4,441 research outputs found
First detection of GeV emission from an ultraluminous infrared galaxy: Arp 220 as seen with the Fermi Large Area Telescope
Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays by
colliding with the dense interstellar medium (ISM). Arp 220 is the nearest
ultra luminous infrared galaxy (ULIRG) that has star-formation at extreme
levels, so it has long been predicted to emit high-energy gamma-rays. However,
no evidence of gamma-ray emission was found despite intense efforts of search.
Here we report the discovery of high-energy gamma-ray emission above 200 MeV
from Arp 220 at a confidence level of using 7.5 years of
\textsl {Fermi} Large Area Telescope observations. The gamma-ray emission shows
no significant variability over the observation period and it is consistent
with the quasi-linear scaling relation between the gamma-ray luminosity and
total infrared luminosity for star-forming galaxies, suggesting that these
gamma-rays arise from CR interactions. As the high density medium of Arp 220
makes it an ideal CR calorimeter, the gamma-ray luminosity can be used to
measure the efficiency of powering CRs by supernova (SN) remnants given a known
supernova rate in Arp 220. We find that this efficiency is about
for CRs above 1 GeV.Comment: Accepted by ApJL, 6 pages, 3 figure
End effect of corroded steel bar in concrete specimen during corrosion test by galvanostatic method
The galvanostatic method is a commonly used accelerated corrosion method in studying the durability of concrete structures caused by steel corrosion. If this method is not adequately controlled, the ends of steel bars will be corroded seriously (this phenomenon is called end effect), which is not common in natural corrosion. How to effectively control the end effect during electrified corrosion is helpful to make the characteristics of electrified corrosion more similar to that of natural corrosion. In this paper, experimental and electromagnetic numerical simulation methods are used to study the influence of cathode and corrosion medium coverage area along the longitudinal direction on the end effect of steel bar before producing corrosion crack. The results showed that the wider the coverage area of the corrosive medium (NaCl solution with a mass concentration of 3%), the longer the corrosion area of reinforcement along the longitudinal direction, the more prone the end effect is. The change of cathode length has no noticeable effect on the corrosion area of reinforcement along the longitudinal direction when the corrosion medium coverage area is constant. Increasing the distance between the end of the corrosion medium coverage area and the end of the reinforcement can effectively avoid the end effect
End effect of corroded steel bar in concrete specimen during corrosion test by galvanostatic method
Corrosion of steel bars is the main durability problem of concrete structures, which seriously endangers the safety and service life of reinforced concrete (RC) structures. When studying such problems, the electrifying corrosion method is often used to obtain corroded components. How to effectively control the corrosion area of steel bars is the key to improve the applicability of the corrosion method. In this paper, experimental and electromagnetic numerical simulation methods are used to study the influence of cathode and corrosion medium coverage area along the longitudinal direction on the corrosion area of steel bar before concrete protective layer corrosion cracking. The results show that the numerical simulation method based on electromagnetism can be used to predict the corrosion area of reinforcement before concrete corrosion cracking. When the resistivity of corrosive medium is much lower than that of concrete, the wider the coverage area, the longer the corrosion area of reinforcement along longitudinal direction, the more prone the end effect is, and the change of cathode length has no obvious effect on the corrosion area of reinforcement along longitudinal direction when the corrosion medium coverage area is constant. Increasing the distance between the end of the corrosion medium coverage area and the end of the reinforcement can effectively avoid the end effect
Simulating and understanding the gap outflow and oceanic response over the Gulf of Tehuantepec during GOTEX
17 USC 105 interim-entered record; under review.Tehuantepecer is a strong mountain gap wind traveling through Chivela Pass into eastern Pacific coastin southern Mexico, most commonly between October and February and brings huge impacts on local and surrounding meteorology and oceanography. Gulf of Tehuantepec EXperiment (GOTEX) was conducted in February 2004 to enhance the understanding of the strong offshore gap wind, ocean cooling, vertical circulations and interactions among them. The gap wind event during GOTEX was simulated using the U.S. Navy Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPSĀ® ). The simulations are compared and validated with the observations retrieved from several satellites (GOES 10ā12, MODIS/Aqua/Terra, TMI, and QuikSCAT) and Airborne EXpendable BathyThermograph (AXBT). The study shows that the gap wind outflow has a fanlike pattern expending from the coast and with a strong diurnal variability. The surface wind stress and cooling along the axis of the gap wind outflow caused intense upwelling and vertical mixing in the upper ocean; both contributed to the cooling of the ocean mixed layer under the gap wind. The cooling pattern of sea surface temperature (SST) also reflects