2,972 research outputs found
Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82
We present a new approach to analysing the dependence of quasar variability
on rest-frame wavelengths. We exploited the spectral archive of the Sloan
Digital Sky Survey (SDSS) to create a sample of more than 9000 quasars in the
Stripe 82. The quasar catalogue was matched with the Light Motion Curve
Catalogue for SDSS Stripe 82 and individual first-order structure functions
were computed. The structure functions are used to create a variability
indicator that is related to the same intrinsic timescales for all quasars (1
to 2 yr in the rest frame). We study the variability ratios for adjacent SDSS
filter bands as a function of redshift. While variability is almost always
stronger in the bluer passband compared to the redder, the variability ratio
depends on whether strong emission lines contribute to either one band or the
other. The variability ratio-redshift relations resemble the corresponding
colour index-redshift relations. From the comparison with Monte Carlo
simulations of variable quasar spectra we find that the observed variability
ratio-redshift relations are closely fitted assuming that (a) the r.m.s.
fluctuation of the quasar continuum follows a power law-dependence on the
intrinsic wavelength with an exponent -2 (i.e., bluer when brighter) and (b)
the variability of the emission line flux is only about 10% of that of the
underlying continuum. These results, based upon the photometry of more than
8000 quasars, confirm the previous findings by Wilhite et al. (2005) from 315
quasars with repeated SDSS spectroscopy. Finally, we find that quasars with
unusual spectra and weak emission lines tend to have less variability than
conventional quasars. This trend is opposite to what is expected from the
dilution effect of variability due to line emission and may be indicative of
high Eddington ratios in these unconventinal quasars.Comment: Accepted for publication in Astronomy and Astrophysic
Changes of statistical structural fluctuations unveils an early compacted degraded stage of PNS myelin
Degradation of the myelin sheath is a common pathology underlying
demyelinating neurological diseases from Multiple Sclerosis to
Leukodistrophies. Although large malformations of myelin ultrastructure in the
advanced stages of Wallerian degradation is known, its subtle structural
variations at early stages of demyelination remains poorly characterized. This
is partly due to the lack of suitable and non-invasive experimental probes
possessing sufficient resolution to detect the degradation. Here we report the
feasibility of the application of an innovative non-invasive local structure
experimental approach for imaging the changes of statistical structural
fluctuations in the first stage of myelin degeneration. Scanning micro X-ray
diffraction, using advances in synchrotron x-ray beam focusing, fast data
collection, paired with spatial statistical analysis, has been used to unveil
temporal changes in the myelin structure of dissected nerves following
extraction of the Xenopus laevis sciatic nerve. The early myelin degeneration
is a specific ordered compacted phase preceding the swollen myelin phase of
Wallerian degradation. Our demonstration of the feasibility of the statistical
analysis of SmXRD measurements using biological tissue paves the way for
further structural investigations of degradation and death of neurons and other
cells and tissues in diverse pathological states where nanoscale structural
changes may be uncovered.Comment: 16 pages, 6 figure
High-precision multi-band measurements of the angular clustering of X-ray sources
In this paper we present the two-point angular correlation function of the
X-ray source population of 1063 XMM-Newton observations at high Galactic
latitudes, comprising up to ~30000 sources over a sky area of 125.5 sq. deg, in
three energy bands: 0.5-2 (soft), 2-10 (hard), and 4.5-10 (ultrahard) keV. We
have measured the angular clustering of our survey and find significant
positive clustering signals in the soft and hard bands, and a marginal
clustering detection in the ultrahard band. We find dependency of the
clustering strength on the flux limit and no significant differences in the
clustering properties between sources with high hardness ratios and those with
low hardness ratios. Our results show that obscured and unobscured objects
share similar clustering properties and therefore they both reside in similar
environments, in agreement with the unified model of AGN. We deprojected the
angular clustering parameters via Limber's equation to compute their typical
spatial lengths. From that we have inferred the typical mass of the dark matter
haloes in which AGN at redshifts of ~1 are embedded. The short AGN lifetimes
derived suggest that AGN activity might be a transient phase that can be
experienced several times by a large fraction of galaxies throughout their
lives.Comment: 14 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Obesity, unhappiness, and the challenge of affluence : theory and evidence
