The X-ray Telescope of CAST

The Cern Axion Solar Telescope (CAST) is in operation and taking data since 2003. The main objective of the CAST experiment is to search for a hypothetical pseudoscalar boson, the axion, which might be produced in the core of the sun. The basic physics process CAST is based on is the time inverted Primakoff effect, by which an axion can be converted into a detectable photon in an external electromagnetic field. The resulting X-ray photons are expected to be thermally distributed between 1 and 7 keV. The most sensitive detector system of CAST is a pn-CCD detector combined with a Wolter I type X-ray mirror system. With the X-ray telescope of CAST a background reduction of more than 2 orders off magnitude is achieved, such that for the first time the axion photon coupling constant g_agg can be probed beyond the best astrophysical constraints g_agg < 1 x 10^-10 GeV^-1.Comment: 19 pages, 25 figures and images, replaced by the revised version accepted for publication in New Journal of Physic

Reliability of the automatic procedures for locating earthquakes in southwestern Alps and northern Apennines (Italy)

International audienceReliable automatic procedure for locating earthquake in quasi-real time is strongly needed for seismic warning system, earthquake preparedness, and producing shaking maps. The reliability of an automatic location algorithm is influenced by several factors such as errors in picking seismic phases, network geometry, and velocity model uncertainties. The main purpose of this work is to investigate the performances of different automatic procedures to choose the most suitable one to be applied for the quasi-real-time earthquake locations in northwestern Italy. The reliability of two automatic-picking algorithms (one based on the Characteristic Function (CF) analysis, CF picker, and the other one based on the Akaike's information criterion (AIC), AIC picker) and two location methods (“Hypoellipse” and “NonLinLoc” codes) is analysed by comparing the automatically determined hypocentral coordinates with reference ones. Reference locations are computed by the “Hypoellipse” code considering manually revised data and tested using quarry blasts. The comparison is made on a dataset composed by 575 seismic events for the period 2000–2007 as recorded by the Regional Seismic network of Northwestern Italy. For P phases, similar results, in terms of both amount of detected picks and magnitude of travel time differences with respect to manual picks, are obtained applying the AIC and the CF picker; on the contrary, for S phases, the AIC picker seems to provide a significant greater number of readings than the CF picker. Furthermore, the “NonLinLoc” software (applied to a 3D velocity model) is proved to be more reliable than the “Hypoellipse” code (applied to layered 1D velocity models), leading to more reliable automatic locations also when outliers (wrong picks) are present

Whole-genome sequences of Malawi cichlids reveal multiple radiations interconnected by gene flow.

The hundreds of cichlid fish species in Lake Malawi constitute the most extensive recent vertebrate adaptive radiation. Here we characterize its genomic diversity by sequencing 134 individuals covering 73 species across all major lineages. The average sequence divergence between species pairs is only 0.1-0.25%. These divergence values overlap diversity within species, with 82% of heterozygosity shared between species. Phylogenetic analyses suggest that diversification initially proceeded by serial branching from a generalist Astatotilapia-like ancestor. However, no single species tree adequately represents all species relationships, with evidence for substantial gene flow at multiple times. Common signatures of selection on visual and oxygen transport genes shared by distantly related deep-water species point to both adaptive introgression and independent selection. These findings enhance our understanding of genomic processes underlying rapid species diversification, and provide a platform for future genetic analysis of the Malawi radiation

Precise hypocentre relocation of microearthquakes in a high-temperature geothermal field: the Torfajokull central volcano, Iceland

The Torfaj¨okull volcanic system is one of approximately 30 active volcanoes comprising the neovolcanic zones of Iceland. The central volcano of the system is the largest silicic centre in Iceland with a caldera of approximately 12 km diameter. Its high-temperature geothermal system is one of the most powerful in Iceland. Torfaj¨okull is a source of persistent seismicity, where both high- and low-frequency earthquakes occur. To study this microseismicity in detail, a temporary array of 20 broad-band seismic stations was deployed between 2002 June and November. These temporary stations were embedded in the permanent South Iceland Lowland (SIL) network and data from nine adjacent SIL stationswere included in this study.Aminimum one-dimensional (1-D) velocity model with station corrections was computed for earthquake relocation by inverting manually picked P- and S-wave arrival times from events occurring in the Torfaj¨okull volcanic centre and its surroundings. High-frequency earthquakes from the Torfaj¨okull volcanic centre were then relocated calculating a non-linear, probabilistic solution to the earthquake location problem. Subsequently, we correlated the waveforms of these 121 events (∼2000 observations) to define linked events, calculated the relative traveltime difference between event pairs and solved for the hypocentral separation between these events. The resulting high-resolution pattern shows a tighter clustering in epicentre and focal depth when compared with original locations. Earthquakes are mainly located beneath the caldera with hypocentres between 1 and 6 km depth and lie almost exclusively within the geothermal system. A sharp cut-off in seismicity at 3 km suggests either that there is a marked temperature increase or that this is a structural boundary. No seismic activity was observed in the fissure swarms to the northeast (NE) and southwest (SW) of the volcanic centre

Low-frequency earthquakes at the Torfajokull volcano, south Iceland

Geodynamics geophysics and tectonics; Petrology igneous metamorphic and volcanic studies