32,545 research outputs found
The Progenitors of Type Ia Supernovae: Are They Supersoft Sources?
In a canonical model, the progenitors of Type Ia supernovae (SNe Ia) are
accreting, nuclear-burning white dwarfs (NBWDs), which explode when the white
dwarf reaches the Chandrasekhar mass, M_C. Such massive NBWDs are hot (kT ~100
eV), luminous (L ~ 10^{38} erg/s), and are potentially observable as luminous
supersoft X-ray sources (SSSs). During the past several years, surveys for soft
X-ray sources in external galaxies have been conducted. This paper shows that
the results falsify the hypothesis that a large fraction of progenitors are
NBWDs which are presently observable as SSSs. The data also place limits on
sub-M_C models. While Type Ia supernova progenitors may pass through one or
more phases of SSS activity, these phases are far shorter than the time needed
to accrete most of the matter that brings them close to M_C.Comment: submitted to ApJ 18 November 2009; 17 pages, 2 figure
Incremental Predictive Process Monitoring: How to Deal with the Variability of Real Environments
A characteristic of existing predictive process monitoring techniques is to
first construct a predictive model based on past process executions, and then
use it to predict the future of new ongoing cases, without the possibility of
updating it with new cases when they complete their execution. This can make
predictive process monitoring too rigid to deal with the variability of
processes working in real environments that continuously evolve and/or exhibit
new variant behaviors over time. As a solution to this problem, we propose the
use of algorithms that allow the incremental construction of the predictive
model. These incremental learning algorithms update the model whenever new
cases become available so that the predictive model evolves over time to fit
the current circumstances. The algorithms have been implemented using different
case encoding strategies and evaluated on a number of real and synthetic
datasets. The results provide a first evidence of the potential of incremental
learning strategies for predicting process monitoring in real environments, and
of the impact of different case encoding strategies in this setting
The Progenitors of Type Ia Supernovae: II. Are they Double-Degenerate Binaries? The Symbiotic Channel
In order for a white dwarf (WD) to achieve the Chandrasekhar mass, M_C, and
explode as a Type Ia supernova (SNIa), it must interact with another star,
either accreting matter from or merging with it. The failure to identify the
types of binaries which produce SNeIa is the "progenitor problem". Its solution
is required if we are to utilize the full potential of SNeIa to elucidate basic
cosmological and physical principles. In single-degenerate models, a WD
accretes and burns matter at high rates. Nuclear-burning WDs (NBWDs) with mass
close to M_C are hot and luminous, potentially detectable as supersoft x-ray
sources (SSSs). In previous work we showed that > 90-99% of the required number
of progenitors do not appear as SSSs during most of the crucial phase of mass
increase. The obvious implication is that double-degenerate (DD) binaries form
the main class of progenitors. We show in this paper, however, that many
binaries that later become DDs must pass through a long-lived NBWD phase during
which they are potentially detectable as SSSs. The paucity of SSSs is therefore
not a strong argument in favor of DD models. Those NBWDs that are the
progenitors of DD binaries are likely to appear as symbiotic binaries for
intervals > 10^6 years. In fact, symbiotic pre-DDs should be common, whether or
not the WDs eventually produce SNeIa. The key to solving the progenitor problem
lies in understanding the appearance of NBWDs. Most do not appear as SSSs most
of the time. We therefore consider the evolution of NBWDs to address the
question of what their appearance may be and how we can hope to detect them.Comment: 24 pages; 5 figures; submitted to Ap
Telehealth Utilization among Low Income Population during COVID-19: An Analysis of COVID-19 Research Database
The COVID-19 Research Database is a public data platform. This platform is a result of private and public partnerships across industries to facilitate data sharing and promote public health research. We analyzed its linked database and examined claims of 2,850,831 unique persons to investigate demographic, socio-economic, and behavioral causes for telehealth utilization in the low-income population. Our results suggest that patients who had higher education, income, and full-time employment were more likely to use telehealth. Patients who had unhealthy behaviors such as smoking were less likely to use telehealth. Our findings suggest that interventions to bolster education, employment, and healthy behaviors should be considered to promote the use of telehealth services
Protocol for electrophysiological monitoring of carotid endarterectomies.
