6,545 research outputs found
Distribution and partitioning of heavy metals in estuarine sediment cores and implications for the use of sediment quality standards
International audienceTotal metal concentrations in surface sediments and historically contaminated sediments were determined in sediment cores collected from three estuaries (Thames, Medway and Blackwater) in south-east England. The partitioning behaviour of metals in these sediments was also determined using a sequential extraction scheme. These data were then compared with sediment quality values (SQVs) to determine the potential ecotoxicological risk to sediment dwelling organisms. When total metal concentrations in surface sediments are examined, no risk to biota in any of the estuaries is indicated. However, when historically contaminated sediments at depth are also considered, risks to biota are apparent and are greatest for the Thames, followed by the Medway and then the Blackwater. This suggests that regulatory authorities should examine vertical metal profiles, particularly in estuaries that are experiencing low sediment accumulation rates where historically contaminated sediments are in the shallow sub-surface zone and where erosion or dredging activities may take place. When metal partitioning characteristics are also considered, the risk to biota is comparable for the Medway and the Blackwater with the potentially bioavailable fraction presenting no ecotoxicological risk. Conversely, over 70% of metals are labile in the Thames Estuary sediments and toxic effects are probable. This suggests that the application of SQVs using total sediment metal concentrations may over- or under-estimate the risk to biota in geochemically dissimilar estuarine sediments. Keywords: sediment quality values, estuarine sediments, metal contamination, partitioning, sequential extraction</p
SDSSJ14584479+3720215: A Benchmark JHK Blazar Light Curve from the 2MASS Calibration Scans
Active galactic nuclei (AGNs) are well-known to exhibit flux variability
across a wide range of wavelength regimes, but the precise origin of the
variability at different wavelengths remains unclear. To investigate the
relatively unexplored near-IR variability of the most luminous AGNs, we conduct
a search for variability using well sampled JHKs-band light curves from the
2MASS survey calibration fields. Our sample includes 27 known quasars with an
average of 924 epochs of observation over three years, as well as one
spectroscopically confirmed blazar (SDSSJ14584479+3720215) with 1972 epochs of
data. This is the best-sampled NIR photometric blazar light curve to date, and
it exhibits correlated, stochastic variability that we characterize with
continuous auto-regressive moving average (CARMA) models. None of the other 26
known quasars had detectable variability in the 2MASS bands above the
photometric uncertainty. A blind search of the 2MASS calibration field light
curves for AGN candidates based on fitting CARMA(1,0) models (damped-random
walk) uncovered only 7 candidates. All 7 were young stellar objects within the
{\rho} Ophiuchus star forming region, five with previous X-ray detections. A
significant {\gamma}-ray detection (5{\sigma}) for the known blazar using 4.5
years of Fermi photon data is also found. We suggest that strong NIR
variability of blazars, such as seen for SDSSJ14584479+3720215, can be used as
an efficient method of identifying previously-unidentified {\gamma}-ray
blazars, with low contamination from other AGN.Comment: 6 pages, 3 figures, ApJ Accepte
High-speed 2D light-sheet fluorescence microscopy enables quantification of spatially varying calcium dynamics in ventricular cardiomyocytes
Introduction: Reduced synchrony of calcium release and t-tubule structure organization in individual cardiomyocytes has been linked to loss of contractile strength and arrhythmia. Compared to confocal scanning techniques widely used for imaging calcium dynamics in cardiac muscle cells, light-sheet fluorescence microscopy enables fast acquisition of a 2D plane in the sample with low phototoxicity. Methods: A custom light-sheet fluorescence microscope was used to achieve dual-channel 2D timelapse imaging of calcium and the sarcolemma, enabling calcium sparks and transients in left and right ventricle cardiomyocytes to be correlated with the cell microstructure. Imaging electrically stimulated dual-labelled cardiomyocytes immobilized with para-nitroblebbistatin, a non-phototoxic, low fluorescence contraction uncoupler, with sub-micron resolution at 395 fps over a 38 μm × 170 µm FOV allowed characterization of calcium spark morphology and 2D mapping of the calcium transient time-to-half-maximum across the cell. Results: Blinded analysis of the data revealed sparks with greater amplitude in left ventricle myocytes. The time for the calcium transient to reach half-maximum amplitude in the central part of the cell was found to be, on average, 2 ms shorter than at the cell ends. Sparks co-localized with t-tubules were found to have significantly longer duration, larger area and spark mass than those further away from t-tubules. Conclusion: The high spatiotemporal resolution of the microscope and automated image-analysis enabled detailed 2D mapping and quantification of calcium dynamics of n = 60 myocytes, with the findings demonstrating multi-level spatial variation of calcium dynamics across the cell, supporting the dependence of synchrony and characteristics of calcium release on the underlying t-tubule structure
The chaotic solar cycle II. Analysis of cosmogenic 10Be data
Context. The variations of solar activity over long time intervals using a
solar activity reconstruction based on the cosmogenic radionuclide 10Be
measured in polar ice cores are studied. Methods. By applying methods of
nonlinear dynamics, the solar activity cycle is studied using solar activity
proxies that have been reaching into the past for over 9300 years. The
complexity of the system is expressed by several parameters of nonlinear
dynamics, such as embedding dimension or false nearest neighbors, and the
method of delay coordinates is applied to the time series. We also fit a damped
random walk model, which accurately describes the variability of quasars, to
the solar 10Be data and investigate the corresponding power spectral
distribution. The periods in the data series were searched by the Fourier and
wavelet analyses. The solar activity on the long-term scale is found to be on
the edge of chaotic behavior. This can explain the observed intermittent period
of longer lasting solar activity minima. Filtering the data by eliminating
variations below a certain period (the periods of 380 yr and 57 yr were used)
yields a far more regular behavior of solar activity. A comparison between the
results for the 10Be data with the 14C data shows many similarities. Both
cosmogenic isotopes are strongly correlated mutually and with solar activity.
