1,878 research outputs found
The Low Quiescent X-Ray Luminosity of the Transient X-Ray Burster EXO 1747-214
We report on X-ray and optical observations of the X-ray burster EXO
1747-214. This source is an X-ray transient, and its only known outburst was
observed in 1984-1985 by the EXOSAT satellite. We re-analyzed the EXOSAT data
to derive the source position, column density, and a distance upper limit using
its peak X-ray burst flux. We observed the EXO 1747-214 field in 2003 July with
the Chandra X-ray Observatory to search for the quiescent counterpart. We found
one possible candidate just outside the EXOSAT error circle, but we cannot rule
out the possibility that the source is unrelated to EXO 1747-214. Our
conclusion is that the upper limit on the unabsorbed 0.3-8 keV luminosity is L
< 7E31 erg/s, making EXO 1747-214 one of the faintest neutron star transients
in quiescence. We compare this luminosity upper limit to the quiescent
luminosities of 19 neutron star and 14 black hole systems and discuss the
results in the context of the differences between neutron stars and black
holes. Based on the theory of deep crustal heating by Brown and coworkers, the
luminosity implies an outburst recurrence time of >1300 yr unless some form of
enhanced cooling occurs within the neutron star. The position of the possible
X-ray counterpart is consistent with three blended optical/IR sources with
R-magnitudes between 19.4 and 19.8 and J-magnitudes between 17.2 and 17.6. One
of these sources could be the quiescent optical/IR counterpart of EXO 1747-214.Comment: 7 pages, accepted by the Astrophysical Journa
RNA secondary structure design
We consider the inverse-folding problem for RNA secondary structures: for a
given (pseudo-knot-free) secondary structure find a sequence that has that
structure as its ground state. If such a sequence exists, the structure is
called designable. We implemented a branch-and-bound algorithm that is able to
do an exhaustive search within the sequence space, i.e., gives an exact answer
whether such a sequence exists. The bound required by the branch-and-bound
algorithm are calculated by a dynamic programming algorithm. We consider
different alphabet sizes and an ensemble of random structures, which we want to
design. We find that for two letters almost none of these structures are
designable. The designability improves for the three-letter case, but still a
significant fraction of structures is undesignable. This changes when we look
at the natural four-letter case with two pairs of complementary bases:
undesignable structures are the exception, although they still exist. Finally,
we also study the relation between designability and the algorithmic complexity
of the branch-and-bound algorithm. Within the ensemble of structures, a high
average degree of undesignability is correlated to a long time to prove that a
given structure is (un-)designable. In the four-letter case, where the
designability is high everywhere, the algorithmic complexity is highest in the
region of naturally occurring RNA.Comment: 11 pages, 10 figure
New archaeointensity data from Italy and geomagnetic ïŹeld intensity variation in the Italian Peninsula
We present new archaeointensity results from three Italian kilns situated at Ascoli Satriano,
Vagnari and Fontanetto Po obtained with the Thellier modified by Coe double heating method.
These data complement the directional results previously published. All sites are dated on
the basis of archaeological information and/or thermoluminescence dating. The results are
corrected for the anisotropy of the thermoremanent magnetization and the cooling rate effects.
The new data are compared with previously published archaeointensity data from Italy and
nearby countries within 900 km radius from Viterbo. An initial data set including archaeointesity
data mainly coming from Italy, France, Switzerland, Czech Republic, Slovakia, Hungary,
Greece and Bulgaria has been compiled. After the application of strict selection criteria, the
most reliable data have been used for the calculation of a preliminary Italian intensity secular
variation (SV) curve for the last 3000 yr. The new curve covers the 300 BCâ400 AD
and 1200â1900 AD periods. It is established by means of sliding windows of 200 yr shifted
by 100 yr. The lack of reliable data for the 1000â200 BC and 400â1200 AD time intervals
does not permit the calculation of a continuous curve. Clearly, more high-quality archaeointensity
data from Italy and Europe are still needed to draw a robust intensity SV curve for
the Italian Peninsula that could be used for archaeomagnetic dating in combination with the directional data
Reduction of Two-Dimensional Dilute Ising Spin Glasses
The recently proposed reduction method is applied to the Edwards-Anderson
model on bond-diluted square lattices. This allows, in combination with a
graph-theoretical matching algorithm, to calculate numerically exact ground
states of large systems. Low-temperature domain-wall excitations are studied to
determine the stiffness exponent y_2. A value of y_2=-0.281(3) is found,
consistent with previous results obtained on undiluted lattices. This
comparison demonstrates the validity of the reduction method for bond-diluted
spin systems and provides strong support for similar studies proclaiming
accurate results for stiffness exponents in dimensions d=3,...,7.Comment: 7 pages, RevTex4, 6 ps-figures included, for related information, see
http://www.physics.emory.edu/faculty/boettcher
Seeing motion and apparent motion
In apparent motion experiments, participants are presented with what is in fact a succession of two brief stationary stimuli at two different locations, but they report an impression of movement. Philosophers have recently debated whether apparent motion provides evidence in favour of a particular account of the nature of temporal experience. I argue that the existing discussion in this area is premised on a mistaken view of the phenomenology of apparent motion and, as a result, the space of possible philosophical positions has not yet been fully explored. In particular, I argue that the existence of apparent motion is compatible with an account of the nature of temporal experience that involves a version of direct realism. In doing so, I also argue against two other claims often made about apparent motion, viz. that apparent motion is the psychological phenomenon that underlies motion experience in the cinema, and that apparent motion is subjectively indistinguishable from real motion
