40,860 research outputs found
The variability of the Crab Nebula in radio: No radio counterpart to gamma-ray flares
We present new Jansky Very Large Array (VLA) radio images of the Crab Nebula
at 5.5 GHz, taken at two epochs separated by 6 days about two months after a
gamma-ray flare in 2012 July. We find no significant change in the Crab's radio
emission localized to a region of <2 light-months in radius, either over the
6-day interval between our present observations or between the present
observations and ones from 2001. Any radio counterpart to the flare has a radio
luminosity of <~ times that of the nebula. Comparing our
images to one from 2001, we do however find changes in radio brightness, up to
10% in amplitude, which occur on decade timescales throughout the nebula. The
morphology of the changes is complex suggesting both filamentary and knotty
structures. The variability is stronger, and the timescales likely somewhat
shorter, nearer the centre of the nebula. We further find that even with the
excellent uv~coverage and signal-to-noise of the VLA, deconvolution errors are
much larger than the noise, being up to 1.2% of peak brightness of the nebula
in this particular case.Comment: Accepted to MNRAS; 13 pages, 6 figure
Linear Precoding for Broadcast Channels with Confidential Messages under Transmit-Side Channel Correlation
In this paper, we analyze the performance of regularized channel inversion
(RCI) precoding in multiple-input single-output (MISO) broadcast channels with
confidential messages under transmit-side channel correlation. We derive a
deterministic equivalent for the achievable per-user secrecy rate which is
almost surely exact as the number of transmit antennas and the number of users
grow to infinity in a fixed ratio, and we determine the optimal regularization
parameter that maximizes the secrecy rate. Furthermore, we obtain deterministic
equivalents for the secrecy rates achievable by: (i) zero forcing precoding and
(ii) single user beamforming. The accuracy of our analysis is validated by
simulations of finite-size systems.Comment: to appear IEEE Communications Letter
Differential quadrature method for space-fractional diffusion equations on 2D irregular domains
In mathematical physics, the space-fractional diffusion equations are of
particular interest in the studies of physical phenomena modelled by L\'{e}vy
processes, which are sometimes called super-diffusion equations. In this
article, we develop the differential quadrature (DQ) methods for solving the 2D
space-fractional diffusion equations on irregular domains. The methods in
presence reduce the original equation into a set of ordinary differential
equations (ODEs) by introducing valid DQ formulations to fractional directional
derivatives based on the functional values at scattered nodal points on problem
domain. The required weighted coefficients are calculated by using radial basis
functions (RBFs) as trial functions, and the resultant ODEs are discretized by
the Crank-Nicolson scheme. The main advantages of our methods lie in their
flexibility and applicability to arbitrary domains. A series of illustrated
examples are finally provided to support these points.Comment: 25 pages, 25 figures, 7 table
Dust-to-gas ratio, factor and CO-dark gas in the Galactic anticentre: an observational study
We investigate the correlation between extinction and H~{\sc i} and CO
emission at intermediate and high Galactic latitudes (|b|>10\degr) within the
footprint of the Xuyi Schmidt Telescope Photometric Survey of the Galactic
anticentre (XSTPS-GAC) on small and large scales. In Paper I (Chen et al.
2014), we present a three-dimensional dust extinction map within the footprint
of XSTPS-GAC, covering a sky area of over 6,000\,deg at a spatial angular
resolution of 6\,arcmin. In the current work, the map is combined with data
from gas tracers, including H~{\sc i} data from the Galactic Arecibo L-band
Feed Array H~{\sc i} survey and CO data from the Planck mission, to constrain
the values of dust-to-gas ratio and CO-to-
conversion factor for the entire GAC
footprint excluding the Galactic plane, as well as for selected star-forming
regions (such as the Orion, Taurus and Perseus clouds) and a region of diffuse
gas in the northern Galactic hemisphere. For the whole GAC footprint, we find
\, and \,. We have also
investigated the distribution of "CO-dark" gas (DG) within the footprint of GAC
and found a linear correlation between the DG column density and the -band
extinction: . The mass fraction of DG is found to be toward
the Galactic anticentre, which is respectively about 23 and 124 per cent of the
atomic and CO-traced molecular gas in the same region. This result is
consistent with the theoretical work of Papadopoulos et al. but much larger
than that expected in the cloud models by Wolfire et al.Comment: 11 pages, 7 figures, accepted for publication in MNRA
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Directionality and complexity of L1 transfer in L2 acquisition: Evidence from L2 Chinese discourse
Abstract
First language (L1) transfer is a common phenomenon in second language (L2) acquisition. However, it will be argued in this article that although there are indeed pervasive influences of learners’ L1 in L2 acquisition, L1 transfer is not everywhere and it can be directional. We compare data from Chang’s (2001b. Discourse effects on the second language acquisition of English and Chinese dative structures. Honolulu: University of Hawai’i at Manoa PhD dissertation, 2004. Discourse effects on EFL learners’ production of dative constructions. Journal of Kaohsiung University of Applied Sciences 33. 145–169.) studies of Chinese-speaking learners of English with data of our study of English-speaking learners of Chinese to examine whether their L2 English discourse and L2 Chinese discourse are equally influenced by their L1 discourse rules. We focus on learners’ answers to wh-questions with a double object construction or a prepositional object construction. The results demonstrate that L1 transfer takes place in Chinese-speaking learners’ L2 English discourse but not in English-speaking learners’ L2 Chinese discourse. This directionality of L1 transfer is accounted for on the basis of computational complexity of linguistic structures involved and on an economical consideration.</jats:p
Spectroscopic Observations of Planetary Nebulae in the Northern Spur of M31
We present spectroscopy of three planetary nebulae (PNe) in the Northern Spur
of the Andromeda Galaxy (M31) obtained with the Double Spectrograph on the 5.1
m Hale Telescope at the Palomar Observatory. The samples are selected from the
observations of Merrett et al. Our purpose is to investigate formation of the
substructures of M31 using PNe as a tracer of chemical abundances. The [O III]
4363 auroral line is detected in the spectra of two objects, enabling
temperature determinations. Ionic abundances are derived from the observed
collisionally excited lines, and elemental abundances of nitrogen, oxygen, and
neon as well as sulphur and argon are estimated. Correlations between oxygen
and the alpha-element abundance ratios are studied, using our sample and the
M31 disk and bulge PNe from the literature. In one of the three PNe, we
observed relatively higher oxygen abundance compared to the disk sample in M31
at similar galactocentric distances. The results of at least one of the three
Northern Spur PNe might be in line with the proposed possible origin of the
Northern Spur substructure of M31, i.e. the Northern Spur is connected to the
Southern Stream and both substructures comprise the tidal debris of the
satellite galaxies of M31.Comment: 5 tables, 17 figures; accepted for publication in Ap
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