3,326 research outputs found
Convergence rates of the DPG method with reduced test space degree
This paper presents a duality theorem of the Aubin-Nitsche type for
discontinuous Petrov Galerkin (DPG) methods. This explains the numerically
observed higher convergence rates in weaker norms. Considering the specific
example of the mild-weak (or primal) DPG method for the Laplace equation, two
further results are obtained. First, the DPG method continues to be solvable
even when the test space degree is reduced, provided it is odd. Second, a
non-conforming method of analysis is developed to explain the numerically
observed convergence rates for a test space of reduced degree
XMM-Newton, Chandra, and CGPS observations of the Supernova Remnants G85.4+0.7 and G85.9-0.6
We present an XMM-Newton detection of two low radio surface brightness SNRs,
G85.4+0.7 and G85.9-0.6, discovered with the Canadian Galactic Plane Survey
(CGPS). High-resolution XMM-Newton images revealing the morphology of the
diffuse emission, as well as discrete point sources, are presented and
correlated with radio and Chandra images. The new data also permit a
spectroscopic analysis of the diffuse emission regions, and a spectroscopic and
timing analysis of the point sources. Distances have been determined from HI
and CO data to be 3.5 +/- 1.0 kpc for SNR G85.4+0.7 and 4.8 +/- 1.6 kpc for SNR
G85.9-0.6. The SNR G85.4+0.7 is found to have a temperature of ~12-13 MK and a
0.5-2.5 keV luminosity of ~1-4 x 10^33 D(3.5)^2 erg/s (where D(3.5) is the
distance in units of 3.5 kpc), with an electron density n_e of
~0.07-0.16(fD(3.5))^-1/2 cm^-3 (where f is the volume filling factor), and a
shock age of ~9-49(fD(3.5))^1/2 kyr. The SNR G85.9-0.6 is found to have a
temperature of ~15-19 MK and a 0.5-2.5 keV luminosity of ~1-4 x 10^34 D(4.8)^2
erg/s (where D(4.8) is the distance in units of 4.8 kpc), with an electron
density n_e of ~0.04-0.10(fD(4.8))^-1/2 cm^-3 and a shock age of
~12-42(fD(4.8))^1/2 kyr. Based on the data presented here, none of the point
sources appears to be the neutron star associated with either SNR.Comment: 30 pages using emulateapj, 16 figures with quality reduced for
astro-ph only. The original version with high-resolution figures can be
downloaded from:
http://www.physics.umanitoba.ca/~samar/astro-ph/G85s-ms09102007.pdf To appear
in ApJ (Jan 20 2008 issue, v673, n1
The Connection between Supernova Remnants and the Galactic Magnetic Field: A Global Radio Study of the Axisymmetric Sample
The study of supernova remnants (SNRs) is fundamental to understanding the
chemical enrichment and magnetism in galaxies, including our own Milky Way. In
an effort to understand the connection between the morphology of SNRs and the
Galactic magnetic field (GMF), we have examined the radio images of all known
SNRs in our Galaxy and compiled a large sample that have an "axisymmetric"
morphology, which we define to mean SNRs with a "bilateral" or "barrel"-shaped
morphology, in addition to one-sided shells. We selected the cleanest examples
and model each of these at their appropriate Galactic position using two GMF
models, those of Jansson & Farrar (2012a), which includes a vertical halo
component, and Sun et al. (2008) that is oriented entirely parallel to the
plane. Since the magnitude and relative orientation of the magnetic field
changes with distance from the sun, we analyse a range of distances, from 0.5
to 10 kpc in each case. Using a physically motivated model of a SNR expanding
into the ambient GMF, we find the models using Jansson & Farrar (2012a) are
able to reproduce observed morphologies of many SNRs in our sample. These
results strongly support the presence of an off-plane, vertical component to
the GMF, and the importance of the Galactic field on SNR morphology. Our
approach also provides a potential new method for determining distances to
SNRs, or conversely, distances to features in the large-scale GMF if SNR
distances are known.Comment: 24 pages, 8 figures plus one 5-page appendix figure, 3 tables,
accepted to A&
Discovery of an Energetic Pulsar Associated with SNR G76.9+1.0
We report the discovery of PSR J2022+3842, a 24 ms radio and X-ray pulsar in
the supernova remnant G76.9+1.0, in observations with the Chandra X-ray
telescope, the Robert C. Byrd Green Bank Radio Telescope, and the Rossi X-ray
Timing Explorer (RXTE). The pulsar's spin-down rate implies a rotation-powered
