2,066 research outputs found
Single Superconducting Split-Ring Resonator Electrodynamics
We investigate the microwave electrodynamic properties of a single
superconducting thin film split-ring resonator (SRR). The experiments were
performed in an all-Nb waveguide, with Nb wires and Nb SRRs. Transmission data
showed a high-Q stopband for a single Nb SRR ( at 4.2 K)
below , and no such feature for a Cu SRR, or closed Nb loops, of similar
dimensions. Adding SRRs increased the bandwidth, but decreased the insertion
loss of the features. Placing the Nb SRR into an array of wires produced a
single, elementary negative-index passband ( at 4.2 K).
Changes in the features due to the superconducting kinetic inductance were
observed. Models for the SRR permeability, and the wire dielectric response,
were used to fit the data.Comment: 4 pages, 3 figures, RevTex, submitted to Applied Physics Letters.
Updated version includes mention of bianisotropy, better looking figures, and
different temperature dat
Tunability of Superconducting Metamaterials
Metamaterials are artificial structures with unique electromagnetic
properties, such as relative dielectric permittivity and magnetic permeability
with values less than 1, or even negative. Because these properties are so
sensitive to loss, we have developed metamaterials comprised of superconducting
waveguides, wires, and split-ring resonators. An important requirement for
applications of these metamaterials is the ability to tune the frequency at
which the unique electromagnetic response occurs. In this paper we present
three methods (unique to superconductors) to accomplish this tuning:
temperature, dc magnetic field, and rf magnetic field. Data are shown for dc
and rf magnetic field tuning of a single Nb split-ring resonator (SRR). It was
found that the dc field tuning was hysteritic in the resonant frequency data,
while the quality factor, , was less hystertic. The rf power tuning showed
no hysteresis, but did show supression of the at high power.
Magneto-optical images reveal inhomogeneous magnetic vortex entry in the dc
field tuning, and laser scanning photoresponse images for a
YBaCuO SRR reveals the current distribution in the rings.Comment: RexTEX, 4 pages of text with 6 figures plus 1/5 page of references,
submitted for the 2006 Applied Superconductivity Conference; Revised edition:
spelling corrections, and we removed mention of measuring the Current Density
and replaced this with a more explicit definition of what we measure (with
reference
BAT AGN spectroscopic survey - XV: the high frequency radio cores of ultra-hard X-ray selected AGN
We have conducted 22 GHz radio imaging at 1 arcsec resolution of 100 low-redshift AGN selected at 14–195 keV by the Swift-BAT. We find a radio core detection fraction of 96 per cent, much higher than lower frequency radio surveys. Of the 96 radio-detected AGN, 55 have compact morphologies, 30 have morphologies consistent with nuclear star formation, and 11 have sub-kpc to kpc-scale jets. We find that the total radio power does not distinguish between nuclear star formation and jets as the origin of the radio emission. For 87 objects, we use optical spectroscopy to test whether AGN physical parameters are distinct between radio morphological types. We find that X-ray luminosities tend to be higher if the 22 GHz morphology is jet-like, but find no significant difference in other physical parameters. We find that the relationship between the X-ray and core radio luminosities is consistent with the L_R/L_X ∼ 10⁻⁵ of coronally active stars. We further find that the canonical fundamental planes of black hole activity systematically overpredict our radio luminosities, particularly for objects with star formation morphologies
The BAT AGN Spectroscopic Survey -- XVIII. Searching for Supermassive Black Hole Binaries in the X-rays
Theory predicts that a supermassive black hole binary (SMBHB) could be
observed as a luminous active galactic nucleus (AGN) that periodically varies
on the order of its orbital timescale. In X-rays, periodic variations could be
caused by mechanisms including relativistic Doppler boosting and shocks. Here
we present the first systematic search for periodic AGNs using hard X-ray
light curves (14-195 keV) from the first 105 months of the Swift Burst Alert
Telescope (BAT) survey (2004-2013). We do not find evidence for periodic AGNs
in Swift-BAT, including the previously reported SMBHB candidate
MCG+1111032. We find that the null detection is consistent with the
combination of the upper-limit binary population in AGNs in our adopted model,
their expected periodic variability amplitudes, and the BAT survey
characteristics. We have also investigated the detectability of SMBHBs against
normal AGN X-ray variability in the context of the eROSITA survey. Under our
assumptions of a binary population and the periodic signals they produce which
have long periods of hundreds of days, up to % true periodic binaries can
be robustly distinguished from normal variable AGNs with the ideal uniform
sampling. However, we demonstrate that realistic eROSITA sampling is likely to
be insensitive to long-period binaries because longer observing gaps reduce
their detectability. In contrast, large observing gaps do not diminish the
prospect of detecting binaries of short, few-day periods, as 19% can be
successfully recovered, the vast majority of which can be identified by the
first half of the survey.Comment: 17 pages, including 8 figures and 4 tables. Accepted for publication
in Ap
The Accretion History of AGNs. I. Supermassive Black Hole Population Synthesis Model
As matter accretes onto the central supermassive black holes in active galactic nuclei (AGNs), X-rays are emitted. We present a population synthesis model that accounts for the summed X-ray emission from growing black holes; modulo the efficiency of converting mass to X-rays, this is effectively a record of the accreted mass. We need this population synthesis model to reproduce observed constraints from X-ray surveys: the X-ray number counts, the observed fraction of Compton-thick AGNs [log (N H/cm−2) > 24], and the spectrum of the cosmic X-ray background (CXB), after accounting for selection biases. Over the past decade, X-ray surveys by XMM-Newton, Chandra, NuSTAR, and Swift-BAT have provided greatly improved observational constraints. We find that no existing X-ray luminosity function (XLF) consistently reproduces all these observations. We take the uncertainty in AGN spectra into account and use a neural network to compute an XLF that fits all observed constraints, including observed Compton-thick number counts and fractions. This new population synthesis model suggests that, intrinsically, 50% ± 9% (56% ± 7%) of all AGNs within z sime 0.1 (1.0) are Compton-thick
Interactions Between Moderate- and Long-Period Giant Planets: Scattering Experiments for Systems in Isolation and with Stellar Flybys
The chance that a planetary system will interact with another member of its
host star's nascent cluster would be greatly increased if gas giant planets
form in situ on wide orbits. In this paper, we explore the outcomes of
planet-planet scattering for a distribution of multiplanet systems that all
have one of the planets on an initial orbit of 100 AU. The scattering
experiments are run with and without stellar flybys. We convolve the outcomes
with distributions for protoplanetary disk and stellar cluster sizes to
generalize the results where possible. We find that the frequencies of large
mutual inclinations and high eccentricities are sensitive to the number of
planets in a system, but not strongly to stellar flybys. However, flybys do
play a role in changing the low and moderate portions of the mutual inclination
distributions, and erase dynamically cold initial conditions on average.
