2,305 research outputs found
The Retail FX Trader: Random Trading and the Negative Sum Game
With the internet boom of early 2000 making access to trading the Foreign Exchange (FX) market far simpler for members of the general public, the growth of 'retail' FX trading continues, with daily transaction volumes as high as $200 billion. Potential new entrants to the retail FX trading world may come from the recent UK pension deregulations, further increasing the volumes. The attraction of FX trading is that it offers high returns and whilst it has been understood that it is high-risk in nature, the rewards are seen as being commensurately high for the 'skilled and knowledgeable' trader who has an edge over other market participants. This paper analyses a number of independent sources of data and previous research, to examine the profitability of the Retail FX trader and compares the results with that of a simulated random trading models. This paper finds evidence to suggest that whilst approximately 20% of traders can expect to end up with a profitable account, around 40% might expect their account to be subject to a margin call. This paper finds a strong correlation between the overall profitability of traders and impact of the cost of the bid-ask spread, whilst finding little if any evidence that retail FX traders, when viewed as a group, are achieving results better than that from random trading
XMM-Newton observation of the long-period polar V1309 Ori: The case for pure blobby accretion
Using XMM-Newton we have obtained the first X-ray observation covering a
complete orbit of the longest period polar, V1309 Ori. The X-ray light curve is
dominated by a short, bright phase interval with EPIC pn count rates reaching
up to 15 cts/sec per 30 sec resolution bin. The bright phase emission is well
described by a single blackbody component with kT_bb = (45 +- 3) eV. The
absence of a bremsstrahlung component at photon energies above 1 keV yields a
flux ratio F_bb/F_br > 6700. This represents the most extreme case of a soft
X-ray excess yet observed in an AM Herculis star. The bright, soft X-ray
emission is subdivided into a series of individual flare events supporting the
hypothesis that the soft X-ray excess in V1309 is caused by accretion of dense
blobs. In addition to the bright phase emission, a faint, hard X-ray component
is visible throughout the binary orbit with an almost constant count rate of
0.01 cts/sec. Spectral modelling indicates that this emission originates from a
complex multi-temperature plasma. At least three components of an optically
thin plasma with temperatures kT= 0.065, 0.7, and 2.9 keV are required to fit
the observed flux distribution. The faint phase emission is occulted during the
optical eclipse. Eclipse ingress lasts about 15--20 min and is substantially
prolonged beyond nominal ingress of the white dwarf. This and the comparatively
low plasma temperature provide strong evidence that the faint-phase emission is
not thermal bremsstrahlung from a post-shock accretion column above the white
dwarf. A large fraction of the softer faint-phase emission could be explained
by scattering of photons from the blackbody component in the infalling material
above the accretion region. The remaining hard X-ray flux could be produced in
the coupling region, so far unseen in other polars.Comment: 10 pages, 5 figures, A&A publishe
Numerical Simulations of the Onset and Stability of Dynamical Mass Transfer in Binaries
Hydrodynamical simulations of semi-detached, polytropic binary stars are
presented in an effort to study the onset and stability of dynamical mass
transfer events. Initial, synchronously rotating equilibrium models are
constructed using a self-consistent-field technique and then evolved with an
Eulerian hydrodynamics code in a fully self-consistent manner. We describe code
improvements introduced over the past few years that permit us to follow
dynamical mass-transfer events through more than 30 orbits. Mass-transfer
evolutions are presented for two different initial configurations: A
dynamically unstable binary with initial mass ratio (donor/accretor) that leads to a complete merger in orbits; and a
double-degenerate binary with initial mass ratio that, after some
initial unstable growth of mass transfer, tends to separate as the