Finally, we find that a series of damped random walk models provides a good fit
to the 10Be data with a fixed characteristic time scale of 1000 years, which is
roughly consistent with the quasi-periods found by the Fourier and wavelet
analyses.Comment: 8 pages, 11 figure
Merger of white dwarf-neutron star binaries: Prelude to hydrodynamic simulations in general relativity
White dwarf-neutron star binaries generate detectable gravitational
radiation. We construct Newtonian equilibrium models of corotational white
dwarf-neutron star (WDNS) binaries in circular orbit and find that these models
terminate at the Roche limit. At this point the binary will undergo either
stable mass transfer (SMT) and evolve on a secular time scale, or unstable mass
transfer (UMT), which results in the tidal disruption of the WD. The path a
given binary will follow depends primarily on its mass ratio. We analyze the
fate of known WDNS binaries and use population synthesis results to estimate
the number of LISA-resolved galactic binaries that will undergo either SMT or
UMT. We model the quasistationary SMT epoch by solving a set of simple ordinary
differential equations and compute the corresponding gravitational waveforms.
Finally, we discuss in general terms the possible fate of binaries that undergo
UMT and construct approximate Newtonian equilibrium configurations of merged
WDNS remnants. We use these configurations to assess plausible outcomes of our
future, fully relativistic simulations of these systems. If sufficient WD
debris lands on the NS, the remnant may collapse, whereby the gravitational
waves from the inspiral, merger, and collapse phases will sweep from LISA
through LIGO frequency bands. If the debris forms a disk about the NS, it may
fragment and form planets.Comment: 28 pages, 25 figures, 6 table
Remote-refocusing light-sheet fluorescence microscopy enables 3D imaging of electromechanical coupling of hiPSC-derived and adult cardiomyocytes in co-culture
Improving cardiac function through stem-cell regenerative therapy requires functional and structural integration of the transplanted cells with the host tissue. Visualizing the electromechanical interaction between native and graft cells necessitates 3D imaging with high spatio-temporal resolution and low photo-toxicity. A custom light-sheet fluorescence microscope was used for volumetric imaging of calcium dynamics in co-cultures of adult rat left ventricle cardiomyocytes and human induced pluripotent stem cell-derived cardiomyocytes. Aberration-free remote refocus of the detection plane synchronously to the scanning of the light sheet along the detection axis enabled fast dual-channel 3D imaging at subcellular resolution without mechanical sample disturbance at up to 8 Hz over a ∼300 µm × 40 µm × 50 µm volume. The two cell types were found to undergo electrically stimulated and spontaneous synchronized calcium transients and contraction. Electromechanical coupling improved with co-culture duration, with 50% of adult-CM coupled after 24 h of co-culture, compared to 19% after 4 h (p = 0.0305). Immobilization with para-nitroblebbistatin did not prevent calcium transient synchronization, with 35% and 36% adult-CM coupled in control and treated samples respectively (p = 0.91), indicating that electrical coupling can be maintained independently of mechanotransduction
A Consistent Picture Emerges: A Compact X-ray Continuum Emission Region in the Gravitationally Lensed Quasar SDSS J0924+0219
We analyze the optical, UV, and X-ray microlensing variability of the lensed
quasar SDSS J0924+0219 using six epochs of Chandra data in two energy bands
(spanning 0.4-8.0 keV, or 1-20 keV in the quasar rest frame), 10 epochs of
F275W (rest-frame 1089A) Hubble Space Telescope data, and high-cadence R-band
(rest-frame 2770A) monitoring spanning eleven years. Our joint analysis
provides robust constraints on the extent of the X-ray continuum emission
region and the projected area of the accretion disk. The best-fit half-light
radius of the soft X-ray continuum emission region is between 5x10^13 and 10^15
cm, and we find an upper limit of 10^15 cm for the hard X-rays. The best-fit
soft-band size is about 13 times smaller than the optical size, and roughly 7
GM_BH/c^2 for a 2.8x10^8 M_sol black hole, similar to the results for other
systems. We find that the UV emitting region falls in between the optical and
X-ray emitting regions at 10^14 cm < r_1/2,UV < 3x10^15 cm. Finally, the
optical size is significantly larger, by 1.5*sigma, than the theoretical
thin-disk estimate based on the observed, magnification-corrected I-band flux,
suggesting a shallower temperature profile than expected for a standard disk.Comment: Replaced with accepted version to Ap
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