luminosity Edot = 1.2 x 10^{38} erg/s, a surface dipole magnetic field strength
B_s = 1.0 x 10^{12} G, and a characteristic age of 8.9 kyr. PSR J2022+3842 is
thus the second-most energetic Galactic pulsar known, after the Crab, as well
as the most rapidly-rotating young, radio-bright pulsar known. The radio
pulsations are highly dispersed and broadened by interstellar scattering, and
we find that a large (delta-f / f ~= 1.9 x 10^{-6}) spin glitch must have
occurred between our discovery and confirmation observations. The X-ray pulses
are narrow (0.06 cycles FWHM) and visible up to 20 keV, consistent with
magnetospheric emission from a rotation-powered pulsar. The Chandra X-ray image
identifies the pulsar with a hard, unresolved source at the midpoint of the
double-lobed radio morphology of SNR G76.9+1.0 and embedded within faint,
compact X-ray nebulosity. The spatial relationship of the X-ray and radio
emissions is remarkably similar to extended structure seen around the Vela
pulsar. The combined Chandra and RXTE pulsar spectrum is well-fitted by an
absorbed power-law model with column density N_H = (1.7\pm0.3) x 10^{22}
cm^{-2} and photon index Gamma = 1.0\pm0.2; it implies that the Chandra
point-source flux is virtually 100% pulsed. For a distance of 10 kpc, the X-ray
luminosity of PSR J2022+3842 is L_X(2-10 keV) = 7.0 x 10^{33} erg s^{-1}.
Despite being extraordinarily energetic, PSR J2022+3842 lacks a bright X-ray
wind nebula and has an unusually low conversion efficiency of spin-down power
to X-ray luminosity, L_X/Edot = 5.9 x 10^{-5}.Comment: 8 pages in emulateapj format. Minor changes (including a shortened
abstract) to reflect the version accepted for publicatio
Multiplexed communication over a high-speed quantum channel
In quantum information systems it is of particular interest to consider the
best way in which to use the non-classical resources consumed by that system.
Quantum communication protocols are integral to quantum information systems and
are amongst the most promising near-term applications of quantum information
science. Here we show that a multiplexed, digital quantum communications system
supported by comb of vacuum squeezing has a greater channel capacity per photon
than a source of broadband squeezing with the same analogue bandwidth. We
report on the time-resolved, simultaneous observation of the first dozen teeth
in a 2.4 GHz comb of vacuum squeezing produced by a sub-threshold OPO, as
required for such a quantum communications channel. We also demonstrate
multiplexed communication on that channel
Chandra Confirmation of a Pulsar Wind Nebula in DA 495
As part of a multiwavelength study of the unusual radio supernova remnant DA
495, we present observations made with the Chandra X-ray Observatory. Imaging
and spectroscopic analysis confirms the previously detected X-ray source at the
heart of the annular radio nebula, establishing the radiative properties of two
key emission components: a soft unresolved source with a blackbody temperature
of 1 MK consistent with a neutron star, surrounded by a nonthermal nebula 40''
in diameter exhibiting a power-law spectrum with photon index Gamma =
1.6+/-0.3, typical of a pulsar wind nebula. The implied spin-down luminosity of
the neutron star, assuming a conversion efficiency to nebular flux appropriate
to Vela-like pulsars, is ~10^{35} ergs/s, again typical of objects a few tens
of kyr old. Morphologically, the nebular flux is slightly enhanced along a
direction, in projection on the sky, independently demonstrated to be of
significance in radio polarization observations; we argue that this represents
the orientation of the pulsar spin axis. At smaller scales, a narrow X-ray
feature is seen extending out 5'' from the point source, a distance consistent
with the sizes of resolved wind termination shocks around many Vela-like
pulsars. Finally, we argue based on synchrotron lifetimes in the estimated
nebular magnetic field that DA 495 represents a rare pulsar wind nebula in
which electromagnetic flux makes up a significant part, together with particle
flux, of the neutron star's wind, and that this high magnetization factor may
account for the nebula's low luminosity.Comment: 26 pages, 5 figures, AASTeX preprint style. Accepted for publication
in The Astrophysical Journa
Can optical squeezing be generated via polarization self-rotation in a thermal vapour cell?
