1,592 research outputs found
A Terraced Scanning Superconducting Quantum Interference Device Susceptometer with Sub-Micron Pickup Loops
Superconducting Quantum Interference Devices (SQUIDs) can have excellent spin
sensitivity depending on their magnetic flux noise, pick-up loop diameter, and
distance from the sample. We report a family of scanning SQUID susceptometers
with terraced tips that position the pick-up loops 300 nm from the sample. The
600 nm - 2 um pickup loops, defined by focused ion beam, are integrated into a
12-layer optical lithography process allowing flux-locked feedback, in situ
background subtraction and optimized flux noise. These features enable a
sensitivity of ~70 electron spins per root Hertz at 4K.Comment: See http://stanford.edu/group/moler/publications.html for an
auxiliary document containing additional fabrication details and discussio
SUMSS: A Wide-Field Radio Imaging Survey of the Southern Sky. I. Science goals, survey design and instrumentation
The Molonglo Observatory Synthesis Telescope, operating at 843 MHz with a 5
square degree field of view, is carrying out a radio imaging survey of the sky
south of declination -30 deg. This survey (the Sydney University Molonglo Sky
Survey, or SUMSS) produces images with a resolution of 43" x 43" cosec(Dec.)
and an rms noise level of about 1 mJy/beam. SUMSS is therefore similar in
sensitivity and resolution to the northern NRAO VLA Sky Survey (NVSS; Condon et
al. 1998). The survey is progressing at a rate of about 1000 square degrees per
year, yielding individual and statistical data for many thousands of weak radio
sources. This paper describes the main characteristics of the survey, and
presents sample images from the first year of observation.Comment: 27 pages, 12 figures (figures 2, 8, 10 in jpg format); AJ, in pres
PKS 1018-42: A Powerful Kinetically Dominated Quasar
We have identified PKS 1018-42 as a radio galaxy with extraordinarily
powerful jets, over twice as powerful as any 3CR source of equal or lesser
redshift except for one (3C196). It is perhaps the most intrinsically powerful
extragalactic radio source in the, still poorly explored, Southern Hemisphere.
PKS 1018-42 belongs to the class of FR II objects that are kinetically
dominated, the jet kinetic luminosity, (calculated at 151 MHz), is 3.4 times larger than the
total thermal luminosity (IR to X-ray) of the accretion flow, . It is the fourth most kinetically dominated
quasar that we could verify from existing radio data. From a review of the
literature, we find that kinetically dominated sources such as PKS 1018-42 are
rare, and list the 5 most kinetically dominated sources found from our review.
Our results for PKS 1018-42 are based on new observations from the Australia
Telescope Compact Array.Comment: To appear in ApJ Letter
Gas and Dust Emission from the Nuclear Region of the Circinus Galaxy
Simultaneous modeling of the line and continuum emission from the nuclear
region of the Circinus galaxy is presented. Composite models which include the
combined effect of shocks and photoionization from the active center and from
the circumnuclear star forming region are considered. The effects of dust
reradiation, bremsstrahlung from the gas and synchrotron radiation are treated
consistently. The proposed model accounts for two important observational
features. First, the high obscuration of Circinus central source is produced by
high velocity and dense clouds with characteristic high dust-to-gas ratios.
Their large velocities, up to 1500 km\s, place them very close to the active
center. Second, the derived size of the line emitting region is well in
agreement with the observed limits for the coronal and narrow line region of
Circinus.Comment: 36 pages, LaTex (including 4 Tables and 9 figures), removed from
Abstract To appear in "The Astrophysical Journal
Dynamical Stability of Six-Dimensional Warped Brane-Worlds
We study a generalization of the Randall-Sundrum mechanism for generating the
weak/Planck hierarchy, which uses two rather than one warped extra dimension,
and which requires no negative tension branes. A 4-brane with one exponentially
large compact dimension plays the role of the Planck brane. We investigate the
dynamical stability with respect to graviton, graviphoton and radion modes. The
radion is shown to have a tachyonic instability for certain models of the
4-brane stress-energy, while it is stable in others, and massless in a special
case. If stable, its mass is in the milli-eV range, for parameters of the model
which solve the hierarchy problem. The radion is shown to couple to matter with
gravitational strength, so that it is potentially detectable by
submillimeter-range gravity experiments. The radion mass can be increased using
a bulk scalar field in the manner of Goldberger and Wise, but only to order
MeV, due to the effect of the large extra dimension. The model predicts a
natural scale of 10^{13} GeV on the 4-brane, making it a natural setting for
inflation from the ultraviolet brane.Comment: 28 pages, 7 figure
Effect of an extreme flood event on solute transport and resilience of a mine water treatment system in a mineralised catchment
Extreme rainfall events are predicted to become more frequent with climate change and can have a major bearing on instream solute and pollutant transport in mineralised catchments. The Coledale Beck catchment in north-west England was subject to an extreme rainfall event in December 2015 that equated to a 1 in 200-year event. The catchment contains the UK's first passive metal mine water treatment system, and as such had been subject to intense monitoring of solute dynamics before and after commissioning. Due to this monitoring record, the site provides a unique opportunity to assess the effects of a major storm event on (1) catchment-scale solute transport, and (2) the resilience of the new and novel passive treatment system to extreme events. Monitoring suggests a modest decline in treatment efficiency over time that is not synchronous with the storm event and explained instead by changes in system hydraulic efficiency. There was no apparent flushing of the mine system during the event that could potentially have compromised treatment system performance. Analysis of metal transport in the catchment downstream of the mine suggests relatively subtle changes in instream chemistry with modest but statistically-significant reductions in zinc in the lower catchment irrespective of flow condition after the extreme event, but most parameters of interest show no significant change. Increased export of colloidal iron and aluminium is associated with major landslips in the mid-catchment after the storm and provide fresh sorption sites to attenuate dissolved zinc more rapidly in these locations, corroborated by laboratory experiments utilising site materials to investigate the attenuation/release of metals from stream and terrestrial sediments. The data are important as they show both the resilience of passive mine water treatment systems to extreme events and the importance of catchment-scale monitoring to ensure continued effectiveness of treatment initiatives after major perturbation
