395 research outputs found
SST-GATE: A dual mirror telescope for the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) will be the world's first open
observatory for very high energy gamma-rays. Around a hundred telescopes of
different sizes will be used to detect the Cherenkov light that results from
gamma-ray induced air showers in the atmosphere. Amongst them, a large number
of Small Size Telescopes (SST), with a diameter of about 4 m, will assure an
unprecedented coverage of the high energy end of the electromagnetic spectrum
(above ~1TeV to beyond 100 TeV) and will open up a new window on the
non-thermal sky. Several concepts for the SST design are currently being
investigated with the aim of combining a large field of view (~9 degrees) with
a good resolution of the shower images, as well as minimizing costs. These
include a Davies-Cotton configuration with a Geiger-mode avalanche photodiode
(GAPD) based camera, as pioneered by FACT, and a novel and as yet untested
design based on the Schwarzschild-Couder configuration, which uses a secondary
mirror to reduce the plate-scale and to allow for a wide field of view with a
light-weight camera, e.g. using GAPDs or multi-anode photomultipliers. One
objective of the GATE (Gamma-ray Telescope Elements) programme is to build one
of the first Schwarzschild-Couder prototypes and to evaluate its performance.
The construction of the SST-GATE prototype on the campus of the Paris
Observatory in Meudon is under way. We report on the current status of the
project and provide details of the opto-mechanical design of the prototype, the
development of its control software, and simulations of its expected
performance.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil). All CTA contributions at arXiv:1307.223
Post-Tanner spreading of nematic droplets
The quasistationary spreading of a circular liquid drop on a solid substrate
typically obeys the so-called Tanner law, with the instantaneous base radius
R(t) growing with time as R ~ t^{1/10} -- an effect of the dominant role of
capillary forces for a small-sized droplet. However, for droplets of nematic
liquid crystals, a faster spreading law sets in at long times, so that R ~
t^alpha with alpha significantly larger than the Tanner exponent 1/10. In the
framework of the thin film model (or lubrication approximation), we describe
this "acceleration" as a transition to a qualitatively different spreading
regime driven by a strong substrate-liquid interaction specific to nematics
(antagonistic anchoring at the interfaces). The numerical solution of the thin
film equation agrees well with the available experimental data for nematics,
even though the non-Newtonian rheology has yet to be taken into account. Thus
we complement the theory of spreading with a post-Tanner stage, noting that the
spreading process can be expected to cross over from the usual
capillarity-dominated stage to a regime where the whole reservoir becomes a
diffusive film in the sense of Derjaguin.Comment: 15 pages, 4 figures, accepted in JPCM special issu
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Flow-induced dynamic surface tension effects at nanoscale
The aim of this study is to investigate flow-induced dynamic surface tension effects, similar to the well-known Marangoni phenomena, but solely generated by the nanoscale topography of the substrates. The flow-induced surface tension effects are examined on the basis of a sharp interface theory. It is demonstrated how nanoscale objects placed at the boundary of the flow domain result in the generation of substantial surface forces acting on the bulk flow
Gamma-ray Observations Under Bright Moonlight with VERITAS
Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive
photomultiplier tube (PMT) cameras. Exposure to high levels of background
illumination degrades the efficiency of and potentially destroys these
photo-detectors over time, so IACTs cannot be operated in the same
configuration in the presence of bright moonlight as under dark skies. Since
September 2012, observations have been carried out with the VERITAS IACTs under
bright moonlight (defined as about three times the night-sky-background (NSB)
of a dark extragalactic field, typically occurring when Moon illumination >
35%) in two observing modes, firstly by reducing the voltage applied to the
PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to
the cameras. This has allowed observations at up to about 30 times previous NSB
