16 research outputs found
A 33 year constancy of the X-ray coronae of AR Lac and eclipse diagnosis of scale height
Extensive X-ray and extreme ultraviolet (EUV) photometric observations of the
eclipsing RS CVn system AR Lac were obtained over the years 1997 to 2013 with
the Chandra X-ray Observatory Extreme Ultraviolet Explorer. During primary
eclipse, HRC count rates decrease by ~40%. A similar minimum is seen during one
primary eclipse observed by EUVE but not in others owing to intrinsic source
variability. Little evidence for secondary eclipses is present in either the
X-ray or EUV data, reminiscent of earlier X-ray and EUV observations. Primary
eclipses allow us to estimate the extent of a spherically symmetric corona on
the primary G star of about 1.3Rsun, or 0.86Rstar, and indicate the G star is
likely brighter than the K component by a factor of 2-5. Brightness changes not
attributable to eclipses appear to be dominated by stochastic variability and
are generally non-repeating. X-ray and EUV light curves cannot therefore be
reliably used to reconstruct the spatial distribution of emission assuming only
eclipses and rotational modulation are at work. Moderate flaring is observed,
where count rates increase by up to a factor of three above quiescence.
Combined with older ASCA, Einstein, EXOSAT, ROSAT and Beppo-SAX observations,
the data show that the level of quiescent coronal emission at X-ray wavelengths
has remained remarkably constant over 33 years, with no sign of variation due
to magnetic cycles. Variations in base level X-ray emission seen by Chandra
over 13 years are only ~10%, while variations back to pioneering Einstein
observations in 1980 amount to a maximum of 45% and more typically about 15%.Comment: To appear in the Astrophysical Journa
Doppler Shifts and Broadening and the Structure of the X-ray Emission from Algol
In a study of Chandra High Energy Transmission Grating spectra of Algol, we
clearly detect Doppler shifts caused by the orbital motion of Algol B. These
data provide the first definitive proof that the X-ray emission of Algol is
dominated by the secondary, in concordance with expectations that Algol A (B8)
is X-ray dark. The measured Doppler shifts are slightly smaller than expected,
implying an effective orbital radius of about 10 Rsolar, instead of 11.5 Rsolar
for the Algol B center of mass. This could be caused by a small contribution of
X-ray flux from Algol A (10-15%), possibly through accretion. The more likely
explanation is an asymmetric corona biased toward the system center of mass by
the tidal distortion of the surface of Algol B. Analysis of the strongest lines
indicates excess line broadening of ~150 km/s above that expected from thermal
motion and surface rotation. Possible explanations include turbulence, flows or
explosive events, or rotational broadening from a radially extended corona. We
favor the latter scenario and infer that a significant component of the corona
at temperatures <10^7 K has a scale height of order the stellar radius. This is
supported by the shape of the X-ray lightcurve and the shallow dip at secondary
eclipse. We also examine the O VII intercombination and forbidden lines in a
Low Energy Transmission Grating Spectrograph observation and find no change in
their relative line fluxes as the system goes from quadrature to primary
eclipse. Since these lines are strongly affected by UV irradiation from Algol
A, this supports the conjecture that the corona of Algol B at temperatures of
several million K must be significantly extended and/or located toward the
poles to avoid being shadowed from Algol A during primary eclipse.Comment: 36 pages, 10 figure
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Chandra Multiwavelength Project. II. First Results of XâRay Source Properties
The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near-on-axis X-ray-bright sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the log N- log S relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA, and Chandra Deep Field surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 Ï) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few Ă 1020 cm2, suggesting that they are typical broadline AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude that it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio, and X-ray colors.Astronom
A Domain-Agnostic Approach for Characterization of Lifelong Learning Systems
Despite the advancement of machine learning techniques in recent years,
state-of-the-art systems lack robustness to "real world" events, where the
input distributions and tasks encountered by the deployed systems will not be
limited to the original training context, and systems will instead need to
adapt to novel distributions and tasks while deployed. This critical gap may be
addressed through the development of "Lifelong Learning" systems that are
capable of 1) Continuous Learning, 2) Transfer and Adaptation, and 3)
Scalability. Unfortunately, efforts to improve these capabilities are typically
treated as distinct areas of research that are assessed independently, without
regard to the impact of each separate capability on other aspects of the
system. We instead propose a holistic approach, using a suite of metrics and an
evaluation framework to assess Lifelong Learning in a principled way that is
agnostic to specific domains or system techniques. Through five case studies,
we show that this suite of metrics can inform the development of varied and
complex Lifelong Learning systems. We highlight how the proposed suite of
metrics quantifies performance trade-offs present during Lifelong Learning
system development - both the widely discussed Stability-Plasticity dilemma and
the newly proposed relationship between Sample Efficient and Robust Learning.