temperature advection by the nearby ocean eddies to have a crescent shape. Two sensitivity experiments were conducted to understand the relative roles of the wind stress and heat flux on the ocean cooling. The control has more cooling right under the gap flow region than either the wind-stress-only or the heat-flux-only experiment. Overall, the wind stress has a slightly larger effect in bringing down the ocean temperature near the surface and plays a more important role in local ocean circulations beneath the mixed layer. The impact of surface heat flux on the ocean is more limited to the top 30 m within the mixed layer and is symmetric to the gap flow region by cooling the ocean under the gap flow region and reducing the warming on both sides. The effect of surface wind stress is to induce more cooling in the mixed layer under the gap wind through upwelling associated with Ekman divergence at the surface. Its effect deeper down is antisymmetric related to the nearby thermocline dome by inducing more upwelling to the east side of the gap flow region and more downwelling on the west side. Diagnostics from the mixed layer heat budget for the control and sensitivity experiments confirm that the surface heat flux has more influence on the broader area and the wind stress has more influence in a deeper region.This research is supported by Office of Naval Research (ONR) through the Department Research Initiative Predictability of Seasonal and Intraseasonal Oscillations (PE061153N). Computational resources were supported in part by a grant of HPC time from the Department of Defense Major Shared Resource Centers, Stennis Space Center, Mississippi
Continuum field theory of 3D topological orders with emergent fermions and braiding statistics
Universal topological data of topologically ordered phases can be captured by
topological quantum field theory in continuous space time by taking the limit
of low energies and long wavelengths. While previous continuum
field-theoretical studies of topological orders in D real space focus on
either self-statistics, braiding statistics, shrinking rules, fusion rules or
quantum dimensions, it is yet to systematically put all topological data
together in a unified continuum field-theoretical framework. Here, we construct
the topological field theory with twisted terms (e.g., and )
as well as a -matrix term, in order to simultaneously explore all such
topological data and reach anomaly-free topological orders. Following the
spirit of the famous -matrix Chern-Simons theory of D topological orders,
we present general formulas and systematically show how the -matrix
term confines topological excitations, and how self-statistics of particles is
transmuted between bosonic one and fermionic one. In order to reach
anomaly-free topological orders, we explore, within the present continuum
field-theoretical framework, how the principle of gauge invariance
fundamentally influences possible realizations of topological data. More
concretely, we present the topological actions of (i) particle-loop braidings
with emergent fermions, (ii) multiloop braidings with emergent fermions, and
(iii) Borromean-Rings braidings with emergent fermions, and calculate their
universal topological data. Together with the previous efforts, our work paves
the way toward a more systematic and complete continuum field-theoretical
analysis of exotic topological properties of D topological orders. Several
interesting future directions are also discussed
Non-Abelian Fusion, Shrinking and Quantum Dimensions of Abelian Gauge Fluxes
Braiding and fusion rules of topological excitations are indispensable
topological invariants in topological quantum computation and topological
orders. While excitations in 2D are always particle-like anyons, those in 3D
incorporate not only particles but also loops -- spatially nonlocal objects --
making it novel and challenging to study topological invariants in higher
dimensions. While 2D fusion rules have been well understood from bulk
Chern-Simons field theory and edge conformal field theory, it is yet to be
thoroughly explored for 3D fusion rules from higher dimensional bulk
topological field theory. Here, we perform a field-theoretical study on (i) how
loops that carry Abelian gauge fluxes fuse and (ii) how loops are shrunk into
particles in the path integral, which generates fusion rules, loop-shrinking
rules, and descendent invariants, e.g., quantum dimensions. We first assign a
gauge-invariant Wilson operator to each excitation and determine the number of
distinct excitations through equivalence classes of Wilson operators. Then, we
adiabatically shift two Wilson operators together to observe how they fuse and
are split in the path integral; despite the Abelian nature of the gauge fluxes
carried by loops, their fusions may be of non-Abelian nature. Meanwhile, we
adiabatically deform world-sheets of unknotted loops into world-lines and
examine the shrinking outcomes; we find that the resulting loop-shrinking rules
are algebraically consistent to fusion rules. Interestingly, fusing a pair of
loop and anti-loop may generate multiple vacua, but fusing a pair of anyon and
anti-anyon in 2D has one vacuum only. By establishing a field-theoretical
ground for fusion and shrinking in 3D, this work leaves intriguing directions,
e.g., symmetry enrichment, quantum gates, and physics of braided monoidal
2-category of 2-group.Comment: Title adjusted. Abstract, Intro and Discussions revised. about 30
pages, 5 figures. 9 table
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