Near zero stroke rates can be achieved in carotid endarterectomy (CEA) surgery with selective shunting and electrophysiological neuromonitoring. though false negative rates as high as 40% have been reported. We sought to determine if improved training for interpretation of the monitoring signals can advance the efficacy of selective shunting with electrophysiological monitoring across multiple centers, and determine if other factors could contribute to the differences in reports. Processed and raw beta band (12.5-30 Hz) electroencephalogram (EEG) and median and tibial nerve somatosensory evoked potentials (SSEP) were monitored in 668 CEA cases at six surgical centers. A decrease in amplitude of 50% or more in any EEG or SSEP channel was the criteria for shunting or initiating a neuroprotective protocol. A reduction of 50% or greater in the beta band of the EEG or amplitude of the SSEP was observed in 150 cases. No patient showed signs of a cerebral infarct after surgery. Selective shunting based on EEG and SSEP monitoring can reduce CEA intraoperative stroke rate to a near zero level if trained personnel adopted standardized protocols. We also found that the rapid administration of a protective stroke protocol by attending anesthesiologists was an important aspect of this success rate
Supersoft sources in M 31: Comparing the XMM-Newton Deep Survey, ROSAT and Chandra catalogues
To investigate the transient nature of supersoft sources (SSSs) in M 31, we
compared SSS candidates of the XMM-Newton Deep Survey, ROSAT PSPC surveys and
the Chandra catalogues in the same field. We found 40 SSSs in the XMM-Newton
observations. While 12 of the XMM-Newton sources were brighter than the
limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ~10
yr earlier. Five correlate with recent optical novae which explains why they
were not detected by ROSAT. The remaining 28 XMM-Newton SSSs have fluxes below
the ROSAT detection threshold. Nevertheless we found one correlation with a
ROSAT source, which had significantly larger fluxes than during the XMM-Newton
observations. Ten of the XMM-Newton SSSs were detected by Chandra with <1-~6 yr
between the observations. Five were also classified as SSSs by Chandra. Of the
30 ROSAT SSSs three were confirmed with XMM-Newton, while for 11 sources other
classifications are suggested. Of the remaining 16 sources one correlates with
an optical nova. Of the 42 Chandra very-soft sources five are classified as
XMM-Newton SSSs, while for 22 we suggest other classifications. Of the
remaining 15 sources, nine are classified as transient by Chandra, one of them
correlates with an optical nova. These findings underlined the high variability
of the sources of this class and the connection between SSSs and optical novae.
Only three sources, were detected by all three missions as SSSs. Thus they are
visible for more than a decade, despite their variability.Comment: 6 pages, 1 figure, Proc. of workshop "SuperSoft X-ray Sources - New
Developments", ESTEC/Villafranca, May 2009, accepted for publication in
Aston.Nach
Time variability of X-ray sources in the M 31 centre field
We present an extension to our XMM-Newton X-ray source catalogue of M 31,
containing 39 newly found sources. In order to classify and identify more of
the sources we search for X-ray time variability in XMM-Newton archival data of
the M 31 centre field.
As a source list we used our extended catalogue based on observations
covering the time span from June 2000 to July 2004. We then determined the flux
or at least an upper limit at the source positions for each observation.
Deriving the flux ratios for the different observations and searching for the
maximum flux difference we determined variability factors. We also calculated
the significance of the flux ratios.
Using hardness ratios, X-ray variability and cross correlations with
catalogues in the X-ray, optical, infrared and radio regimes, we detected three
super soft source candidates, one supernova remnant and six supernova remnant
candidates, one globular cluster candidate, three X-ray binaries and four X-ray
binary candidates. Additionally we identified one foreground star candidate and
classified fifteen sources with hard spectra, which may either be X-ray
binaries or Crab-like supernova remnants in M 31 or background active galactic
nuclei. The remaining five sources stay unidentified or without classification.
Based on the time variability results we suggest six sources, which were
formerly classified as "hard", to be X-ray binary candidates. The
classification of one other source (XMMM31 J004236.7+411349) as a supernova
remnant, has to be rejected due to the distinct time variability we found. We
now classify this source as a foreground star.Comment: 13 pages, 6 figures, accepted for publication in A&
Quasistatic rheology and the origins of strain
Features of rheological laws applied to solid-like granular materials are
recalled and confronted to microscopic approaches via discrete numerical
simulations. We give examples of model systems with very similar equilibrium
stress transport properties -- the much-studied force chains and force
distribution -- but qualitatively different strain responses to stress
increments. Results on the stability of elastoplastic contact networks lead to
the definition of two different rheological regimes, according to whether a
macroscopic fragility property (propensity to rearrange under arbitrary small
stress increments in the thermodynamic limit) applies. Possible consequences
are discussed.Comment: Published in special issue of "Comptes-Rendus Physique" on granular
material
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