9,052 research outputs found
Density Evolution for Asymmetric Memoryless Channels
Density evolution is one of the most powerful analytical tools for
low-density parity-check (LDPC) codes and graph codes with message passing
decoding algorithms. With channel symmetry as one of its fundamental
assumptions, density evolution (DE) has been widely and successfully applied to
different channels, including binary erasure channels, binary symmetric
channels, binary additive white Gaussian noise channels, etc. This paper
generalizes density evolution for non-symmetric memoryless channels, which in
turn broadens the applications to general memoryless channels, e.g. z-channels,
composite white Gaussian noise channels, etc. The central theorem underpinning
this generalization is the convergence to perfect projection for any fixed size
supporting tree. A new iterative formula of the same complexity is then
presented and the necessary theorems for the performance concentration theorems
are developed. Several properties of the new density evolution method are
explored, including stability results for general asymmetric memoryless
channels. Simulations, code optimizations, and possible new applications
suggested by this new density evolution method are also provided. This result
is also used to prove the typicality of linear LDPC codes among the coset code
ensemble when the minimum check node degree is sufficiently large. It is shown
that the convergence to perfect projection is essential to the belief
propagation algorithm even when only symmetric channels are considered. Hence
the proof of the convergence to perfect projection serves also as a completion
of the theory of classical density evolution for symmetric memoryless channels.Comment: To appear in the IEEE Transactions on Information Theor
Gamma-Ray Burst Spectral Features: Interpretation as X-ray Emission From A Photoionized Plasma
Numerous reports have been made of features, either in emission or
absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally
interpreted in the context of Galactic neutron star models as cyclotron line
emission and annihilation features, the recent demonstration that
the majority of GRBs lie at cosmological distances make these explanations
unlikely. In this letter, we adopt a relativistic fireball model for
cosmological GRBs in which dense, metal rich blobs or filaments of plasma are
entrained in the relativistic outflow. In the context of this model, we
investigate the conditions under which broadband features, similar to those
detected, can be observed. We find a limited region of parameter space capable
of reproducing the observed GRB spectra. Finally, we discuss possible
constraints further high-energy spectral observations could place on fireball
model parameters.Comment: Accepted for publication in Astrophysical Journal Letters Four pages,
2 figure
How Common Are Magnetars? The Consequences of Magnetic-Field Decay
Ultramagnetized neutron stars or magnetars have been invoked to explain
several astrophysical phenomena. We examine how the magnetic field of a
magnetar will decay over time and how this decay affects the cooling of the
object. We find that for sufficiently strong nascent fields, field decay alters
the cooling evolution significantly relative to similarly magnetized neutron
stars with constant fields. As a result, old magnetars can be expected to be
bright in the soft X-ray band. The soft X-ray source RXJ~0720.43125 may well
be the nearest such old magnetar.Comment: 7 pages, 1 figure, accepted for publication in Ap. J. Letter
On the reliability of parameter estimates in the first observing run of Advanced LIGO
Accurate parameter estimation is key to maximizing the scientific impact of gravitational-wave astronomy. Parameters of a binary merger are typically estimated using Bayesian inference. It is necessary to make several assumptions when doing so, one of which is that the the detectors output stationary Gaussian noise. We test the validity of these assumptions by performing percentile-percentile tests in both simulated Gaussian noise and real detector data in the first observing run of Advanced LIGO (O1). We add simulated signals to 512s of data centered on each of the three events detected in O1 -- GW150914, GW151012, and GW151226 -- and check that the recovered credible intervals match statistical expectations. We find that we are able to recover unbiased parameter estimates in the real detector data, indicating that the assumption of Gaussian noise does not adversely effect parameter estimates. However, we also find that both the parallel-tempered emcee sampler emcee_pt and the nested sampler dynesty struggle to produced unbiased parameter estimates for GW151226-like signals, even in simulated Gaussian noise. The emcee_pt sampler does produce unbiased estimates for GW150914-like signals. This highlights the importance of performing percentile-percentile tests in different targeted areas of parameter space
Hot forming of silicon sheet, silicon sheet growth development for the large area silicon sheet task of the low cost silicon solar array project
The hot workability of polycrystalline silicon was studied. Uniaxail stress-strain curves are given for strain rates in the range of .0001 to .1/sec and temperatures from 1100 to 1380 C. At the highest strain rates at 1380 C axial strains in excess of 20% were easily obtainable without cracking. After deformations of 36%, recrystallization was completed within 0.1 hr at 1380 C. When the recrystallization was complete, there was still a small volume fraction of unrecyrstallized material which appeared very stable and may degrade the electronic properties of the bulk materials. Texture measurements showed that the as-produced vapor deposited polycrystalline rods have a 110 fiber texture with the 110 direction parallel to the growth direction and no preferred orientation about this axis. Upon axial compression perpendicular to the growth direction, the former 110 fiber axis changed to 111 and the compression axis became 110 . Recrystallization changed the texture to 110 along the former fiber axis and to 100 along the compression axis
Towards a public analysis database for LHC new physics searches using MadAnalysis 5
We present the implementation, in the MadAnalysis 5 framework, of several
ATLAS and CMS searches for supersymmetry in data recorded during the first run
of the LHC. We provide extensive details on the validation of our
implementations and propose to create a public analysis database within this
framework.Comment: 20 pages, 15 figures, 5 recast codes; version accepted by EPJC (Dec
22, 2014) including a new section with guidelines for the experimental
collaborations as well as for potential contributors to the PAD;
complementary information can be found at
http://madanalysis.irmp.ucl.ac.be/wiki/PhysicsAnalysisDatabas
The Host Galaxy of GRB980703 at Radio Wavelengths - a Nuclear Starburst in a ULIRG
We present radio observations of GRB980703 at 1.43, 4.86, and 8.46 GHz for
the period of 350 to 1000 days after the burst. These radio data clearly
indicate that there is a persistent source at the position of GRB980703 with a
flux density of approximately 70 Jy at 1.43 GHz, and a spectral index,
, where . We show that emission
from the afterglow of GRB980703 is expected to be one to two orders of
magnitude fainter, and therefore cannot account for these observations. We
interpret this persistent emission as coming from the host galaxy --- the first
example of a gamma-ray burst (GRB) host detection at radio wavelengths. We show
that emission from an AGN is unlikely, and find that it can be explained as a
result of a star-formation rate (SFR) of massive stars (M>5M) of 90
M/yr, which gives a total SFR of M/yr. Using the
correlation between the radio and far-IR (FIR) luminosities of star-forming
galaxies, we find that the host of GRB980703 is at the faint end of the class
of Ultra Luminous Infrared Galaxies (ULIRGs), with L_{FIR}\sim few\times
10^{12} L. From the radio measurements of the offset between the burst
and the host, and the size of the host, we conclude that GRB980703 occurred
near the center of the galaxy in a region of maximum star formation. A
comparison of the properties of this galaxy with radio and optical surveys at a
similar redshift () reveals that the host of GRB980703 is an
average star-forming galaxy. This result has significant implications for the
potential use of a GRB-selected galaxy sample for the study of galaxies and the
IGM at high redshifts.Comment: Submitted to Ap
- …