677 research outputs found
High resolution bathymetric survey on the NW slope of Walvis Ridge, offshore Namibia
Expedition 17/1 of the German research vessel R/V MARIA S. MERIAN, carried out geophysical
surveys and experiments between November and December 2010 in the area
around Walvis Ridge, Southeast Atlantic Ocean. Among the data collected, a high-resolution
bathymetric dataset aquired on the northwestern slope of the ridge offers some important
preliminary insights into the tectonic evolution of the ridge and the adjoining lower
continental slopes and ocean basin. The NE-SW trending Walvis Ridge has a trapezoid
shape and is likely built up by thick sequences of plateau basalts, with top of basement
rocks inclined to the south. Sediments are almost absent on the NW side of the ridge, preserving
a fascinating mountainscape formed early in the tectonic history, most probably
on-land. This interpretation is supported by clear denudational features, like steep cliffs up
to 150 m high, and deeply incised valleys, defining paleo-drainages. Isolated, flat-topped
guyots seaward of the ocean-continent boundary attest to a later history of wave abrasion
and progressive subsidence of Walvis Ridge
H.E.S.S. discovery of very-high-energy gamma-ray emission of PKS 1440-389
Blazars are the most abundant class of known extragalactic very-high-energy
(VHE, E>100 GeV) gamma-ray sources. However, one of the biggest difficulties in
investigating their VHE emission resides in their limited number, since less
than 60 of them are known by now. In this contribution we report on H.E.S.S.
observations of the BL Lac object PKS 1440-389. This source has been selected
as target for H.E.S.S. based on its high-energy gamma-ray properties measured
by Fermi-LAT. The extrapolation of this bright, hard-spectrum gamma-ray blazar
into the VHE regime made a detection on a relatively short time scale very
likely, despite its uncertain redshift. H.E.S.S. observations were carried out
with the 4-telescope array from February to May 2012 and resulted in a clear
detection of the source. Contemporaneous multi-wavelength data are used to
construct the spectral energy distribution of PKS 1440-389 which can be
described by a simple one-zone synchrotron-self Compton model.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
Periodicities in the emplacement of large igneous provinces through the Phanerozoic: Relations to ocean chemistry and marine biodiversity evolution
AbstractLarge igneous provinces (LIPs) are considered a relevant cause for mass extinctions of marine life throughout Earthâs history. Their flood basalts and associated intrusions can cause significant release of SO4 and CO2 and consequently, cause major environmental disruptions. Here, we reconstruct the long-term periodic pattern of LIP emplacement and its impact on ocean chemistry and biodiversity from ÎŽ34Ssulfate of the last 520 Ma under particular consideration of the preservation limits of LIP records. A combination of cross-wavelet and other time-series analysis methods has been applied to quantify a potential chain of linkage between LIP emplacement periodicity, geochemical changes and the Phanerozoic marine genera record. We suggest a mantle plume cyclicity represented by LIP volumes (V) of V=â(350â770)Ă103km3sin(2Ït/170Ma)+(300â650)Ă103km3sin(2Ït/64.5Ma+2.3) for t=time in Ma. A shift from the 64.5Ma to a weaker âŒ28â35Ma LIP cyclicity during the Jurassic contributes together with probably independent changes in the marine sulfur cycle to less ocean anoxia, and a general stabilization of ocean chemistry and increasing marine biodiversity throughout the last âŒ135Ma. The LIP cycle pattern is coherent with marine biodiversity fluctuations corresponding to a reduction of marine biodiversity of âŒ120genera/Ma at âŒ600Ă103km3 LIP eruption volume. The 62â65Ma LIP cycle pattern as well as excursion in ÎŽ34Ssulfate and marine genera reduction suggest a not-yet identified found LIP event at âŒ440â450Ma
Observations of the Crab Nebula with H.E.S.S. Phase II
The High Energy Stereoscopic System (H.E.S.S.) phase I instrument was an
array of four mirror area Imaging Atmospheric Cherenkov
Telescopes (IACTs) that has very successfully mapped the sky at photon energies
above GeV. Recently, a telescope was added to
the centre of the existing array, which can be operated either in standalone
mode or jointly with the four smaller telescopes. The large telescope lowers
the energy threshold for gamma-ray observations to several tens of GeV, making
the array sensitive at energies where the Fermi-LAT instrument runs out of
statistics. At the same time, the new telescope makes the H.E.S.S. phase II
instrument. This is the first hybrid IACT array, as it operates telescopes of
different size (and hence different trigger rates) and different field of view.
In this contribution we present results of H.E.S.S. phase II observations of
the Crab Nebula, compare them to earlier observations, and evaluate the
performance of the new instrument with Monte Carlo simulations.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
Detection and Imaging of Defects Especially Materials with Small UT Transducers Using Broad-Band Holography
Since conventional single frequency acoustical holography provides only poor axial resolution, this concept was improved with the multifrequency holography to enhance the imaging quality. This leads to long data acquisition times because of the need to measure each frequency. A further step towards a fast imaging system with good spatial resolution is broadband holography. Here, one illuminates the object with broadband signals in a single measurement procedure
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