116,558 research outputs found
Lutz-Kelker bias in pulsar parallax measurements
Lutz & Kelker showed that parallax measurements are systematically
overestimated because they do not properly account for the larger volume of
space that is sampled at smaller parallax values. We apply their analysis to
neutron stars, incorporating the bias introduced by the intrinsic radio
luminosity function and a realistic Galactic population model for neutron
stars. We estimate the bias for all published neutron star parallax
measurements and find that measurements with less than ~95% certainty, are
likely to be significantly biased. Through inspection of historic parallax
measurements, we confirm the described effects in optical and radio
measurements, as well as in distance estimates based on interstellar dispersion
measures. The potential impact on future tests of relativistic gravity through
pulsar timing and on X-ray--based estimates of neutron star radii is briefly
discussed.Comment: 9 pages, 3 tables, 1 figure. Accepted for publication in MNRA
A monopole solution from noncommutative multi-instantons
We extend the relation between instanton and monopole solutions of the
selfduality equations in SU(2) gauge theory to noncommutative space-times.
Using this approach and starting from a noncommutative multi-instanton solution
we construct a U(2) monopole configuration which lives in 3 dimensional
ordinary space. This configuration resembles the Wu-Yang monopole and satisfies
the selfduality (Bogomol'nyi) equations for a U(2) Yang-Mills-Higgs system.Comment: 19 pages; title and abstract changed, brane interpretation corrected.
Version to appear in JHE
A New Electric Field in Asymmetric Magnetic Reconnection
We present a theory and numerical evidence for the existence of a previously
unexplored in-plane electric field in collisionless asymmetric magnetic
reconnection. This electric field, dubbed the "Larmor electric field," is
associated with finite Larmor radius effects and is distinct from the known
Hall electric field. Potentially, it could be an important indicator for the
upcoming Magnetospheric Multiscale (MMS) mission to locate reconnection sites
as we expect it to appear on the magnetospheric side, pointing Earthward, at
the dayside magnetopause reconnection site.Comment: 5 pages, 3 figures, to be published in Physical Review Letter
A search for rotating radio transients and fast radio bursts in the Parkes high-latitude pulsar survey
Discoveries of rotating radio transients and fast radio bursts (FRBs) in
pulsar surveys suggest that more of such transient sources await discovery in
archival data sets. Here we report on a single-pulse search for dispersed radio
bursts over a wide range of Galactic latitudes (|b| < ) in data
previously searched for periodic sources by Burgay et al. We re-detected 20 of
the 42 pulsars reported by Burgay et al. and one rotating radio transient
reported by Burke-Spolaor. No FRBs were discovered in this survey. Taking into
account this result, and other recent surveys at Parkes, we corrected for
detection sensitivities based on the search software used in the analyses and
the different backends used in these surveys and find that the all-sky FRB
event rate for sources with a fluence above 4.0 Jy ms at 1.4 GHz to be FRBs day sky, where the
uncertainties represent a confidence interval. While this rate is lower
than inferred from previous studies, as we demonstrate, this combined event
rate is consistent with the results of all systematic FRB searches at Parkes to
date and does not require the need to postulate a dearth of FRBs at
intermediate latitudes.Comment: Accepted, 10 pages, 6 figure
High Energy Collisions of Black Holes Numerically Revisited
We use fully nonlinear numerical relativity techniques to study high energy
head-on collision of nonspinning, equal-mass black holes to estimate the
maximum gravitational radiation emitted by these systems. Our simulations
include improvements in the construction of initial data, subsequent full
numerical evolutions, and the computation of waveforms at infinity. The new
initial data significantly reduces the spurious radiation content, allowing for
initial speeds much closer to the speed of light, i.e. . Using
these new techniques, We estimate the maximum radiated energy from head-on
collisions to be . This value
differs from the second-order perturbative and zero-frequency-limit
analytic computations, but is close to those obtained by thermodynamic
arguments and by previous numerical estimates .Comment: 11 pages, 10 figure
Finite Temperature Retarded and Advanced Fields
By employing retarded and advanced propagators, Aurenche and Becherrawy
showed how to rewrite the real-time thermal Feynman rules so that the
temperature dependence is removed from the free propagators and transferred to
the vertices. The present paper introduces retarded and advanced field
operators and incorporates all temperature dependence into the interaction term
of the Hamiltonian. Perturbative expansions of the Green functions in the
Hamiltonian formulation give the correct results order by order in perturbation
theory.