Wide-orbit planets can be mixed throughout the planetary system, and in some
cases, can potentially become hot Jupiters, which we demonstrate using
scattering experiments that include a tidal damping model. If planets form on
wide orbits in situ, then there will be discernible differences in the proper
motion distributions of a sample of wide-orbit planets compared with a pure
scattering formation mechanism. Stellar flybys can enhance the frequency of
ejections in planetary systems, but auto-ionization is likely to remain the
dominant source of free-floating planets.Comment: Accepted for publication by Ap
BASS XXXIX: Swift-BAT AGN with changing-look optical spectra
Changing-look (CL) AGN are unique probes of accretion onto supermassive black
holes (SMBHs), especially when simultaneous observations in complementary
wavebands allow investigations into the properties of their accretion flows. We
present the results of a search for CL behaviour in 412 Swift-BAT detected AGN
with multiple epochs of optical spectroscopy from the BAT AGN Spectroscopic
Survey (BASS). 125 of these AGN also have 14-195 keV ultra-hard X-ray
light-curves from Swift-BAT which are contemporaneous with the epochs of
optical spectroscopy. Eight CL events are presented for the first time, where
the appearance or disappearance of broad Balmer line emission leads to a change
in the observed Seyfert type classification. Combining with known events from
the literature, 21 AGN from BASS are now known to display CL behaviour. Nine CL
events have 14-195 keV data available, and five of these CL events can be
associated with significant changes in their 14-195 keV flux from BAT. The
ultra-hard X-ray flux is less affected by obscuration and so these changes in
the 14-195 keV band suggest that the majority of our CL events are not due to
changes in line-of-sight obscuration. We derive a CL rate of 0.7-6.2 per cent
on 10-25 year time-scales, and show that many transitions happen within at most
a few years. Our results motivate further multi-wavelength observations with
higher cadence to better understand the variability physics of accretion onto
SMBHs.Comment: 14 pages, 7 figures, plus appendix. v2: updated references. Published
in MNRA
A population of luminous accreting black holes with hidden mergers
Major galaxy mergers are thought to play an important part in fuelling the
growth of supermassive black holes. However, observational support for this
hypothesis is mixed, with some studies showing a correlation between merging
galaxies and luminous quasars and others showing no such association. Recent
observations have shown that a black hole is likely to become heavily obscured
behind merger-driven gas and dust, even in the early stages of the merger, when
the galaxies are well separated (5 to 40 kiloparsecs). Merger simulations
further suggest that such obscuration and black-hole accretion peaks in the
final merger stage, when the two galactic nuclei are closely separated (less
than 3 kiloparsecs). Resolving this final stage requires a combination of
high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray
observations to detect highly obscured sources. However, large numbers of
obscured luminous accreting supermassive black holes have been recently
detected nearby (distances below 250 megaparsecs) in X-ray observations. Here
we report high-resolution infrared observations of hard-X-ray-selected black
holes and the discovery of obscured nuclear mergers, the parent populations of
supermassive-black-hole mergers. We find that obscured luminous black holes
(bolometric luminosity higher than 2x10^44 ergs per second) show a significant
(P<0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a
sample of inactive galaxies with matching stellar masses and star formation
rates (1.1 per cent), in agreement with theoretical predictions. Using
hydrodynamic simulations, we confirm that the excess of nuclear mergers is
indeed strongest for gas-rich major-merger hosts of obscured luminous black
holes in this final stage.Comment: To appear in the 8 November 2018 issue of Nature. This is the
authors' version of the wor
Optics with an Atom Laser Beam
We report on the atom optical manipulation of an atom laser beam. Reflection,
focusing and its storage in a resonator are demonstrated. Precise and versatile
mechanical control over an atom laser beam propagating in an inhomogeneous
magnetic field is achieved by optically inducing spin-flips between atomic
ground states with different magnetic moment. The magnetic force acting on the
atoms can thereby be effectively switched on and off. The surface of the atom
optical element is determined by the resonance condition for the spin-flip in
the inhomogeneous magnetic field. A mirror reflectivity of more than 98% is
measured
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