associated with a lower mass young star in the cluster.Comment: 6 pages, 2 embedded figures, accepted to ApJLetter
NGC 3576 and NGC 3603: Two Luminous Southern HII Regions Observed at High Resolution with the Australia Telescope Compact Array
NGC 3576 (G291.28-0.71; l=291.3o, b=-0.7o) and NGC 3603 (G291.58-0.43;
l=291.6o, b=-0.5o) are optically visible, luminous HII regions located at
distances of 3.0 kpc and 6.1 kpc, respectively. We present 3.4 cm Australian
Telescope Compact Array (ATCA) observations of these two sources in the
continuum and the H90a, He90a, C90a and H113b recombination lines with an
angular resolution of 7" and a velocity resolution of 2.6 km/s. All four
recombination lines are detected in the integrated profiles of the two sources.
Broad radio recombination lines are detected in both NGC 3576 (DV_{FWHM}>= 50
km/s) and NGC 3603 (DV_{FWHM}>=70 km/s). In NGC 3576 a prominent N-S velocity
gradient (~30 km/s/pc) is observed, and a clear temperature gradient (6000 K to
8000 K) is found from east to west, consistent with a known IR color gradient
in the source. In NGC 3603, the H90a, He90a and the H113b lines are detected
from 13 individual sources. The Y^+ (He/H) ratios in the two sources range from
0.08+/-0.04 to 0.26+/-0.10. We compare the morphology and kinematics of the
ionized gas at 3.4 cm with the distribution of stars, 10 micron emission and
H_2O, OH, and CH_3OH maser emission. These comparisons suggest that both NGC
3576 and NGC 3603 have undergone sequential star formation.Comment: 24 pages, 12 Postscript figure
Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138
We present three epochs of VLBA observations of Galactic HI absorption toward
the quasar 3C~138 with resolutions of 20 mas (~ 10 AU). This analysis includes
VLBA data from observations in 1999 and 2002 along with a reexamination of 1995
VLBA data. Improved data reduction and imaging techniques have led to an order
of magnitude improvement in sensitivity compared to previous work. With these
new data we confirm the previously detected milliarcsecond scale spatial
variations in the HI opacity at the level of Delta(tau_{max}) =0.50 \pm 0.05.
The typical size scale of the optical depth variations is ~ 50 mas or 25 AU. In
addition, for the first time we see clear evidence for temporal variations in
the HI opacity over the seven year time span of our three epochs of data. We
also attempted to detect the magnetic field strength in the HI gas using the
Zeeman effect. From this analysis we have been able to place a 3 sigma upper
limit on the magnetic field strength per pixel of ~45 muG. We have also been
able to calculate for the first time the plane of sky covering fraction of the
small scale HI gas of ~10%. We also find that the line widths of the
milliarcsecond sizescale HI features are comparable to those determined from
previous single dish measurements toward 3C~138, suggesting that the opacity
variations cannot be due to changes in the HI spin temperature. From these
results we favor a density enhancement interpretation for the small scale HI
structures, although these enhancements appear to be of short duration and are
unlikely to be in equilibrium.Comment: 34 pages, 8 figures. Figures 3 & 4 are in color. Accepted to A
Wide area detection system: Conceptual design study
An integrated sensor for traffic surveillance on mainline sections of urban freeways is described. Applicable imaging and processor technology is surveyed and the functional requirements for the sensors and the conceptual design of the breadboard sensors are given. Parameters measured by the sensors include lane density, speed, and volume. The freeway image is also used for incident diagnosis
Hyperbaric oxygen in the prevention of osteoradionecrosis of the jaws
The document attached has been archived with permission from the Australian Dental Association (23rd Jan 2008). An external link to the publisherâs copy is included.Patients who have had their jaws irradiated as part of management of head and neck malignancy are at risk of osteoradionecrosis (ORN) following tooth extraction. Thirty-seven patients with a history of irradiation to the jaws were managed during a four year period. Twenty-nine patients received hyperbaric oxygen therapy (HBO) consisting of 20 treatments after. Only one (4 per cent) developed ORN. Seven patients who did not have HBO and one who did (15 per cent) developed ORN. The need for prophylactic treatment with HBO is discussed. It is recommended that prophylactic HBO is used prior to surgery for irradiated facial bone.S. Vudiniabola, C. Pirone, J. Williamson and A. N. Gos
Tunnel junctions of unconventional superconductors
The phenomenology of Josephson tunnel junctions between unconventional
superconductors is developed further. In contrast to s-wave superconductors,
for d-wave superconductors the direction dependence of the tunnel matrix
elements that describe the barrier is relevant. We find the full I-V
characteristics and comment on the thermodynamical properties of these
junctions. They depend sensitively on the relative orientation of the
superconductors. The I-V characteristics differ from the normal s-wave RSJ-like
behavior.Comment: 4 pages, revtex, 4 (encapsulated postscript) figures (figures
replaced
A Radio Transient 0.1 pc from Sagittarius A*
We report the discovery of a transient radio source 2.7 arcsec (0.1 pc
projected distance) South of the Galactic Center massive black hole,
Sagittarius A*. The source flared with a peak of at least 80 mJy in March 2004.
The source was resolved by the Very Large Array into two components with a
separation of ~0.7 arcsec and characteristic sizes of ~0.2 arcsec. The two
components of the source faded with a power-law index of 1.1 +/- 0.1. We detect
an upper limit to the proper motion of the Eastern component of ~3 x 10^3 km
s^-1 relative to Sgr A*. We detect a proper motion of ~10^4 km s^-1 for the
Western component relative to Sgr A*. The transient was also detected at X-ray
wavelengths with the Chandra X-ray Observatory and the XMM-Newton telescope and
given the designation CXOGC J174540.0-290031. The X-ray source falls in between
the two radio components. The maximum luminosity of the X-ray source is ~10^36
erg s^-1, significantly sub-Eddington. The radio jet flux density predicted by
the X-ray/radio correlation for X-ray binaries is orders of magnitude less than
the measured flux density. We conclude that the radio transient is the result
of a bipolar jet originating in a single impulsive event from the X-ray source
and interacting with the dense interstellar medium of the Galactic Center.Comment: accepted in ApJ; 24 pages; 8 figure
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