1,528,655 research outputs found
This is What it Means to be a DACA Recipient
Since 2012, over 800,000 DREAMers, like ourselves, have been given the legal right to work, apply for a driver’s license, and, most importantly, live without the fear of deportation. We complete background checks and pay $495 in fees every two years to maintain our DACA (Deferred Action for Childhood Arrivals) status. [excerpt
Kelvin-Helmholtz instabilities at the sloshing cold fronts in the Virgo cluster as a measure for the effective ICM viscosity
Sloshing cold fronts (CFs) arise from minor merger triggered gas sloshing.
Their detailed structure depends on the properties of the intra-cluster medium
(ICM): hydrodynamical simulations predict the CFs to be distorted by
Kelvin-Helmholtz instabilities (KHIs), but aligned magnetic fields, viscosity,
or thermal conduction can suppress the KHIs. Thus, observing the detailed
structure of sloshing CFs can be used to constrain these ICM properties. Both
smooth and distorted sloshing CFs have been observed, indicating that the KHI
is suppressed in some clusters, but not in all. Consequently, we need to
address at least some sloshing clusters individually before drawing general
conclusions about the ICM properties. We present the first detailed attempt to
constrain the ICM properties in a specific cluster from the structure of its
sloshing CF. Proximity and brightness make the Virgo cluster an ideal target.
We combine observations and Virgo-specific hydrodynamical sloshing simulations.
Here we focus on a Spitzer-like temperature dependent viscosity as a mechanism
to suppress the KHI, but discuss the alternative mechanisms in detail. We
identify the CF at 90 kpc north and north-east of the Virgo center as the best
location in the cluster to observe a possible KHI suppression. For viscosities
10% of the Spitzer value KHIs at this CF are suppressed. We describe
in detail the observable signatures at low and high viscosities, i.e. in the
presence or absence of KHIs. We find indications for a low ICM viscosity in
archival XMM-Newton data and demonstrate the detectability of the predicted
features in deep Chandra observations.Comment: Accepted for ApJ; 15 pages, 11 figures. A movie can be found here:
http://www.hs.uni-hamburg.de/DE/Ins/Per/Roediger/research.html#Virgo-viscou
Launch Vehicle Description
The Thorad-Agena is a two-stage launch vehicle consisting of a Thorad first-stage and an Agena second-stage, connected by a booster adapter. The composite vehicle, including the shroud and the booster adapter, is about 33 meters (109 ft) long. The total weight at lift-off is approximately 91 625 kilograms (202 000 lbm)
Tool repairs tube components in situ
Two versions of a portable tool repair the seats of tube fittings and the flared ends of tubing. Each version operates on the principle of lapping to remove imperfections from tube and fitting interfacing surfaces
Superconducting switch permits measurement of small voltages at cryogenic temperatures
Dual-coil, superconducting, on-off switch measures small, thermoelectrically generated voltages produced by thermocouples in a liquid helium bath. Placed in a shunt configuration between the thermocouple and the measuring device, the measuring device sees the sum of the voltage to be measured and the spurious thermoelectric voltages
Non-Gaussian statistics, maxwellian derivation and stellar polytropes
In this letter we discuss the Non-gaussian statistics considering two
aspects. In the first, we show that the Maxwell's first derivation of the
stationary distribution function for a dilute gas can be extended in the
context of Kaniadakis statistics. The second one, by investigating the stellar
system, we study the Kaniadakis analytical relation between the entropic
parameter and stellar polytrope index . We compare also the
Kaniadakis relation with proposed in the Tsallis
framework.Comment: 10 pages, 1 figur
Fidelity of the surface code in the presence of a bosonic bath
We study the resilience of the surface code to decoherence caused by the
presence of a bosonic bath. This approach allows us to go beyond the standard
stochastic error model commonly used to quantify decoherence and error
threshold probabilities in this system. The full quantum mechanical system-bath
dynamics is computed exactly over one quantum error correction cycle. Since all
physical qubits interact with the bath, space-time correlations between errors
are taken into account. We compute the fidelity of the surface code as a
function of the quantum error correction time. The calculation allows us to map
the problem onto an Ising-like statistical spin model with two-body
interactions and a fictitious temperature which is related to the inverse bath
coupling constant. The model departs from the usual Ising model in the sense
that interactions can be long ranged and can involve complex exchange
couplings; in addition, the number of allowed configurations is restricted by
the syndrome extraction. Using analytical estimates and numerical calculations,
we argue that, in the limit of an infinite number of physical qubits, the spin
model sustain a phase transition which can be associated to the existence of an
error threshold in the surface code. An estimate of the transition point is
given for the case of nearest-neighbor interactions.Comment: 15 pages, 5 figure
Orbital order, stacking defects and spin-fluctuations in the -electron molecular solid RbO
We examine magnon and orbiton behavior in localized O anti-bonding
molecular orbitals using an effective Kugel-Khomskii Hamiltonian
derived from a two band Hubbard model with hopping parameters taken from {\em
ab initio} density functional calculations. The considerable difference between
intraband and interband hoppings leads to a strong coupling between the spin
wave dispersion and the orbital ground state, providing a straightforward way
of experimentally determining the orbital ground state from the measured magnon
dispersion. The near degeneracy of different orbital ordered states leads to
stacking defects which further modulate spin-fluctuation spectra. Proliferation
of orbital domains disrupts long-range magnetic order, thus causing a
significant reduction in the observed N\'eel temperature.Comment: 5 pages, 2 figure
Transmission Line Analogy for Relativistic Poynting-Flux Jets
Radio emission, polarization, and Faraday rotation maps of the radio jet of
the galaxy 3C 303 have shown that one knot of this jet carries a {\it
galactic}-scale electric current and that it is magnetically dominated. We
develop the theory of magnetically dominated or Poynting-flux jets by making an
analogy of a Poynting jet with a transmission line or waveguide carrying a net
current and having a potential drop across it (from the jet's axis to its
radius) and a definite impedance which we derive. Time-dependent but not
necessarily small perturbations of a Poynting-flux jet are described by the
"telegrapher's equations." These predict the propagation speed of disturbances
and the effective wave impedance for forward and backward propagating wave
components. A localized disturbance of a Poynting jet gives rise to localized
dissipation in the jet which may explain the enhanced synchrotron radiation in
the knots of the 3C 303 jet, and also in the apparently stationary knot HST-1
in the jet near the nucleus of the nearby galaxy M87. For a relativistic
Poynting jet on parsec scales, the reflected voltage wave from an inductive
termination or load can lead to a backward propagating wave which breaks down
the magnetic insulation of the jet giving . At the
threshold for breakdown, , positive and negative
particles are directly accelerated in the direction which is
approximately along the jet axis. Acceleration can occur up to Lorentz factors
. This particle acceleration mechanism is distinct from that in
shock waves and that in magnetic field reconnection.Comment: 8 pages, 6 figure
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