625 research outputs found
Viscosity in spherically symmetric accretion
The influence of viscosity on the flow behaviour in spherically symmetric
accretion, has been studied here. The governing equation chosen has been the
Navier-Stokes equation. It has been found that at least for the transonic
solution, viscosity acts as a mechanism that detracts from the effectiveness of
gravity. This has been conjectured to set up a limiting scale of length for
gravity to bring about accretion, and the physical interpretation of such a
length-scale has been compared with the conventional understanding of the
so-called "accretion radius" for spherically symmetric accretion. For a
perturbative presence of viscosity, it has also been pointed out that the
critical points for inflows and outflows are not identical, which is a
consequence of the fact that under the Navier-Stokes prescription, there is a
breakdown of the invariance of the stationary inflow and outflow solutions --
an invariance that holds good under inviscid conditions. For inflows, the
critical point gets shifted deeper within the gravitational potential well.
Finally, a linear stability analysis of the stationary inflow solutions, under
the influence of a perturbation that is in the nature of a standing wave, has
indicated that the presence of viscosity induces greater stability in the
system, than has been seen for the case of inviscid spherically symmetric
inflows.Comment: 7 pages. Minor changes made in the version published in MNRA
Antimicrobial Resistance and Virulence Genes in Enterococcus faecium and Enterococcus faecalis from Humans and Retail Red Meat
The emergence of antimicrobial-resistant and virulent enterococci is a major public health concern. While enterococci are commonly found in food of animal origin, the knowledge on their zoonotic potential is limited. The aim of this study was to determine and compare the antimicrobial susceptibility and virulence traits of Enterococcus faecalis and Enterococcus faecium isolates from human clinical specimens and retail red meat in Slovenia. A total of 242 isolates were investigated: 101 from humans (71 E. faecalis, 30 E. faecium) and 141 from fresh beef and pork (120 E. faecalis, 21 E. faecium). The susceptibility to 12 antimicrobials was tested using a broth microdilution method, and the presence of seven common virulence genes was investigated using PCR. In both species, the distribution of several resistance phenotypes and virulence genes was disparate for isolates of different origin. All isolates were susceptible to daptomycin, linezolid, teicoplanin, and vancomycin. In both species, the susceptibility to antimicrobials was strongly associated with a food origin and the multidrug resistance, observed in 29.6% of E. faecalis and 73.3% E. faecium clinical isolates, with a clinical origin (Fisher's exact test). Among meat isolates, in total 66.0% of E. faecalis and E. faecium isolates were susceptible to all antimicrobials tested and 32.6% were resistant to either one or two antimicrobials. In E. faecalis, several virulence genes were significantly associated with a clinical origin; the most common (31.0%) gene pattern included all the tested genes except hyl. In meat isolates, the virulence genes were detected in E. faecalis only and the most common pattern included ace, efaA, and gelE (32.5%), of which gelE showed a statistically significant association with a clinical origin. These results emphasize the importance of E. faecalis in red meat as a reservoir of virulence genes involved in its persistence and human infections with reported severe outcomes
Uniqueness of the mass in the radiating regime
The usual approaches to the definition of energy give an ambiguous result for
the energy of fields in the radiating regime. We show that for a massless
scalar field in Minkowski space-time the definition may be rendered unambiguous
by adding the requirement that the energy cannot increase in retarded time. We
present a similar theorem for the gravitational field, proved elsewhere, which
establishes that the Trautman-Bondi energy is the unique (up to a
multiplicative factor) functional, within a natural class, which is monotonic
in time for all solutions of the vacuum Einstein equations admitting a smooth
``piece'' of conformal null infinity Scri.Comment: 8 pages, revte
NGC300 X-1 and IC10 X-1: a new breed of black hole binary?
[ABRIDGED] IC10 X-1 has recently been confirmed as a black hole (BH) +
Wolf-Rayet (WR) X-ray binary, and NGC300 X-1 is thought to be. IC10 X-1 and
NGC300 X-1 have similar X-ray properties, with luminosities ~10^38 erg/s, and
orbital periods ~30 hr. We investigate similarities between these two, as well
as differences between them and the known Galactic BH binary systems. We have
examined XMM-Newton observations of NGC300 X-1 and IC10 X-1. We extracted
lightcurves and spectra; power density spectra (PDS) were constructed from the
lightcurves, and the X-ray emission spectra were modeled. Each source exhibits
PDS that are characteristic of disc-accreting X-ray binaries (XBs) in the high
state. In this state, Galactic XBs with known BH primaries have soft, thermal
emission; however the emission spectra of our targets are predominantly
non-thermal. Furthermore, the Observation 1 spectrum of NGC300 X-1 is
strikingly similar to that of IC10 X-1. The remarkable similarity between the
behaviour of NGC300 X-1 in Observation 1 and that of IC10 X-1 lends strong
evidence for NGC300 X-1 being a (BH+WR) binary. The unusual spectra of NGC300
X-1 and IC10 X-1 may be due to these systems existing in a persistently high
state, whereas all known BH LMXBs are transient. BH XBs in a persistent high
state could retain their corona, and hence exhibit a large non-thermal
component. LMC X-1 is a BH XB that has only been observed in the high state,
and its spectrum is remarkably similar to those of our targets. We therefore
classify NGC300 X-1, IC10 X-1 and perhaps LMC X-1 as a new breed of BH XB,
defined by their persistently high accretion rates and consequent stable disc
configuration and corona. This scenario may also explain the lack of
ultraluminous X-ray sources in the canonical soft state.Comment: Approved for publication in A&A. 8 pages, 5 figure
Bondi-Sachs metrics and Photon Rockets
We study the Bondi-Sachs rockets with nonzero cosmological constant. We
observe that the acceleration of the systems arises naturally in the asymptotic
symmetries of (anti-) de Sitter spacetimes. Assuming the validity of the
concepts of energy and mass previously introduced in asymptotically flat
spacetimes, we find that the emission of pure radiation energy balances the
loss of the Bondi mass in certain special families of the Bondi-Sachs rockets,
so in these there is no gravitational radiation.Comment: 12 pages, to appear in General Relativity and Gravitatio
Stochasticity, decoherence and an arrow of time from the discretization of time?