mass-transfer rate levels off.Comment: 47 pages, 11 figures, submitted to the Astrophysical Journal. See
http://www.phys.lsu.edu/faculty/tohline/astroph/dmtf05 for high resolution
figures and mpeg animation
Enhanced activity of massive black holes by stellar capture assisted by a self-gravitating accretion disc
We study the probability of close encounters between stars from a nuclear
cluster and a massive black hole. The gravitational field of the system is
dominated by the black hole in its sphere of influence. It is further modified
by the cluster mean field (a spherical term) and a gaseous disc/torus (an
axially symmetric term) causing a secular evolution of stellar orbits via Kozai
oscillations. Intermittent phases of large eccentricity increase the chance
that stars become damaged inside the tidal radius of the central hole. Such
events can produce debris and lead to recurring episodes of enhanced accretion
activity. We introduce an effective loss cone and associate it with tidal
disruptions during the high-eccentricity phases of the Kozai cycle. By
numerical integration of the trajectories forming the boundary of the loss cone
we determine its shape and volume. We also include the effect of relativistic
advance of pericentre. The potential of the disc has the efffect of enlarging
the loss cone and, therefore, the predicted number of tidally disrupted stars
should grow by factor of ~10^2. On the other hand, the effect of the cluster
mean potential together with the relativistic pericentre advance act against
the eccentricity oscillations. In the end we expect the tidal disruption events
to be approximately ten times more frequent in comparison with the model in
which the three effects -- the cluster mean field, the relativistic pericentre
advance, and the Kozai mechanism -- are all ignored. The competition of
different influences suppresses the predicted star disruption rate as the black
hole mass increases. Hence, the process under consideration is more important
for intermediate-mass black holes, M_bh~10^4M_s.Comment: 10 pages, 5 figures; Astronomy & Astrophysics accepte
Region of magnetic dominance near a rotating black hole
This is a brief contribution in which a simplified criterion of the relevance
of the test-particle approximation describing motion of material near a
magnetized black hole is discussed. Application to processes of the dissipative
collimation of astronomical jets (as proposed by de Felice and Curir, 1992) is
mentioned.Comment: 11 pages, to appear in General Relativity and Gravitation, also
available (with additional illustrations) at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
X-ray spectroscopy and photometry of the long-period polar AI Tri with XMM-Newton
Context. The energy balance of cataclysmic variables with strong magnetic
fields is a central subject in understanding accretion processes on magnetic
white dwarfs. With XMM-Newton, we perform a spectroscopic and photometric study
of soft X-ray selected polars during their high states of accretion. Aims. On
the basis of X-ray and optical observations of the magnetic cataclysmic
variable AI Tri, we derive the properties of the spectral components, their
flux contributions, and the physical structure of the accretion region in soft
polars. Methods. We use multi-temperature approaches in our xspec modeling of
the spectra to describe the physical conditions and the structures of the
post-shock accretion flow and the accretion spot on the white-dwarf surface. In
addition, we investigate the accretion geometry of the system by a timing
analysis of the photometric data. Results. Flaring soft X-ray emission from the
heated surface of the white dwarf dominates the X-ray flux during roughly 70%
of the binary cycle. This component deviates from a single black body and can
be described by a superimposition of mildly absorbed black bodies with a
Gaussian temperature distribution. In addition, weaker hard X-ray emission is
visible nearly all the time. The spectrum from the cooling post-shock accretion
flow is most closely fitted by a combination of thermal plasma mekal models
with temperature profiles adapted from prior stationary two-fluid hydrodynamic
calculations. The soft X-ray light curves show a dip during the bright phase,
which can be interpreted as self-absorption in the accretion stream.
