240 research outputs found
Spectral properties of one-dimensional Fermi systems after an interaction quench
We show that the single-particle spectral properties of gapless
one-dimensional Fermi systems in the Luttinger liquid state reached at
intermediate times after an abrupt quench of the two-particle interaction are
highly indicative of the unusual nonequilibrium nature of this state. The line
shapes of the momentum integrated and resolved spectral functions strongly
differ from their ground state as well as finite temperature equilibrium
counterparts. Using an energy resolution improved version of radio-frequency
spectroscopy of quasi one-dimensional cold Fermi gases it should be possible to
experimentally identify this nonequilibrium state by its pronounced spectral
signatures.Comment: 5 pages, 3 figure
The cosmic radio dipole: Bayesian estimators on new and old radio surveys
The cosmic radio dipole is an anisotropy in the number counts of radio
sources, analogous to the dipole seen in the cosmic microwave background (CMB).
Measurements of source counts of large radio surveys have shown that though the
radio dipole is generally consistent in direction with the CMB dipole, the
amplitudes are in tension. These observations present an intriguing puzzle as
to the cause of this discrepancy, with a true anisotropy breaking with the
assumptions of the cosmological principle, invalidating the most common
cosmological models that are built on these assumptions. We present a novel set
of Bayesian estimators to determine the cosmic radio dipole and compare the
results with commonly used methods on the Rapid ASKAP Continuum Survey (RACS)
and the NRAO VLA Sky Survey (NVSS) radio surveys. In addition, we adapt the
Bayesian estimators to take into account systematic effects known to affect
such large radio surveys, folding information such as the local noise floor or
array configuration directly into the parameter estimation. The enhancement of
these estimators allows us to greatly increase the amount of sources used in
the parameter estimation, yielding tighter constraints on the cosmic radio
dipole estimation than previously achieved with NVSS and RACS. We extend the
estimators further to work on multiple catalogues simultaneously, leading to a
combined parameter estimation using both NVSS and RACS. The result is a dipole
estimate that perfectly aligns with the CMB dipole in terms of direction but
with an amplitude that is three times as large, and a significance of
4.8. This new dipole measurement is made to an unprecedented level of
precision for radio sources, which is only matched by recent results using
infrared quasars.Comment: 14 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
Review of recent developments of the functional renormalization group for systems out of equilibrium
We recapitulate recent developments of the functional renormalization group (FRG) approach to the steady state of systems out of thermal equilibrium. In particular, we discuss second-order truncation schemes which account for the frequency-dependence of the two particle vertex and which incorporate inelastic processes. Our focus is on two different types of one-dimensional fermion chains: (i) infinite, open systems which feature a translation symmetry, and (ii) finite systems coupled to left and right reservoirs. In addition to giving a detailed and unified review of the technical derivation of the FRG schemes, we briefly summarize some of the key physical results. In particular, we compute the non-equilibrium phase diagram and analyze the fate of the Berezinskii–Kosterlitz–Thouless transition in the infinite, open system
OpenCL Actors - Adding Data Parallelism to Actor-based Programming with CAF
The actor model of computation has been designed for a seamless support of
concurrency and distribution. However, it remains unspecific about data
parallel program flows, while available processing power of modern many core
hardware such as graphics processing units (GPUs) or coprocessors increases the
relevance of data parallelism for general-purpose computation.
In this work, we introduce OpenCL-enabled actors to the C++ Actor Framework
(CAF). This offers a high level interface for accessing any OpenCL device
without leaving the actor paradigm. The new type of actor is integrated into
the runtime environment of CAF and gives rise to transparent message passing in
distributed systems on heterogeneous hardware. Following the actor logic in
CAF, OpenCL kernels can be composed while encapsulated in C++ actors, hence
operate in a multi-stage fashion on data resident at the GPU. Developers are
thus enabled to build complex data parallel programs from primitives without
leaving the actor paradigm, nor sacrificing performance. Our evaluations on
commodity GPUs, an Nvidia TESLA, and an Intel PHI reveal the expected linear
scaling behavior when offloading larger workloads. For sub-second duties, the
efficiency of offloading was found to largely differ between devices. Moreover,
our findings indicate a negligible overhead over programming with the native
OpenCL API.Comment: 28 page
The SKA and "High-Resolution" Science
"High-resolution", or "long-baseline", science with the SKA and its
precursors covers a broad range of topics in astrophysics. In several research
areas, the coupling between improved brightness sensitivity of the SKA and a
sub-arcsecond resolution would uncover truly unique avenues and opportunities
for studying extreme states of matter, vicinity of compact relativistic
objects, and complex processes in astrophysical plasmas. At the same time, long
baselines would secure excellent positional and astrometric measurements with
the SKA and critically enhance SKA image fidelity at all scales. The latter
aspect may also have a substantial impact on the survey speed of the SKA, thus
affecting several key science projects of the instrument.Comment: JENAM-2010: Invited talk at JENAM session S7: The Square Kilometre
Array: Paving the way for the new 21st century radio astronomy paradigm; 9
page
A Supernova Factory in Mrk 273?