The Molonglo Reference Catalog 1-Jy radio source survey IV. Optical spectroscopy of a complete quasar sample
Optical spectroscopic data are presented here for quasars from the Molonglo
Quasar Sample (MQS), which forms part of a complete survey of 1-Jy radio
sources from the Molonglo Reference Catalogue. The combination of low-frequency
selection and complete identifications means that the MQS is relatively free
from the orientation biases which affect most other quasar samples. To date,
the sample includes 105 quasars and 6 BL Lac objects, 106 of which have now
been confirmed spectroscopically. This paper presents a homogenous set of
low-resolution optical spectra for 79 MQS quasars, the majority of which have
been obtained at the Anglo-Australian Telescope. Full observational details are
given and redshifts, continuum and emission-line data tabulated for all
confirmed quasars.Comment: 40 pages, ApJS in pres
Impact of eddy–wind interaction on eddy demographics and phytoplankton community structure in a model of the North Atlantic Ocean
Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Dynamics of Atmospheres and Oceans 52 (2011): 80-94, doi:10.1016/j.dynatmoce.2011.01.003.Two eddy-resolving (0.1-degree) physical-biological simulations of the North Atlantic Ocean are compared, one with the surface momentum flux computed only from wind velocities and the other using the difference between air and ocean velocity vectors. This difference in forcing has a significant impact on the intensities and relative number of different types of mesoscale eddies in the Sargasso Sea. Eddy/wind interaction significantly reduces eddy intensities and increases the number of mode-water eddies and “thinnies” relative to regular cyclones and anticyclones; it also modifies upward isopycnal displacements at the base of the euphotic zone, increasing them in the centers of mode water eddies and at the edges of cyclones, and decreasing them in the centers of cyclones. These physical changes increase phytoplankton growth rates and biomass in mode-water eddies, bringing the biological simulation into better agreement with field data. These results indicate the importance of including the eddy/wind interaction in simulations of the physics and biology of eddies in the subtropical North Atlantic. However, eddy intensities in the simulation with eddy/wind interaction are lower than observed, which suggests a decrease in horizontal viscosity or an increase in horizontal grid resolution will be necessary to regain the observed level of eddy activity.LAA and DJM gratefully acknowledge the support of NASA grant 07-CARBON07-17. SCD and IDL gratefully acknowledge support from the NSF Center for Microbial Oceanography, Research and Education (C-MORE; NSF EF-0424599)
Radiative Efficiency and Content of Extragalactic Radio Sources: Toward a Universal Scaling Relation Between Jet Power and Radio Power
We present an analysis of the energetics and particle content of the lobes of
24 radio galaxies at the cores of cooling clusters. The radio lobes in these
systems have created visible cavities in the surrounding hot, X-ray-emitting
gas, which allow direct measurement of the mechanical jet power of radio
sources over six decades of radio luminosity, independently of the radio
properties themselves. Using these measurements, we examine the ratio between
radio power and total jet power (the radiative efficiency). We find that jet
(cavity) power increases with radio synchrotron power approximately as P_jet ~
(L_radio)^beta, where 0.35 < beta < 0.70 depending on the bandpass of
measurement and state of the source. However, the scatter about these relations
caused by variations in radiative efficiency spans more than four orders of
magnitude. After accounting for variations in synchrotron break frequency
(age), the scatter is reduced by ~ 50%, yielding the most accurate scaling
relation available between the lobe bolometric radio power and the jet (cavity)
power. We place limits on the magnetic field strengths and particle content of
the radio lobes using a variety of X-ray constraints. We find that the lobe
magnetic field strengths vary between a few to several tens of microgauss
depending on the age and dynamical state of the lobes. If the cavities are
maintained in pressure balance with their surroundings and are supported by
internal fields and particles in equipartition, the ratio of energy in
electrons to heavy particles (k) must vary widely from approximately unity to
4000, consistent with heavy (hadronic) jets.Comment: 20 pages, 11 figures. Accepted for publication in Ap
A high-resolution radio survey of the Vela supernova remnant
This paper presents a high-resolution radio continuum (843 MHz) survey of the
Vela supernova remnant. The contrast between the structures in the central
pulsar-powered nebula of the remnant and the synchrotron radiation shell allows
the remnant to be identified morphologically as a member of the composite
class. The data are the first of a composite remnant at spatial scales
comparable with those available for the Cygnus Loop and the Crab Nebula, and
make possible a comparison of radio, optical and soft X-ray emission from the
resolved shell filaments. The survey, made with the Molonglo Observatory
Synthesis Telescope, covers an area of 50 square degrees at a resolution of
43'' x 60'', while imaging structures on scales up to 30'.Comment: 18 pages, 7 jpg figures (version with ps figures at
http://astro.berkeley.edu/~dbock/papers/); AJ, in pres
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