8,258 research outputs found
Oscillations of Thick Accretion Discs Around Black Holes - II
We present a numerical study of the global modes of oscillation of thick
accretion discs around black holes. We have previously studied the case of
constant distributions of specific angular momentum. In this second paper, we
investigate (i) how the size of the disc affects the oscillation
eigenfrequencies, and (ii) the effect of power-law distributions of angular
momentum on the oscillations. In particular, we compare the oscillations of the
disc with the epicyclic eigenfrequencies of a test particle with different
angular momentum distributions orbiting around the central object. We find that
there is a frequency shift away from the epicyclic eigenfrequency of the test
particle to lower values as the size of the tori is increased. We have also
studied the response of a thick accretion disc to a localized external
perturbation using non constant specific angular momentum distributions within
the disc. We find that in this case it is also possible (as reported previously
for constant angular momentum distributions) to efficiently excite internal
modes of oscillation. In fact we show here that the local perturbations excite
global oscillations (acoustic p modes) closely related to the epicyclic
oscillations of test particles. Our results are particularly relevant in the
context of low mass X-ray binaries and microquasars, and the high frequency
Quasi-Periodic Oscillations (QPOs) observed in them. Our computations make use
of a Smooth Particle Hydrodynamics (SPH) code in azimuthal symmetry, and use a
gravitational potential that mimics the effects of strong gravity.Comment: 10 pages, 8 figures, accepted for publication as a paper in the
Monthly Notices of the Royal Astronomical Societ
Oscillations of Thick Accretion Discs Around Black Holes
We present a numerical study of the response of a thick accretion disc to a
localized, external perturbation with the aim of exciting internal modes of
oscillation. We find that the perturbations efficiently excite global modes
recently identified as acoustic p--modes, and closely related to the epicyclic
oscillations of test particles. The two strongest modes occur at
eigenfrequencies which are in a 3:2 ratio. We have assumed a constant specific
angular momentum distribution within the disc. Our models are in principle
scale--free and can be used to simulate accretion tori around stellar or super
massive black holes.Comment: 4 pages, 4 figures, accepted for publication as a letter in the
Monthly Notices of the Royal Astronomical Societ
A search for radio pulsars and fast transients in M31 using the WSRT
We present the results of the most sensitive and comprehensive survey yet
undertaken for radio pulsars and fast transients in the Andromeda galaxy (M31)
and its satellites, using the Westerbork Synthesis Radio Telescope (WSRT) at a
central frequency of 328 MHz. We used the WSRT in a special configuration
called 8gr8 (eight-grate) mode, which provides a large instantaneous
field-of-view, about 5 square degrees per pointing, with good sensitivity, long
dwell times (up to 8 hours per pointing), and good spatial resolution (a few
arc minutes) for locating sources. We have searched for both periodicities and
single pulses in our data, aiming to detect bright, persistent radio pulsars
and rotating radio transients (RRATs) of either Galactic or extragalactic
origin. Our searches did not reveal any confirmed periodic signals or bright
single bursts from (potentially) cosmological distances. However, we do report
the detection of several single pulse events, some repeating at the same
dispersion measure, which could potentially originate from neutron stars in
M31. One in particular was seen multiple times, including a burst of six pulses
in 2000 seconds, at a dispersion measure of 54.7 pc cm^-3, which potentially
places the origin of this source outside of our Galaxy. Our results are
compared to a range of hypothetical populations of pulsars and RRATs in M31 and
allow us to constrain the luminosity function of pulsars in M31. They also show
that, unless the pulsar population in M31 is much dimmer than in our Galaxy,
there is no need to invoke any violation of the inverse square law of the
distance for pulsar fluxes.Comment: 18 pages, 14 figures, 8 tables. Accepted for publication in the main
journal of MNRA
Phenolics, depsides and triterpenes from the chilean lichen pseudocyphellaria nudata (zahlbr.) D.J. Galloway
Indexación: ScieloThe lichen Pseudocyphellaria nudata is a species endemic to southern South América. From the lichen tallus, methyl orsellinate, 2-methoxy-3,6-dimethyl-4-hydroxybenzaldehyde, methyl-evernate, tenuiorin, hopan-6ß,22-diol and hopan-6α,76,22-triol were isolated and identified as the main lichen constituents. This is the first report of the occurrence of 2-methoxy-3,6-dimethyl-4-hydroxybenzaldehyde in lichens.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=s0717-97072008000300017&nrm=is
Enhancements to the GW space-time method
We describe the following new features which significantly enhance the power
of the recently developed real-space imaginary-time GW scheme (Rieger et al.,
Comp. Phys. Commun. 117, 211 (1999)) for the calculation of self-energies and
related quantities of solids: (i) to fit the smoothly decaying time/energy
tails of the dynamically screened Coulomb interaction and other quantities to
model functions, treating only the remaining time/energy region close to zero
numerically and performing the Fourier transformation from time to energy and
vice versa by a combination of analytic integration of the tails and
Gauss-Legendre quadrature of the remaining part and (ii) to accelerate the
convergence of the band sum in the calculation of the Green's function by
replacing higher unoccupied eigenstates by free electron states (plane waves).