The spectrum of the temperature-dependent Hamiltonian contains the thermal
quasiparticles that produce poles in the propagators.Comment: 27 pages, latex, no figure
The Infrared and Radio Fluxes Densities of Galactic HII Regions
We derive infrared and radio flux densities of all ~1000 known Galactic HII
regions in the Galactic longitude range 17.5 < l < 65 degree. Our sample comes
from the Wide-Field Infrared Survey Explorer (WISE) catalog of Galactic \hii
regions \citep{anderson2014}. We compute flux densities at six wavelengths in
the infrared (GLIMPSE 8 microns, WISE 12 microns and 22 microns, MIPSGAL 24
microns, and Hi-GAL 70 microns and 160 microns) and two in the radio (MAGPIS 20
cm and VGPS 21 cm). All HII region infrared flux densities are strongly
correlated with their ~20 cm flux densities. All HII regions used here,
regardless of physical size or Galactocentric radius, have similar infrared to
radio flux density ratios and similar infrared colors, although the smallest
regions (pc), have slightly elevated IR to radio ratios. The colors
and , and and reliably select
HII regions, independent of size. The infrared colors of ~22 of HII
regions, spanning a large range of physical sizes, satisfy the IRAS color
criteria of \citet{wood1989} for HII regions, after adjusting the criteria to
the wavelengths used here. Since these color criteria are commonly thought to
select only ultra-compact HII regions, this result indicates that the true
ultra-compact HII region population is uncertain. Comparing with a sample of IR
color indices from star-forming galaxies, HII regions show higher
ratios. We find a weak trend of
decreasing infrared to ~20 cm flux density ratios with increasing , in
agreement with previous extragalactic results, possibly indicating a decreased
dust abundance in the outer Galaxy.Comment: 27 pages, 16 figures, 5 table
Rank-based attachment leads to power law graphs
We investigate the degree distribution resulting from graph generation models
based on rank-based attachment. In rank-based attachment, all vertices are
ranked according to a ranking scheme. The link probability of a given vertex is
proportional to its rank raised to the power -a, for some a in (0,1). Through a
rigorous analysis, we show that rank-based attachment models lead to graphs
with a power law degree distribution with exponent 1+1/a whenever vertices are
ranked according to their degree, their age, or a randomly chosen fitness
value. We also investigate the case where the ranking is based on the initial
rank of each vertex; the rank of existing vertices only changes to accommodate
the new vertex. Here, we obtain a sharp threshold for power law behaviour. Only
if initial ranks are biased towards lower ranks, or chosen uniformly at random,
we obtain a power law degree distribution with exponent 1+1/a. This indicates
that the power law degree distribution often observed in nature can be
explained by a rank-based attachment scheme, based on a ranking scheme that can
be derived from a number of different factors; the exponent of the power law
can be seen as a measure of the strength of the attachment
Current-induced reversal in magnetic nanopillars passivated by silicon
We demonstrate that magnetic multilayer nanopillars can be efficiently
protected from oxidation by coating with silicon. Both the protected and the
oxidized nanopillars exhibit an increase of reversal current at cryogenic
temperatures. However the magnetic excitation onset current increases only in
the oxidized samples. We show that oxidized nanopillars exhibit anomalous
switching statistics at low temperature, providing a simple test for the
quality of magnetic nanodevices.Comment: 3 pages, 4 figure
Effects of hydrogen/deuterium absorption on the magnetic properties of Co/Pd multilayers
The effects of hydrogen (H2) and deuterium (D2) absorption were studied in
two Co/Pd multilayers with perpendicular magnetic anisotropy (PMA) using
polarized neutron reflectivity (PNR). PNR was measured in an external magnetic
field H applied in the plane of the sample with the magnetization M confined in
the plane for {\mu}_o H= 6.0 T and partially out of plane at 0.65 T. Nominal
thicknesses of the Co and Pd layers were 2.5 {\AA} and 21 {\AA}, respectively.
Because of these small values, the actual layer chemical composition,
thickness, and interface roughness parameters were determined from the nuclear
scattering length density profile ({\rho}_n) and its derivative obtained from
both x-ray reflectivity and PNR, and uncertainties were determined using Monte
Carlo analysis. The PNR {\rho}_n showed that although D2 absorption occurred
throughout the samples, absorption in the multilayer stack was modest (0.02 D
per Pd atom) and thus did not expand. Direct magnetometry showed that H2
absorption decreased the total M at saturation and increased the component of M
in the plane of the sample when not at saturation. The PNR magnetic scattering
length density ({\rho}_m) revealed that the Pd layers in the multilayer stack
were magnetized and that their magnetization was preferentially modified upon
D2 absorption. In one sample, a modulation of M with twice the multilayer
period was observed at {\mu}_o H= 0.65 T, which increased upon D2 absorption.
These results indicate that H2 or D2 absorption decreases both the PMA and
total magnetization of the samples. The lack of measurable expansion during
absorption indicates that these changes are primarily governed by modification
of the electronic structure of the material.Comment: to appear in Physics review B, 201
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