2,068 research outputs found
Gravitational Microlensing by the Ellis Wormhole
A method to calculate light curves of the gravitational microlensing of the
Ellis wormhole is derived in the weak-field limit. In this limit, lensing by
the wormhole produces one image outside the Einstein ring and one other image
inside. The weak-field hypothesis is a good approximation in Galactic lensing
if the throat radius is less than . The light curves calculated
have gutters of approximately 4% immediately outside the Einstein ring crossing
times. The magnification of the Ellis wormhole lensing is generally less than
that of Schwarzschild lensing. The optical depths and event rates are
calculated for the Galactic bulge and Large Magellanic Cloud fields according
to bound and unbound hypotheses. If the wormholes have throat radii between 100
and , are bound to the galaxy, and have a number density that is
approximately that of ordinary stars, detection can be achieved by reanalyzing
past data. If the wormholes are unbound, detection using past data is
impossible.Comment: 27 pages, 4 figures, ApJ accepte
Comments on the paper ``Bare Quark Surfacees of Strange Stars and Electron-Positron Pair Emission''
In a recent paper (Ushov, PRL, 80, 230, 1998), it has been claimed that the
bare surface of a strange star can emit electron-positron pairs of luminosity
\~10^{51} ergs/s for about 10s. If true, obviously, this mechanism may explain
the origin of cosmic Gamma Ray Bursts. However, we point out that such a
mechanism is does not work because (i) if pair production really occurs the
supposed pre-existing supercritical electric field will be quenched and this
discharge process may at best release ~10^{24} ergs of electromagnetic energy,
and (ii) there is no way by which the trapped core thermal energy of few
10^{52} ergs can be transmitted electromagnetically on a time scale of ~10s or
even on a much larger time scale. The only way the hot core can cool on a time
scale of ~10 s or much shorter is by the well known process of emission of
nu-antinu pairs.Comment: Final version accepted in Phy. Rev. Lett. Main conclusion that the
mechanism by Usov does not work remains unchanged,
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Air cooling : an experimental method of evaluating the cooling effect of air streams on air-cooled cylinders
In this report is described an experimental method which the writer has evolved for dealing with air-cooled engines, and some of the data obtained by its means. Methods of temperature measurement and cooling are provided
Interbacterial competition mediated by the type VIIb secretion system
\ua9 2023 The Authors. Successful occupancy of a given niche requires the colonising bacteria to interact extensively with the biotic and abiotic environment, including other resident microbes. Bacteria have evolved a range of protein secretion machines for this purpose with eleven such systems identified to date. The type VIIb secretion system (T7SSb) is utilised by Bacillota to secrete a range of protein substrates, including antibacterial toxins targeting closely related strains, and the system as a whole has been implicated in a range of activities such as iron acquisition, intercellular signalling, host colonisation and virulence. This review covers the components and secretion mechanism of the T7SSb, the substrates of these systems and their roles in Gram-positive bacteria, with a focus on interbacterial competition
Finding outlier light-curves in catalogs of periodic variable stars
We present a methodology to discover outliers in catalogs of periodic
light-curves. We use cross-correlation as measure of ``similarity'' between two
individual light-curves and then classify light-curves with lowest average
``similarity'' as outliers. We performed the analysis on catalogs of variable
stars of known type from the MACHO and OGLE projects and established that our
method correctly identifies light-curves that do not belong to those catalogs
as outliers. We show how our method can scale to large datasets that will be
available in the near future such as those anticipated from Pan-STARRS and
LSST.Comment: 16 pages, 24 figure
Gravitational Waves from Sub-lunar Mass Primordial Black Hole Binaries - A New Probe of Extradimensions
In many braneworld models, gravity is largely modified at the electro-weak
scale ~ 1TeV. In such models, primordial black holes (PBHs) with lunar mass M ~
10^{-7}M_sun might have been produced when the temperature of the universe was
at ~ 1TeV. If a significant fraction of the dark halo of our galaxy consists of
these lunar mass PBHs, a huge number of BH binaries will exist in our
neighborhood. Third generation detectors such as EURO can detect gravitational
waves from these binaries, and can also determine their chirp mass. With a new
detector designed to be sensitive at high frequency bands greater than 1 kHz,
the existence of extradimensions could be confirmed.Comment: 4 pages, 1 figure, typos correcte
The development of reading tests for use in a regularly spelled language.
Data are presented on the development of tests of reading skill in rural Tanzanian primary school pupils. Instruction in these schools is in Kiswahili, a regularly spelt language. Using a translation of a standard reading test, children could read aloud all words once they had learnt sound-letter correspondences, regardless of comprehension. In addition, children can appear to pass traditional comprehension tasks by decoding only some of the words. Three graded tests were developed which allow testing of children who either have only some letter knowledge, can read single words, or are proficient readers. The tests require children to both decode and understand the reading material in order to achieve high scores. The tests correlated well with scores on other educational achievement tests, and showed age and school grade differences. It is suggested that these tests are useful measures of reading development in a regularly spelt language. Adaptation to English and validation against standardised instruments is planned
Microlensing as a probe of the Galactic structure; 20 years of microlensing optical depth studies
Microlensing is now a very popular observational astronomical technique. The
investigations accessible through this effect range from the dark matter
problem to the search for extra-solar planets. In this review, the techniques
to search for microlensing effects and to determine optical depths through the
monitoring of large samples of stars will be described. The consequences of the
published results on the knowledge of the Milky-Way structure and its dark
matter component will be discussed. The difficulties and limitations of the
ongoing programs and the perspectives of the microlensing optical depth
technique as a probe of the Galaxy structure will also be detailed.Comment: Accepted for publication in General Relativity and Gravitation.
General Relativity and Gravitation in press (2010) 0
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