7,400 research outputs found
Influence of the r-mode instability on hypercritically accreting neutron stars
We have investigated an influence of the r-mode instability on
hypercritically accreting () neutron stars in
close binary systems during their common envelope phases based on the scenario
proposed by Bethe et al. \shortcite{bethe-brown-lee}. On the one hand neutron
stars are heated by the accreted matter at the stellar surface, but on the
other hand they are also cooled down by the neutrino radiation. At the same
time, the accreted matter transports its angular momentum and mass to the star.
We have studied the evolution of the stellar mass, temperature and rotational
frequency.
The gravitational-wave-driven instability of the r-mode oscillation strongly
suppresses spinning-up of the star, whose final rotational frequency is well
below the mass-shedding limit, typically as small as 10% of that of the
mass-shedding state. On a very short time scale the rotational frequency tends
to approach a certain constant value and saturates there as far as the amount
of the accreted mass does not exceed a certain limit to collapse to a black
hole. This implies that the similar mechanism of gravitational radiation as the
so-called Wagoner star may work in this process. The star is spun up by
accretion until the angular momentum loss by gravitational radiation balances
the accretion torque. The time-integrated dimensionless strain of the radiated
gravitational wave may be large enough to be detectable by the gravitational
wave detectors such as LIGO II.Comment: 6 pages, 3 figure
Two types of softening detected in X-ray afterglows of Swift bursts: internal and external shock origins?
The softening process observed in the steep decay phase of early X-ray
afterglows of Swift bursts has remained a puzzle since its discovery. The
softening process can also be observed in the later phase of the bursts and its
cause has also been unknown. Recently, it was suggested that, influenced by the
curvature effect, emission from high latitudes would shift the Band function
spectrum from higher energy band to lower band, and this would give rise to the
observed softening process accompanied by a steep decay of the flux density.
The curvature effect scenario predicts that the terminating time of the
softening process would be correlated with the duration of the process. In this
paper, based on the data from the UNLV GRB group web-site, we found an obvious
correlation between the two quantities. In addition, we found that the
softening process can be divided into two classes: the early type softening
() and the late type softening ().
The two types of softening show different behaviors in the duration vs.
terminating time plot. In the relation between the variation rates of the flux
density and spectral index during the softening process, a discrepancy between
the two types of softening is also observed. According to their time scales and
the discrepancy between them, we propose that the two types are of different
origins: the early type is of internal shock origin and the late type is of
external shock origin. The early softening is referred to the steep decay just
following the prompt emission, whereas the late decay typically conceives the
transition from flat decay to late afterglow decay. We suspect that there might
be a great difference of the Lorentz factor in two classes which is responsible
for the observed discrepancy.Comment: 20 pages, 5 figures, 2 tables, Accepted for Publication to Journal of
Cosmology and Astroparticle Physics (JCAP
Wave Energy Amplification in a Metamaterial based Traveling Wave Structure
We consider the interaction between a particle beam and a propagating
electromagnetic wave in the presence of a metamaterial. We show that the
introduction of a metamaterial gives rise to a novel dispersion curve which
determines a unique wave particle relationship, via the frequency dependence of
the metamaterial and the novel ability of metamaterials to exhibit simultaneous
negative permittivity and permeability. Using a modified form of Madey's
theorem we find that the novel dispersion of the metamaterial leads to a
amplification of the EM wave power
Fluorescent oxide nanoparticles adapted to active tips for near-field optics
We present a new kind of fluorescent oxide nanoparticles with properties well
suited to active-tip based near-field optics. These particles with an average
diameter in the range 5-10 nm are produced by Low Energy Cluster Beam
Deposition (LECBD) from a YAG:Ce3+ target. They are studied by transmission
electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), classical
photoluminescence, cathodoluminescence and near-field scanning optical
microscopy (NSOM). Particles of extreme photo-stability as small as 10 nm in
size are observed. These emitters are validated as building blocks of active
NSOM tips by coating a standard optical tip with a 10 nm thick layer of
YAG:Ce3+ particles directly in the LECBD reactor and by subsequently performing
NSOM imaging of test surfaces.Comment: Changes made following Referee's comments; added references; one
added figure. See story on this article at:
http://nanotechweb.org/cws/article/tech/3606
Circumstellar Na I and Ca II lines in type IIP supernovae and SN 1998S
We study a possibility of detection of circumstellar absorption lines of Na I
D and Ca II H,K in spectra of type IIP supernovae at the photospheric
epoch. The modelling shows that the circumstellar lines of Na I doublet will
not be seen in type IIP supernovae for moderate wind density, e.g.,
characteristic of SN 1999em, whereas rather pronounced Ca II lines with P Cygni
profile should be detectable. A similar model is used to describe Na I and Ca
II circumstellar lines seen in SN 1998S, type IIL with a dense wind. We show
that line intensities in this supernova are reproduced, if one assumes an
ultraviolet excess, which is caused primarily by the comptonization of
supernova radiation in the shock wave.Comment: To be published in Astronomy Letter
Dynamic Evolution Model of Isothermal Voids and Shocks
We explore self-similar hydrodynamic evolution of central voids embedded in
an isothermal gas of spherical symmetry under the self-gravity. More
specifically, we study voids expanding at constant radial speeds in an
isothermal gas and construct all types of possible void solutions without or
with shocks in surrounding envelopes. We examine properties of void boundaries
and outer envelopes. Voids without shocks are all bounded by overdense shells
and either inflows or outflows in the outer envelope may occur. These
solutions, referred to as type void solutions, are further
divided into subtypes and
according to their characteristic behaviours across the sonic critical line
(SCL). Void solutions with shocks in envelopes are referred to as type
voids and can have both dense and quasi-smooth edges.