Certain intriguing consequences of the discreteness of time on the time
evolution of dynamical systems are discussed. In the discrete-time classical
mechanics proposed here, there is an {\it arrow of time} that follows from the
fact that the replacement of the time derivative by the backward difference
operator alone can preserve the non-negativity of the phase space density. It
is seen that, even for free particles, all the degrees of freedom are {\it
correlated} in principle. The forward evolution of functions of phase space
variables by a finite number of time steps, in this discrete-time mechanics,
depends on the entire continuous-time history in the interval . In
this sense, discrete time evolution is {\it nonlocal} in time from a
continuous-time point of view. A corresponding quantum mechanical treatment is
possible {\it via} the density matrix approach. The interference between
non-degenerate quantum mechanical states decays exponentially. This {\it
decoherence} is present, in principle, for all systems; however, it is of
practical importance only in macroscopic systems, or in processes involving
large energy changes.Comment: 10 pages, no figure
A multi-coloured survey of NGC 253 with XMM-Newton: testing the methods used for creating luminosity functions from low-count data
NGC 253 is a local, star-bursting spiral galaxy with strong X-ray emission
from hot gas, as well as many point sources. We have conducted a spectral
survey of the X-ray population of NGC 253 using a deep XMM-Newton
observation.NGC 253 only accounts for ~20% of the XMM-Newton EPIC field of
view, allowing us to identify ~100 X-ray sources that are unlikely to be
associated with NGC\thinspace 253. Hence we were able to make a direct estimate
of contamination from e.g. foreground stars and background galaxies.
X-ray luminosity functions (XLFs) of galaxy populations are often used to
characterise their properties. There are several methods for estimating the
luminosities of X-ray sources with few photons. We have obtained spectral fits
for the brightest 140 sources in the 2003 XMM-Newton observation of NGC 253,
and compare the best fit luminosities of those 69 non-nuclear sources
associated with NGC 253 with luminosities derived using other methods.
We find the luminosities obtained from these various methods to vary
systematically by a factor of up to three for the same data; this is largely
due to differences in absorption.
We therefore conclude that assuming Galactic absorption is probably unwise;
rather, one should measure the absorption for the population.
A remarkable correlation has been reported between the XLFs of galaxies and
their star formation rates. However, the XLFs used in that study were obtained
using several different methods. If the sample galaxies were revisited and a
single method were applied, then this correlation may become stronger still.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society (MNRAS). 17 pages, 7 figure
Yang-Mills condensate dark energy coupled with matter and radiation
The coincidence problem is studied for the dark energy model of effective
Yang-Mills condensate in a flat expanding universe during the matter-dominated
stage. The YMC energy is taken to represent the dark energy, which
is coupled either with the matter, or with both the matter and the radiation
components. The effective YM Lagrangian is completely determined by quantum
field theory up to 1-loop order. It is found that under very generic initial
conditions and for a variety of forms of coupling, the existence of the scaling
solution during the early stages and the subsequent exit from the scaling
regime are inevitable. The transition to the accelerating stage always occurs
around a redshift . Moreover, when the Yang-Mills
condensate transfers energy into matter or into both matter and radiation, the
equation of state of the Yang-Mills condensate can cross over -1 around
, and takes on a current value . This is consistent with
the recent preliminary observations on supernovae Ia. Therefore, the
coincidence problem can be naturally solved in the effective YMC dark energy
models.Comment: 24 pages, 18 figure
The First Space-Based Gravitational-Wave Detectors
Gravitational waves provide a laboratory for general relativity and a window
to energetic astrophysical phenomena invisible with electromagnetic radiation.
Several terrestrial detectors are currently under construction, and a
space-based interferometer is envisioned for launch early next century to
detect test-mass motions induced by waves of relatively short wavelength.
Very-long-wavelength gravitational waves can be detected using the plasma in
the early Universe as test masses; the motion induced in the plasma by a wave
is imprinted onto the cosmic microwave background (CMB). While the signature of
gravitational waves on the CMB temperature fluctuations is not unique, the
polarization pattern can be used to unambiguously detect gravitational
radiation. Thus, forthcoming CMB polarization experiments, such as MAP and
Planck, will be the first space-based gravitational-wave detectors.Comment: 5 pages, 3 postscript figure
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