We report on 1.6 and 5.0 GHz observations of the ultraluminous infrared
galaxy (ULIRG) Mrk 273, using the European VLBI Network (EVN) and the
Multi-Element Radio-Linked Interferometer Network (MERLIN). We also make use of
published 1.4 GHz VLBA observations of Mrk 273 by Carilli & Taylor (2000). Our
5 GHz images have a maximum resolution of 5-10 mas, which corresponds to linear
resolutions of 3.5-7 pc at the distance of Mrk 273, and are the most sensitive
high-resolution radio observations yet made of this ULIRG. Component N1, often
pinpointed as a possible AGN, displays a steep spectral index (); hence it is very difficult to reconcile
with N1 being an AGN, and rather suggests that the compact nonthermal radio
emission is produced by an extremely high luminous individual radio supernova
(RSN), or a combination of unresolved emission from nested supernova remnants
(SNR), luminous RSNe, or both. Component N2 is partly resolved out into several
compact radio sources --none of which clearly dominates-- and a region of
extended emission about 30 pc in size. The integrated spectral index of this
region is flat (), which can be interpreted as due to a
superposition of several unresolved components, e.g., RSNe or SNRs, whose radio
emission peaks at different frequencies and is partially free-free absorbed.
The overall extended radio emission from component N is typical of nonthermal,
optically thin radio emission (), and its 1.4 GHz
luminosity ( WHz) is
consistent with being produced by relativistic electrons diffused away from
supernova remnants in an outburst.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society Main Journal. 6 pages, 3 figure
A Virtual Sky with Extragalactic HI and CO Lines for the SKA and ALMA
We present a sky simulation of the atomic HI emission line and the first ten
CO rotational emission lines of molecular gas in galaxies beyond the Milky Way.
The simulated sky field has a comoving diameter of 500/h Mpc, hence the actual
field-of-view depends on the (user-defined) maximal redshift zmax; e.g. for
zmax=10, the field of view yields ~4x4 sqdeg. For all galaxies, we estimate the
line fluxes, line profiles, and angular sizes of the HI and CO emission lines.
The galaxy sample is complete for galaxies with cold hydrogen masses above 10^8
Msun. This sky simulation builds on a semi-analytic model of the cosmic
evolution of galaxies in a Lambda-cold dark matter (LCDM) cosmology. The
evolving CDM-distribution was adopted from the Millennium Simulation, an N-body
CDM-simulation in a cubic box with a side length of 500/h Mpc. This side length
limits the coherence scale of our sky simulation: it is long enough to allow
the extraction of the baryon acoustic oscillations (BAOs) in the galaxy power
spectrum, yet the position and amplitude of the first acoustic peak will be
imperfectly defined. This sky simulation is a tangible aid to the design and
operation of future telescopes, such the SKA, the LMT, and ALMA. The results
presented in this paper have been restricted to a graphical representation of
the simulated sky and fundamental dN/dz-analyzes for peak flux density limited
and total flux limited surveys of HI and CO. A key prediction is that HI will
be harder to detect at redshifts z>2 than predicted by a no-evolution model.
The future verification or falsification of this prediction will allow us to
qualify the semi-analytic models.Comment: 16 pages, 9 figures, 1 tabl
PyCOOL - a Cosmological Object-Oriented Lattice code written in Python
There are a number of different phenomena in the early universe that have to
be studied numerically with lattice simulations. This paper presents a graphics
processing unit (GPU) accelerated Python program called PyCOOL that solves the
evolution of scalar fields in a lattice with very precise symplectic
integrators. The program has been written with the intention to hit a sweet
spot of speed, accuracy and user friendliness. This has been achieved by using
the Python language with the PyCUDA interface to make a program that is easy to
adapt to different scalar field models. In this paper we derive the symplectic
dynamics that govern the evolution of the system and then present the
implementation of the program in Python and PyCUDA. The functionality of the
program is tested in a chaotic inflation preheating model, a single field
oscillon case and in a supersymmetric curvaton model which leads to Q-ball
production. We have also compared the performance of a consumer graphics card
to a professional Tesla compute card in these simulations. We find that the
program is not only accurate but also very fast. To further increase the
usefulness of the program we have equipped it with numerous post-processing
functions that provide useful information about the cosmological model. These
include various spectra and statistics of the fields. The program can be
additionally used to calculate the generated curvature perturbation. The
program is publicly available under GNU General Public License at
https://github.com/jtksai/PyCOOL . Some additional information can be found
from http://www.physics.utu.fi/tiedostot/theory/particlecosmology/pycool/ .Comment: 23 pages, 12 figures; some typos correcte
Prediction of the Cosmic Evolution of the CO-Luminosity Functions
We predict the emission line luminosity functions (LFs) of the first 10
rotational transitions of CO in galaxies at redshift z=0 to z=10. This
prediction relies on a recently presented simulation of the molecular cold gas
content in ~3e7 evolving galaxies based on the Millennium Simulation. We
combine this simulation with a model for the conversion between molecular mass
and CO-line intensities, which incorporates the following mechanisms: (i)
molecular gas is heated by the CMB, starbursts (SBs), and active galactic
nuclei (AGNs); (ii) molecular clouds in dense or inclined galaxies can overlap;
(iii) compact gas can attain a smooth distribution in the densest part of
disks; (iv) CO-luminosities scale with metallicity changes between galaxies;
(v) CO-luminosities are always detected against the CMB. We analyze the
relative importance of these effects and predict the cosmic evolution of the
CO-LFs. The most notable conclusion is that the detection of regular galaxies
(i.e. no AGN, no massive SB) at high z>7 in CO-emission will be dramatically
hindered by the weak contrast against the CMB, in contradiction to earlier
claims that CMB-heating will ease the detection of high-redshift CO. The full
simulation of extragalactic CO-lines and the predicted CO-LFs at any redshift
can be accessed online, prior registration required} and they should be useful
for the modeling of CO-line surveys with future telescopes, such as ALMA, the
LMT, or the SKA.Comment: 8 figures, 1 tabl
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