Further, we make recommendations for the formulation and use of metrics to
guide the continuing development of Lifelong Learning systems and assess their
progress in the future.Comment: To appear in Neural Network
Movement Patterns and Muscular Function Before and After Onset of Sports-Related Groin Pain: A Systematic Review with Meta-analysis
BACKGROUND: Sports-related groin pain (SRGP) is a common entity in rotational sports such as football, rugby and hockey, accounting for 12-18 % of injuries each year, with high recurrence rates and often prolonged time away from sport. OBJECTIVE: This systematic review synthesises movement and muscle function findings to better understand deficits and guide rehabilitation. STUDY SELECTION: Prospective and retrospective cross-sectional studies investigating muscle strength, flexibility, cross-sectional area, electromyographic activation onset and magnitude in patients with SRGP were included. SEARCH METHODS: Four databases (MEDLINE, Web of Knowledge, EBSCOhost and EMBASE) were searched in June 2014. Studies were critiqued using a modified version of the Downs and Black Quality Index, and a meta-analysis was performed. RESULTS: Seventeen studies (14 high quality, 3 low quality; 8 prospective and 9 retrospective) were identified. Prospective findings: moderate evidence indicated decreased hip abduction flexibility as a risk factor for SRGP. Limited or very limited evidence suggested that decreased hip adduction strength during isokinetic testing at ~119°/s was a risk factor for SRGP, but no associations were found at ~30°/s or ~210°/s, or with peak torque angle. Decreased hip abductor strength in angular velocity in ~30°/s but not in ~119°/s and ~210°/s was found as a risk factor for SRGP. No relationships were found with hip internal or external rotation range of movement, nor isokinetic knee extension strength. Decreased isokinetic knee flexion strength also was a potential risk factor for SRGP, at a speed ~60°/s. Retrospective findings: there was strong evidence of decreased hip adductor muscle strength during a squeeze test at 45°, and decreased total hip external rotation range of movement (sum of both legs) being associated with SRGP. There was strong evidence of no relationship to abductor muscle strength nor unilateral hip internal and external rotation range of movement. Moderate evidence suggested that increased abduction flexibility and no change in total hip internal rotation range of movement (sum of both legs) were retrospectively associated with SRGP. Limited or very limited evidence (significant findings only) indicated decreased hip adductor muscle strength during 0° and 30° squeeze tests and during an eccentric hip adduction test, but a decrease in the isometric adductors-to-abductors strength ratio at speed 120°/s; decreased abductors-to-adductors activation ratio in the early phase in the moving leg as well as in all three phases in the weight-bearing leg during standing hip flexion; and increased hip flexors strength during isokinetic and decrease in transversus abdominis muscle resting thickness associated with SRGP. CONCLUSIONS: There were a number of significant movement and muscle function associations observed in athletes both prior to and following the onset of SRGP. The strength of findings was hampered by the lack of consistent terminology and diagnostic criteria, with there being clear guides for future research. Nonetheless, these findings should be considered in rehabilitation and prevention planning
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Chandra Multiwavelength Project. I. First XâRay Source Catalog
The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei, high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra deep surveys and previous missions. We present the first ChaMP catalog, which consists of 991 near on-axis, bright X-ray sources obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. To assess source reliability and positional uncertainty, we perform a series of simulations and also use Chandra data to complement the simulation study. The false source detection rate is found to be as good as or better than expected for a given limiting threshold. On the other hand, the chance of missing a real source is rather complex, depending on the source counts, off-axis distance (or PSF), and background rate. The positional error (95% confidence level) is usually less than 1'' for a bright source, regardless of its off-axis distance, while it can be as large as 4'' for a weak source (~20 counts) at a large off-axis distance (Doff-axis > 8'). We have also developed new methods to find spatially extended or temporary variable sources, and those sources are listed in the catalog.Astronom
Fatigue behavior of subchondral bone under simulated physiological loads of equine athletic training
Jockey Perception of Shoe and Surface Effects on Hoof-Ground Interactions and Implications for Safety in the Galloping Thoroughbred Racehorse
Design and initial performance of the Askaryan Radio Array prototype EeV neutrino detector at the South Pole
We report on studies of the viability and sensitivity of the Askaryan Radio Array (ARA), a new initiative to develop a Teraton-scale ultra-high energy neutrino detector in deep, radio-transparent ice near Amundsen-Scott station at the South Pole. An initial prototype ARA detector system was installed in January 2011, and has been operating continuously since then. We describe measurements of the background radio noise levels, the radio clarity of the ice, and the estimated sensitivity of the planned ARA array given these results, based on the first five months of operation. Anthropogenic radio interference in the vicinity of the South Pole currently leads to a few-percent loss of data, but no overall effect on the background noise levels, which are dominated by the thermal noise floor of the cold polar ice, and galactic noise at lower frequencies. We have also successfully detected signals originating from a 2.5 km deep impulse generator at a distance of over 3 km from our prototype detector, confirming prior estimates of kilometer-scale attenuation lengths for cold polar ice. These are also the first such measurements for propagation over such large slant distances in ice. Based on these data, ARA-37, the Ë200 km2 array now in its initial construction phase, will achieve the highest sensitivity of any planned or existing neutrino detector in the 1016-1019 eV energy range.SCOPUS: ar.jinfo:eu-repo/semantics/publishe