The traversal of an elliptically polarized optical field through a thermal
vapour cell can give rise to a rotation of its polarization axis. This process,
known as polarization self-rotation (PSR), has been suggested as a mechanism
for producing squeezed light at atomic transition wavelengths. In this paper,
we show results of the characterization of PSR in isotopically enhanced
Rubidium-87 cells, performed in two independent laboratories. We observed that,
contrary to earlier work, the presence of atomic noise in the thermal vapour
overwhelms the observation of squeezing. We present a theory that contains
atomic noise terms and show that a null result in squeezing is consistent with
this theory.Comment: 10 pages, 11 figures, submitted to PRA. Please email author for a PDF
file if the article does not appear properl
Searching for the pulsar in G18.95-1.1: Discovery of an X-ray point source and associated synchrotron nebula with Chandra
Using the Chandra X-ray Observatory, we have pinpointed the location of a
faint X-ray point source (CXOUJ182913.1-125113) and an associated diffuse
nebula in the composite supernova remnant G18.95-1.1. These objects appear to
be the long-sought pulsar and its wind nebula. The X-ray spectrum of the point
source is best described by an absorbed powerlaw model with Gamma=1.6 and an
N_H of ~1x10^(22) cm^(-2). This model predicts a relatively low unabsorbed
X-ray luminosity of about L_X (0.5-8.0keV) = 4.1x10^(31)D_2^2 erg s^(-1), where
D_2 is the distance in units of 2kpc. The best-fitted model of the diffuse
nebula is a combination of thermal (kT = 0.48keV) and non-thermal (1.4 < Gamma
< 1.9) emission. The unabsorbed X-ray luminosity of L_X = 5.4x10^(33)D_2^2 erg
s^(-1) in the 0.5-8keV energy band seems to be largely dominated by the thermal
component from the SNR, providing 87% of L_X in this band. No radio or X-ray
pulsations have been reported for CXOUJ182913.1-125113. If we assume an age of
~5300yr for G18.95-1.1 and use the X-ray luminosity for the pulsar and the wind
nebula together with the relationship between spin-down luminosity (via
magnetic dipole radiation) and period, we estimate the pulsar's period to be P
= 0.4s. Compared to other rotation-powered pulsars, a magnetic field of
2.2x10^(13)G is implied by its location in the P-Pdot diagram, a value which is
close to that of the quantum critical field.Comment: 8 pages, 3 Figures, accepted for publication in Ap
A Massive Jet Ejection Event from the Microquasar SS 433 Accompanying Rapid X-Ray Variability
Microquasars occasionally exhibit massive jet ejections which are distinct
from the continuous or quasi-continuous weak jet ejections. Because those
massive jet ejections are rare and short events, they have hardly been observed
in X-ray so far. In this paper, the first X-ray observation of a massive jet
ejection from the microquasar SS 433 with the Rossi X-ray Timing Explorer
(RXTE) is reported. SS 433 undergoing a massive ejection event shows a variety
of new phenomena including a QPO-like feature near 0.1 Hz, rapid time
variability, and shot-like activities. The shot-like activity may be caused by
the formation of a small plasma bullet. A massive jet may be consist of
thousands of those plasma bullets ejected from the binary system. The size,
mass, internal energy, and kinetic energy of the bullets and the massive jet
are estimated.Comment: 21 pages including 5 figures, submitted to Ap
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