Asymptotically, outflows, breezes, inflows, accretions and static outer
envelopes may all surround such type voids. Both cases of
constant and varying temperatures across isothermal shock fronts are analyzed;
they are referred to as types and
void shock solutions. We apply the `phase net matching procedure' to construct
various self-similar void solutions. We also present analysis on void
generation mechanisms and describe several astrophysical applications. By
including self-gravity, gas pressure and shocks, our isothermal self-similar
void (ISSV) model is adaptable to various astrophysical systems such as
planetary nebulae, hot bubbles and superbubbles in the interstellar medium as
well as supernova remnants.Comment: 24 pages, 13 figuers, accepted by ApS
Hubble Space Telescope WFPC2 Imaging of SN 1979C and Its Environment
The locations of supernovae in the local stellar and gaseous environment in
galaxies contain important clues to their progenitor stars. As part of a
program to study the environments of supernovae using Hubble Space Telescope
(HST) imaging data, we have examined the environment of the Type II-L SN 1979C
in NGC 4321 (M100). We place more rigorous constraints on the mass of the SN
progenitor, which may have had a mass M \approx 17--18 M_sun. Moreover, we have
recovered and measured the brightness of SN 1979C, m=23.37 in F439W (~B;
m_B(max) = 11.6), 17 years after explosion. .Comment: 18 pages, 8 figures, submitted to PAS
Combining hydrology and mosquito population models to identify the drivers of Rift Valley fever emergence in semi-arid regions of West Africa
BACKGROUND: Rift Valley fever (RVF) is a vector-borne viral zoonosis of increasing global importance. RVF virus (RVFV) is transmitted either through exposure to infected animals or through bites from different species of infected mosquitoes, mainly of Aedes and Culex genera. These mosquitoes are very sensitive to environmental conditions, which may determine their presence, biology, and abundance. In East Africa, RVF outbreaks are known to be closely associated with heavy rainfall events, unlike in the semi-arid regions of West Africa where the drivers of RVF emergence remain poorly understood. The assumed importance of temporary ponds and rainfall temporal distribution therefore needs to be investigated. METHODOLOGY/PRINCIPAL FINDINGS: A hydrological model is combined with a mosquito population model to predict the abundance of the two main mosquito species (Aedes vexans and Culex poicilipes) involved in RVFV transmission in Senegal. The study area is an agropastoral zone located in the Ferlo Valley, characterized by a dense network of temporary water ponds which constitute mosquito breeding sites. The hydrological model uses daily rainfall as input to simulate variations of pond surface areas. The mosquito population model is mechanistic, considers both aquatic and adult stages and is driven by pond dynamics. Once validated using hydrological and entomological field data, the model was used to simulate the abundance dynamics of the two mosquito species over a 43-year period (1961–2003). We analysed the predicted dynamics of mosquito populations with regards to the years of main outbreaks. The results showed that the main RVF outbreaks occurred during years with simultaneous high abundances of both species. CONCLUSION/SIGNIFICANCE: Our study provides for the first time a mechanistic insight on RVFV transmission in West Africa. It highlights the complementary roles of Aedes vexans and Culex poicilipes mosquitoes in virus transmission, and recommends the identification of rainfall patterns favourable for RVFV amplification
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