593 research outputs found
SPH Simulations of Direct Impact Accretion in the Ultracompact AM CVn Binaries
The ultracompact binary systems V407 Vul (RX J1914.4+2456) and HM Cnc (RX
J0806.3+1527) - a two-member subclass of the AM CVn stars - continue to pique
interest because they defy unambiguous classification. Three proposed models
remain viable at this time, but none of the three is significantly more
compelling than the remaining two, and all three can satisfy the observational
constraints if parameters in the models are tuned. One of the three proposed
models is the direct impact model of Marsh & Steeghs (2002), in which the
accretion stream impacts the surface of a rapidly-rotating primary white dwarf
directly but at a near-glancing angle. One requirement of this model is that
the accretion stream have a high enough density to advect its specific kinetic
energy below the photosphere for progressively more-thermalized emission
downstream, a constraint that requires an accretion spot size of roughly
1.2x10^5 km^2 or smaller. Having at hand a smoothed particle hydrodynamics code
optimized for cataclysmic variable accretion disk simulations, it was
relatively straightforward for us to adapt it to calculate the footprint of the
accretion stream at the nominal radius of the primary white dwarf, and thus to
test this constraint of the direct impact model. We find that the mass flux at
the impact spot can be approximated by a bivariate Gaussian with standard
deviation \sigma_{\phi} = 164 km in the orbital plane and \sigma_{\theta} = 23
km in the perpendicular direction. The area of the the 2\sigma ellipse into
which 86% of the mass flux occurs is roughly 47,400 km^2, or roughly half the
size estimated by Marsh & Steeghs (2002). We discuss the necessary parameters
of a simple model of the luminosity distribution in the post-impact emission
region.Comment: 24 pages, 5 figures, Accepted for publication in Ap
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
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
A new soft X-ray spectral model for polars with an application to AM Herculis
We present a simple heuristic model for the time-averaged soft X-ray
temperature distribution in the accretion spot on the white dwarf in polars.
The model is based on the analysis of the Chandra LETG spectrum of the
prototype polar AM Her and involves an exponential distribution of the emitting
area vs. blackbody temperature a(T) = a0 exp(-T/T0). With one free parameter
besides the normalization, it is mathematically as simple as the single
blackbody, but is physically more plausible and fits the soft X-ray and
far-ultraviolet spectral fluxes much better. The model yields more reliable
values of the wavelength-integrated flux of the soft X-ray component and the
implied accretion rate than reported previously.Comment: 5 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
A burst from the direction of UZ Fornacis with XMM-Newton
The XMM-Newton pointing towards the magnetic cataclysmic variable UZ For
finds the source to be a factor > 10^3 fainter than previous EXOSAT and ROSAT
observations. The source was not detected for the majority of a 22 ksec
exposure with the EPIC cameras, suggesting that the accretion rate either
decreased, or stopped altogether. However a 1.1 ksec burst was detected from UZ
For during the observation. Spectral fits favour optically thin, kT = 4.4 keV
thermal emission. Detection of the burst by the on-board Optical Monitor
indicates that this was most probably an accretion event. The 0.1-10 keV
luminosity of 2.1 x 10^30 erg/s is typical for accretion shock emission from
high state polars and would result from the potential energy release of ~ 10^16
g of gas. There is no significant soft excess due to reprocessing in the white
dwarf atmosphere.Comment: 7 pages, 2 postscript figures, ApJL, in pres
Support vector machine for functional data classification
In many applications, input data are sampled functions taking their values in
infinite dimensional spaces rather than standard vectors. This fact has complex
consequences on data analysis algorithms that motivate modifications of them.