"Where the mask ends and the face begins is not certain": Mediating ethnicity and cheating geography in Jonny Steinberg's Little Liberia
Mixing historical commentary, reportage, biography and personal stories, South
African writer Jonny Steinberg takes up the tale of a fractured African nation and its
diaspora in Little Liberia: An African Odyssey in New York City (2011). The "little
Liberia" founded in New York's urban jungle may have represented, for many of its
inhabitants, a way to "cheat geography" by recreating a home away from home, but
Little Liberia shows the reader it has not allowed them to cheat history. The book
deals with the lives of two inhabitants of Park Hill Avenue on Staten Island, where
nearly everyone is Liberian. Their conflict threatens to implode the community, igniting
suspicions and accusations that had been bottled up since their exile. The article
focuses on the interface of mediated ethnicity and citizenship related to the struggle
for power in the diasporic Liberian community on Staten Island. Attention is also paid
to feelings of identity of Little Liberia's author.DHE
Bird observations in Severnaya Zemlya, Siberia
Fieldwork in different parts of Severnaya Zemlya in 1985, 1991, 1992 and 1993 and aerial surveys in 1994 revealed a limited bird fauna with a total of 17 breeding species. The most numerous breeding birds are cliff-nesting seabirds, comprising little auk (Alle alle), 10 000-80 000 pairs; kittiwake (Rissa tridactyla), 5000-10 000; black guillemot (Cepphus grylle), 1000-5000; ivory gull (Pagophila eburnea), 1000-2000; and glaucous gull (Larus hyperboreus), 500-1000. They breed all over the archipelago, usually in rather small mixed- or single-species colonies. Arctic tern (Sterna paradisaea) 100-500, and herring gull (Larus argentatus) 1-10, breed as solitary pairs or with a few pairs together. Of tundra birds, only brent goose (Branta bernicla), purple sandpiper (Calidris maritima) and snow bunting (Plectrophenax nivalis) are found breeding on most of the major islands. The other tundra species-red-throated diver (Gavia stellata), king eider (Somateria spectabilis), sanderling (Calidris alba), Arctic skua (Stercorarius parasiticus), long-tailed skua (Stercorarius longicaudus), snowy owl (Nyctea scandiaca) and Lapland bunting (Calcarius lapponicus)-breed in small numbers and in limited areas, often not every year. Of the even fewer mammal species, reindeer (Rangifer tarandus) occurs occasionally, while Arctic fox (Alopex lagopus) and collared lemming (Dicrostonyx torquatus) are locally common in some years
Teleworking practice in small and medium-sized firms: Management style and worker autonomy
In an empirical study of teleworking practices amongst small and medium-sized enterprises (SMEs) in West London, organisational factors such as management attitudes, worker autonomy and employment flexibility were found to be more critical than technological provision in facilitating successful implementation. Consequently, we argue that telework in most SMEs appears as a marginal activity performed mainly by managers and specialist mobile workers
Multidisciplinary Approach to Aerospike Nozzle Design
A model of a linear aerospike rocket nozzle that consists of coupled aerodynamic and structural analyses has been developed. A nonlinear computational fluid dynamics code is used to calculate the aerodynamic thrust, and a three-dimensional finite-element model is used to determine the structural response and weight. The model will be used to demonstrate multidisciplinary design optimization (MDO) capabilities for relevant engine concepts, assess performance of various MDO approaches, and provide a guide for future application development. In this study, the MDO problem is formulated using the multidisciplinary feasible (MDF) strategy. The results for the MDF formulation are presented with comparisons against separate aerodynamic and structural optimized designs. Significant improvements are demonstrated by using a multidisciplinary approach in comparison with the single-discipline design strategy
SinemurianâPliensbachian calcareous nannofossil biostratigraphy and organic carbon isotope stratigraphy in the Paris Basin: Calibration to the ammonite biozonation of NW Europe
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Available online 12 December 2016The biostratigraphy of Sinemurian to lower Toarcian calcareous nannofossils has been investigated in the Sancerre-Couy core (Paris Basin), which contains a mixed assemblage of species with affinities to the northern and southern areas of the peri-tethyan realm, thus allowing for the use and calibration of the Mediterranean Province (Italy/S France) and NW Europe (UK) biozonation schemes. This study is based on semi-quantitative analyses of the calcareous nannofossil assemblage performed on 145 samples and the recorded bioevents are calibrated to the NW European Ammonite Zonation and to a new organic carbon isotope curve based on 385 data points. The main bioevents, i.e. the first occurrences of Parhabdolithus liasicus, Crepidolithus pliensbachensis, Crepidolithus crassus, Mitrolithus lenticularis, Similiscutum cruciulus sensu lato, Lotharingius hauffii, Crepidolithus cavus and Lotharingius sigillatus as well as the last occurrence of Parhabdolithus robustus, have been identified. However, we show that a large number of standard biostratigraphic markers show inconsistent occurrences at the base and top of their range, possibly accounting for some of the significant discrepancies observed between the different domains. In addition to the nine main bioevents used for the biozonation of the core, we document an additional 50 distinct bioevents, evaluate their reliability and discuss their potential significance by comparison to previous studies. A total of five significant negative organic carbon isotope excursions are identified and defined in the Paris Basin including the well-documented SinemurianâPliensbachian boundary event. One positive excursion is further defined in the Pliensbachian interval. Our calibration of high-resolution calcareous nannofossil biostratigraphy to ammonite biostratigraphy and organic carbon isotopes represents a new stratigraphic reference for the Lower Jurassic series
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