These improvements make the calculation of larger systems (surfaces, clusters,
defects etc.) accessible.Comment: 10 pages, 6 figure
Scanning tunneling microscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite
We report on a novel scheme to perform efficient simulations of Scanning
Tunneling Microscopy (STM) of molecules weakly bonded to surfaces. Calculations
are based on a tight binding (TB) technique including self-consistency for the
molecule to predict STM imaging and spectroscopy. To palliate the lack of
self-consistency in the tunneling current calculation, we performed first
principles density-functional calculations to extract the geometrical and
electronic properties of the system. In this way, we can include, in the TB
scheme, the effects of structural relaxation upon adsorption on the electronic
structure of the molecule. This approach is applied to the study of
regioregular poly(3-dodecylthiophene) (P3DDT) polymer chains adsorbed on highly
oriented pyrolytic graphite (HOPG). Results of spectroscopic calculations are
discussed and compared with recently obtained experimental datComment: 15 pages plus 5 figures in a tar fil
Time-dependent density functional theory on a lattice
A time-dependent density functional theory (TDDFT) for a quantum many-body
system on a lattice is formulated rigorously. We prove the uniqueness of the
density-to-potential mapping and demonstrate that a given density is
-representable if the initial many-body state and the density satisfy
certain well defined conditions. In particular, we show that for a system
evolving from its ground state any density with a continuous second time
derivative is -representable and therefore the lattice TDDFT is guaranteed
to exist. The TDDFT existence and uniqueness theorem is valid for any connected
lattice, independently of its size, geometry and/or spatial dimensionality. The
general statements of the existence theorem are illustrated on a pedagogical
exactly solvable example which displays all details and subtleties of the proof
in a transparent form. In conclusion we briefly discuss remaining open problems
and directions for a future research.Comment: 12 pages, 1 figur
Granular Scale Magnetic Flux Cancellations in the Photosphere
We investigate the evolution of 5 granular-scale magnetic flux cancellations
just outside the moat region of a sunspot by using accurate spectropolarimetric
measurements and G-band images with the Solar Optical Telescope aboard Hinode.
The opposite polarity magnetic elements approach a junction of the
intergranular lanes and then they collide with each other there. The
intergranular junction has strong red shifts, darker intensities than the
regular intergranular lanes, and surface converging flows. This clearly
confirms that the converging and downward convective motions are essential for
the approaching process of the opposite-polarity magnetic elements. However,
motion of the approaching magnetic elements does not always match with their
surrounding surface flow patterns in our observations. This suggests that, in
addition to the surface flows, subsurface downward convective motions and
subsurface magnetic connectivities are important for understanding the approach
and collision of the opposite polarity elements observed in the photosphere. We
find that the horizontal magnetic field appears between the canceling opposite
polarity elements in only one event. The horizontal fields are observed along
the intergranular lanes with Doppler red shifts. This cancellation is most
probably a result of the submergence (retraction) of low-lying photospheric
magnetic flux. In the other 4 events, the horizontal field is not observed
between the opposite polarity elements at any time when they approach and
cancel each other. These approaching magnetic elements are more concentrated
rather than gradually diffused, and they have nearly vertical fields even while
they are in contact each other. We thus infer that the actual flux cancellation
is highly time dependent events at scales less than a pixel of Hinode SOT
(about 200 km) near the solar surface.Comment: Accepted for publication in the Astrophysical Journa
Development Of An Engineered Bioluminescent Reporter Phage For Detection Of Bacterial Blight Of Crucifers
Bacterial blight, caused by the phytopathogen Pseudomonas cannabina pv. alisalensis, is an emerging disease afflicting important members of the Brassicaceae family. The disease is often misdiagnosed as pepper spot, a much less severe disease caused by the related pathogen Pseudomonas syringae pv. maculicola. We have developed a phage-based diagnostic that can both identify and detect the causative agent of bacterial blight and differentiate the two pathogens. A recombinant >light>-tagged reporter phage was generated by integrating bacterial luxAB genes encoding luciferase into the genome of P. cannabina pv. alisalensis phage PBSPCA1. The PBSPCA1::luxAB reporter phage is viable and stable and retains properties similar to those of the wildtype phage. PBSPCA1::luxAB rapidly and sensitively detects P. cannabina pv. alisalensis by conferring a bioluminescent signal response to cultured cells. Detection is dependent on cell viability. Other bacterial pathogens of Brassica species such as P. syringae pv. maculicola, Pseudomonas marginalis, Pectobacterium carotovorum, Xanthomonas campestris pv. campestris, and X. campestris pv. raphani either do not produce a response or produce significantly attenuated signals with the reporter phage. Importantly, the reporter phage detects P. cannabina pv. alisalensis on diseased plant specimens, indicating its potential for disease diagnosis.National Science Foundation Small Business Innovative Research 1012059U.S. Department of EducationU.S. Department of AgricultureCellular and Molecular Biolog
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