In fact most of the traditional data analysis tools for regression,
classification and clustering have been adapted to functional inputs under the
general name of functional Data Analysis (FDA). In this paper, we investigate
the use of Support Vector Machines (SVMs) for functional data analysis and we
focus on the problem of curves discrimination. SVMs are large margin classifier
tools based on implicit non linear mappings of the considered data into high
dimensional spaces thanks to kernels. We show how to define simple kernels that
take into account the unctional nature of the data and lead to consistent
classification. Experiments conducted on real world data emphasize the benefit
of taking into account some functional aspects of the problems.Comment: 13 page
Chinese mental illness narratives: Controlling the spirit
Through analysis of local level narrative themes and linguistic features and their attendant narrative ‘domains’ and emotive content, this study explicates the discursive forms of Chinese personal narratives about mental illness published in a psychoeducational volume in mainland China. The cultural phenomena underpinning the explicated discursive forms are considered. The study finds that the Chinese mental illness narratives emphasize the problematic nature of the illness experience for the individual concerned, as well as the importance of gaining control over the personal and social disembodiment that mental illness brings about. The language of medicine presents as a dominant linguistic feature of the texts. Narrative authors appear to respond to their illness experience by negotiating and utilizing social relationships and support structures and through active medical intercession. As such, their narratives appear deeply culturally marked despite their psychoeducational intent and institutional context of publication. The study findings bear out the call for more attention to be given to the cultural dimension when analysing illness narratives
Neuroprotective actions of leptin facilitated through balancing mitochondrial morphology and improving mitochondrial function
Authors would like to acknowledge ARUK for supporting this research. YC is Chinese Scholarship recipient. The University of St Andrews is a charity registered in Scotland: No SC013532Mitochondrial dysfunction has a recognised role in the progression of Alzheimer's disease (AD) pathophysiology. Cerebral perfusion becomes increasingly inefficient throughout ageing, leading to unbalanced mitochondrial dynamics. This effect is exaggerated by amyloid β (Aβ) and phosphorylated tau, two hallmark proteins of AD pathology. A neuroprotective role for the adipose‐derived hormone, leptin, has been demonstrated in neuronal cells. However, its effects with relation to mitochondrial function in AD remain largely unknown. To address this question, we have used both a glucose‐serum deprived (CGSD) model of ischaemic stroke in SH‐SY5Y cells and a Aβ1‐42‐treatment model of AD in differentiated hippocampal cells. Using a combination of JC‐1 and MitoRed staining techniques, we show that leptin prevents depolarisation of the mitochondrial membrane and excessive mitochondrial fragmentation induced by both CGSD and Aβ1‐42. Thereafter, we used ELISAs and a number of activity assays to reveal the biochemical underpinnings of these processes. Specifically, leptin was seen to inhibit upregulation of the mitochondrial fission protein Fis1 and downregulation of the mitochondrial fusion protein, Mfn2. Furthermore, leptin was seen to upregulate the expression and activity of the antioxidant enzyme, monoamine oxidase B. Herein we provide the first demonstration that leptin is sufficient to protect against aberrant mitochondrial dynamics and resulting loss of function induced by both CGSD and Aβ1‐42. We conclude that the established neuroprotective actions of leptin may be facilitated through regulation of mitochondrial dynamics.Publisher PDFPeer reviewe
A Detection of an Anti-correlated Hard X-ray Lag in AM Herculis
Context {Earlier cross-correlation studies for AM Her were performed in
various energy range from optical to X-ray and suggested that it mostly shows a
high level of correlation but on occasion it shows a low level of correlation
or uncorrelation.} Aims {To investigate the degree of correlation between soft
(2-4 keV) and hard (9-20 keV) X-rays, we perform the cross-correlation study of
the X-ray data sets of AM Her obtained with {\it RXTE}.}
Methods {We cross-correlate the background-subtracted soft and hard X-ray
light curves using the XRONOS program crosscor and fit a model to the obtained
cross-correlation functions.}
Results {We detect a hard X-ray lag of s in a specific section of
energy-dependent light curve, where the soft X-ray (2-4 keV) intensity
decreases but the hard X-ray (9-20 keV) intensity increases. From a spectral
analysis, we find that the X-ray emission temperature increases during the
anti-correlated intensity variation. In two other observations, the
cross-correlation functions show a low level of correlation, which is
consistent with the earlier results performed in a different energy range.}
Conclusions {We report a detection of an anti-correlated hard X-ray lag of
190 s from the proto-type polar AM Her. The hard X-ray lag is detected
for the first time in the given energy range, and it is the longest lag among
those reported in magnetic cataclysmic variables. We discuss the implications
of our findings regarding the origin of the hard X-ray lag and the
anti-correlated intensity variation.}Comment: Accepted in A&A, 4 page
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