Is affluence a good thing? The book "The Challenge of Affluence" by Avner Offer (2006) argues that economic prosperity weakens self-control and undermines human well-being. Consistent with a pessimistic view, we show that psychological distress has been rising through time in modern Great Britain. Taking over-eating as an example, our data reveal that half the British population view themselves as overweight, and that happiness and mental health are worse among fatter people in Britain and Germany. Comparisons also matter. We discuss problems of inference and argue that longitudinal data are needed. We suggest a theory of obesity imitation where utility depends on relative weight
Simultaneous constraints on bias, normalization and growth index through power spectrum measurements
In this Letter we point out that redshift surveys can break the degeneracy
between the galaxy bias, the power spectrum normalization, \sigma_{8,0} and the
growth factor, without the need for external information by using a simple and
rather general parametrization for the growth rate, the well known
\gamma-parametrization and measuring the power spectrum at least at two
different redshifts. We find that in next-generation surveys like Euclid,
\sigma_{8,0} and \gamma can be measured to within 1% and 5%, respectively,
while the bias b(z) can be measured to within 1-2% in each of 14 equal-width
redshift bins spanning 0.7<=z<=2.Comment: 6 pages, 4 figures, version matching the one published by MNRAS
Letter
Diagnostic amyloid proteomics: experience of the UK National Amyloidosis Centre
Systemic amyloidosis is a serious disease which is caused when normal circulating proteins misfold and aggregate extracellularly as insoluble fibrillary deposits throughout the body. This commonly results in cardiac, renal and neurological damage. The tissue target, progression and outcome of the disease depends on the type of protein forming the fibril deposit, and its correct identification is central to determining therapy. Proteomics is now used routinely in our centre to type amyloid; over the past 7 years we have examined over 2000 clinical samples. Proteomics results are linked directly to our patient database using a simple algorithm to automatically highlight the most likely amyloidogenic protein. Whilst the approach has proved very successful, we have encountered a number of challenges, including poor sample recovery, limited enzymatic digestion, the presence of multiple amyloidogenic proteins and the identification of pathogenic variants. Our proteomics procedures and approaches to resolving difficult issues are outlined
On the variability of quasars: a link between Eddington ratio and optical variability?
Repeat scans by the Sloan Digital Sky Survey (SDSS) of a 278 square degree
stripe along the Celestial equator have yielded an average of over 10
observations each for nearly 8,000 spectroscopically confirmed quasars. Over
2500 of these quasars are in the redshift range such that the CIV emission line
is visible in the SDSS spectrum. Utilising the width of these CIV lines and the
luminosity of the nearby continuum, we estimate black hole masses for these
objects. In an effort to isolate the effects of black hole mass and luminosity
on the photometric variability of our dataset, we create several subsamples by
binning in these two physical parameters. By comparing the ensemble structure
functions of the quasars in these bins, we are able to reproduce the well-known
anticorrelation between luminosity and variability, now showing that this
anticorrelation is independent of the black hole mass. In addition, we find a
correlation between variability and the mass of the central black hole. By
combining these two relations, we identify the Eddington ratio as a possible
driver of quasar variability, most likely due to differences in accretion
efficiency.Comment: 13 pages, 5 figures, Accepted for publication in MNRA
Association of Eating and Sleeping Intervals With Weight Change Over Time: The Daily24 Cohort.
Background We aim to evaluate the association between meal intervals and weight trajectory among adults from a clinical cohort. Methods and Results This is a multisite prospective cohort study of adults recruited from 3 health systems. Over the 6-month study period, 547 participants downloaded and used a mobile application to record the timing of meals and sleep for at least 1 day. We obtained information on weight and comorbidities at each outpatient visit from electronic health records for up to 10 years before until 10 months after baseline. We used mixed linear regression to model weight trajectories. Mean age was 51.1 (SD 15.0) years, and body mass index was 30.8 (SD 7.8) kg/
Growth Index of DGP Model and Current Growth Rate Data
Recently, some efforts focus on differentiating dark energy and modified
gravity with the growth function . In the literature, it is useful
to parameterize the growth rate
with the growth index . In this note, we consider the general DGP model
with any . We confront the growth index of DGP model with currently
available growth rate data and find that the DGP model is still consistent with
it. This implies that more and better growth rate data are required to
distinguish between dark energy and modified gravity.Comment: 12 pages, 1 table, 2 figures, Latex2e; v2: discussions added, Phys.
Lett. B in press; v3: published versio
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