Phase-resolved spectral modeling supports the picture of one-pole accretion and
self-eclipse. One of the optical light curves corresponds to an irregular mode
of accretion. During a short XMM-Newton observation at the same epoch, the
X-ray emission of the system is clearly dominated by the soft component.Comment: A&A, in press; 11 pages, 9 figures, 3 table
Analyzing collaborative learning processes automatically
In this article we describe the emerging area of text classification research focused on the problem of collaborative learning process analysis both from a broad perspective and more specifically in terms of a publicly available tool set called TagHelper tools. Analyzing the variety of pedagogically valuable facets of learners’ interactions is a time consuming and effortful process. Improving automated analyses of such highly valued processes of collaborative learning by adapting and applying recent text classification technologies would make it a less arduous task to obtain insights from corpus data. This endeavor also holds the potential for enabling substantially improved on-line instruction both by providing teachers and facilitators with reports about the groups they are moderating and by triggering context sensitive collaborative learning support on an as-needed basis. In this article, we report on an interdisciplinary research project, which has been investigating the effectiveness of applying text classification technology to a large CSCL corpus that has been analyzed by human coders using a theory-based multidimensional coding scheme. We report promising results and include an in-depth discussion of important issues such as reliability, validity, and efficiency that should be considered when deciding on the appropriateness of adopting a new technology such as TagHelper tools. One major technical contribution of this work is a demonstration that an important piece of the work towards making text classification technology effective for this purpose is designing and building linguistic pattern detectors, otherwise known as features, that can be extracted reliably from texts and that have high predictive power for the categories of discourse actions that the CSCL community is interested in
Global Models of Runaway Accretion in White Dwarf Debris Disks
A growing sample of white dwarfs (WDs) with metal-enriched atmospheres are
accompanied by excess infrared emission, indicating that they are encircled by
a compact dusty disk of solid debris. Such `WD debris disks' are thought to
originate from the tidal disruption of asteroids or other minor bodies, but the
precise mechanism(s) responsible for transporting matter to the WD surface
remains unclear, especially in those systems with the highest inferred metal
accretion rates dM_Z/dt ~ 1e8-1e10 g/s. Here we present global time-dependent
calculations of the coupled evolution of the gaseous and solid components of WD
debris disks. Solids transported inwards (initially due to PR drag) sublimate
at tens of WD radii, producing a source of gas that accretes onto the WD
surface and viscously spreads outwards in radius, where it overlaps with the
solid disk. If the aerodynamic coupling between the solids and gaseous disks is
sufficiently strong (and/or the gas viscosity sufficiently weak), then gas
builds up near the sublimation radius faster than it can viscously spread away.
Since the rate of drag-induced solid accretion increases with gas density, this
results in a runaway accretion process, during which the WD accretion rate
reaches values orders of magnitude higher than can be achieved by PR drag
alone. We explore the evolution of WD debris disks across a wide range of
physical conditions and calculate the predicted distribution of observed
accretion rates dM_Z/dt, finding reasonable agreement with the current sample.
Although the conditions necessary for runaway accretion are at best marginally
satisfied given the minimal level of aerodynamic drag between circular gaseous
and solid disks, the presence of other stronger forms of solid-gas
coupling---such as would result if the gaseous disk is only mildly
eccentric---substantially increase the likelihood of runaway accretion.Comment: 23 pages, 20 figures, submitted to MNRA
Episodic mass loss in binary evolution to the Wolf-Rayet phase: Keck and HST proper motions of RY Scuti's nebula
Binary mass transfer via Roche-lobe overflow (RLOF) is a key channel for
producing stripped-envelope Wolf-Rayet (WR) stars and may be critical to
account for SN Ib/c progenitors. RY Scuti is an extremely rare example of a
massive binary star caught in this brief but important phase. Its toroidal
nebula indicates equatorial mass loss during RLOF, while the mass-gaining star
is apparently embedded in an opaque accretion disk. RY Scuti's toroidal nebula
has two components: an inner ionised double-ring system, and an outer dust
torus that is twice the size of the ionised rings. We present two epochs of
Lband Keck NGS-AO images of the dust torus, plus three epochs of HST images of
the ionised gas rings. Proper motions show that the inner ionised rings and the
outer dust torus came from two separate ejection events roughly 130 and 250 yr
ago. This suggests that RLOF in massive contact binaries can be accompanied by
eruptive and episodic burst of mass loss, reminiscent of LBVs. We speculate
that the repeating outbursts may arise in the mass gainer from instabilities
associated with a high accretion rate. If discrete mass-loss episodes in other
RLOF binaries are accompanied by luminous outbursts, they might contribute to
the population of extragalactic optical transients. When RLOF ends for RY
Scuti, the overluminous mass gainer, currently surrounded by an accretion disk,
will probably become a B[e] supergiant and may outshine the hotter mass-donor
star that should die as a Type Ib/c supernova.Comment: 15 pages, 7 figures, submitted to MNRA
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