121 research outputs found
Mutual information in classical spin models
The total many-body correlations present in finite temperature classical spin
systems are studied using the concept of mutual information. As opposed to
zero-temperature quantum phase transitions, the total correlations are not
maximal at the phase transition, but reach a maximum in the high temperature
paramagnetic phase. The Shannon and Renyi mutual information in both Ising and
Potts models in 2 dimensions are calculated numerically by combining matrix
product states algorithms and Monte Carlo sampling techniques
Twelve and a Half Years of Observations of Centaurus A with RXTE
The Rossi X-ray Timing Explorer has observed the nearest radio galaxy,
Centaurus A, in 13 intervals from 1966 August to 2009 February over the 3--200
keV band. Spectra accumulated over the 13 intervals were well described with an
absorbed power law and iron line. Cut-off power laws and Compton reflection
from cold matter did not provide a better description. For the 2009 January
observation, we set a lower limit on the cut-off energy at over 2 MeV. The
power spectral density function was generated from RXTE/ASM and PCA data, as
well as an XMM-Newton long look, and clear evidence for a break at 18+10-7 days
(68% conf.) was seen. Given Cen A's high black hole mass and very low value of
Lx/LEdd, the break was a factor of 17+/-9 times higher than the break frequency
predicted by the McHardy and co-workers' relation, which was empirically
derived for a sample of objects, which are radio-quiet and accreting at
relatively high values of Lbol/LEdd. We have interpreted our observations in
the context of a clumpy molecular torus. The variability characteristics and
the broadband spectral energy distribution, when compared to Seyferts, imply
that the bright hard X-ray continuum emission may originate at the base of the
jet, yet from behind the absorbing line of sight material, in contrast to what
is commonly observed from blazars.Comment: 56 pages, 12 figures, 4 tables, revised manuscript submitted to The
Astrophysical Journa
X-ray monitoring of the radio and gamma-ray loud Narrow-Line Seyfert 1 Galaxy PKS 2004-447
We present preliminary results of the X-ray analysis of XMM-Newton and Swift
observations as part of a multi-wavelength monitoring campaign in 2012 of the
radio-loud narrow line Seyfert 1 galaxy PKS 2004-447. The source was recently
detected in gamma-rays by Fermi/LAT among only four other galaxies of that
type. The 0.5-10 keV X-ray spectrum is well-described by a simple absorbed
powerlaw (photon index ~ 1.6). The source brightness exhibits variability on
timescales of months to years with indications for spectral variability, which
follows a 'bluer-when-brighter' behaviour, similar to blazars.Comment: Proceedings for the 'Jet 2013' conference. Includes 3 pages, 3
figure
Evidence for different accretion regimes in GRO J1008-57
We present a comprehensive spectral analysis of the BeXRB GRO J1008-57 over a
luminosity range of three orders of magnitude using NuSTAR, Suzaku and RXTE
data. We find significant evolution of the spectral parameters with luminosity.
In particular the photon index hardens with increasing luminosity at
intermediate luminosities between erg s. This
evolution is stable and repeatedly observed over different outbursts. However,
at the extreme ends of the observed luminosity range, we find that the
correlation breaks down, with a significance level of at least . We
conclude that these changes indicate transitions to different accretion
regimes, which are characterized by different deceleration processes, such as
Coulomb or radiation breaking. We compare our observed luminosity levels of
these transitions to theoretical predications and discuss the variation of
those theoretical luminosity values with fundamental neutron star parameters.
Finally, we present detailed spectroscopy of the unique "triple peaked"
outburst in 2014/15 which does not fit in the general parameter evolution with
luminosity. The pulse profile on the other hand is consistent with what is
expected at this luminosity level, arguing against a change in accretion
geometry. In summary, GRO J1008-57 is an ideal target to study different
accretion regimes due to the well constrained evolution of its broad-band
spectral continuum over several orders of magnitude in luminosity.Comment: 13 pages, 7 figures, 3 tables. Accepted for publication in A&
Multimodal brain age prediction fusing morphometric and imaging data and association with cardiovascular risk factors
IntroductionThe difference between the chronological and biological brain age, called the brain age gap (BAG), has been identified as a promising biomarker to detect deviation from normal brain aging and to indicate the presence of neurodegenerative diseases. Moreover, the BAG has been shown to encode biological information about general health, which can be measured through cardiovascular risk factors. Current approaches for biological brain age estimation, and therefore BAG estimation, either depend on hand-crafted, morphological measurements extracted from brain magnetic resonance imaging (MRI) or on direct analysis of brain MRI images. The former can be processed with traditional machine learning models while the latter is commonly processed with convolutional neural networks (CNNs). Using a multimodal setting, this study aims to compare both approaches in terms of biological brain age prediction accuracy and biological information captured in the BAG.MethodsT1-weighted MRI, containing brain tissue information, and magnetic resonance angiography (MRA), providing information about brain arteries, from 1,658 predominantly healthy adults were used. The volumes, surface areas, and cortical thickness of brain structures were extracted from the T1-weighted MRI data, while artery density and thickness within the major blood flow territories and thickness of the major arteries were extracted from MRA data. Independent multilayer perceptron and CNN models were trained to estimate the brain age from the hand-crafted features and image data, respectively. Next, both approaches were fused to assess the benefits of combining image data and hand-crafted features for brain age prediction.ResultsThe combined model achieved a mean absolute error of 4 years between the chronological and predicted biological brain age. Among the independent models, the lowest mean absolute error was observed for the CNN using T1-weighted MRI data (4.2 years). When evaluating the BAGs obtained using the different approaches and imaging modalities, diverging associations between cardiovascular risk factors were found. For example, BAGs obtained from the CNN models showed an association with systolic blood pressure, while BAGs obtained from hand-crafted measurements showed greater associations with obesity markers.DiscussionIn conclusion, the use of more diverse sources of data can improve brain age estimation modeling and capture more diverse biological deviations from normal aging
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