levels (around 80% Moon illumination), resulting in 30% more observing time
between the two modes over the course of a year. These additional observations
have already allowed for the detection of a flare from the 1ES 1727+502 and for
an observing program targeting a measurement of the cosmic-ray positron
fraction. We provide details of these new observing modes and their performance
relative to the standard VERITAS observations
Post-Tanner stages of droplet spreading: the energy balance approach revisited
The spreading of a circular liquid drop on a solid substrate can be described
by the time evolution of its base radius R(t). In complete wetting the
quasistationary regime (far away from initial and final transients) typically
obeys the so-called Tanner law, with R t^alpha_T, alpha_T=1/10. Late-time
spreading may differ significantly from the Tanner law: in some cases the drop
does not thin down to a molecular film and instead reaches an equilibrium
pancake-like shape; in other situations, as revealed by recent experiments with
spontaneously spreading nematic crystals, the growth of the base radius
accelerates after the Tanner stage. Here we demonstrate that these two
seemingly conflicting trends can be reconciled within a suitably revisited
energy balance approach, by taking into account the line tension contribution
to the driving force of spreading: a positive line tension is responsible for
the formation of pancake-like structures, whereas a negative line tension tends
to lengthen the contact line and induces an accelerated spreading (a transition
to a faster power law for R(t) than in the Tanner stage).Comment: 12 pages, 1 figur
Discovery of very-high-energy emission from RGB J2243+203 and derivation of its redshift upper limit
Very-high-energy (VHE; 100 GeV) gamma-ray emission from the blazar RGB
J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the
period between 21 and 24 December 2014. The VERITAS energy spectrum from this
source can be fit by a power law with a photon index of , and a
flux normalization at 0.15 TeV of . The integrated
\textit{Fermi}-LAT flux from 1 GeV to 100 GeV during the VERITAS detection is
, which is an order of
magnitude larger than the four-year-averaged flux in the same energy range
reported in the 3FGL catalog, (). The detection with VERITAS
triggered observations in the X-ray band with the \textit{Swift}-XRT. However,
due to scheduling constraints \textit{Swift}-XRT observations were performed 67
hours after the VERITAS detection, not simultaneous with the VERITAS
observations. The observed X-ray energy spectrum between 2 keV and 10 keV can
be fitted with a power-law with a spectral index of , and the
integrated photon flux in the same energy band is . EBL model-dependent upper limits
of the blazar redshift have been derived. Depending on the EBL model used, the
upper limit varies in the range from z to z
Very-high-energy observations of the binaries V 404 Cyg and 4U 0115+634 during giant X-ray outbursts
Transient X-ray binaries produce major outbursts in which the X-ray flux can
increase over the quiescent level by factors as large as . The low-mass
X-ray binary V 404 Cyg and the high-mass system 4U 0115+634 underwent such
major outbursts in June and October 2015, respectively. We present here
observations at energies above hundreds of GeV with the VERITAS observatory
taken during some of the brightest X-ray activity ever observed from these
systems. No gamma-ray emission has been detected by VERITAS in 2.5 hours of
observations of the microquasar V 404 Cyg from 2015, June 20-21. The upper flux
limits derived from these observations on the gamma-ray flux above 200 GeV of F
cm s correspond to a tiny fraction (about
) of the Eddington luminosity of the system, in stark contrast to that
seen in the X-ray band. No gamma rays have been detected during observations of
4U 0115+634 in the period of major X-ray activity in October 2015. The flux
upper limit derived from our observations is F cm
s for gamma rays above 300 GeV, setting an upper limit on the ratio of
gamma-ray to X-ray luminosity of less than 4%.Comment: Accepted for publication in the Astrophysical Journa
Optical spectroscopy of Blazars for the Cherenkov Telescope Array
Context. Blazars are the most numerous class of High Energy (HE; E about 50
MeV - few 100 GeV) and Very High Energy (VHE; E about 100 GeV - 10 TeV)
gamma-ray emitters. As of today, a measured spectroscopic redshift is available
for only about 50% of gamma-ray BL Lacs, mainly due to the difficulty of
measuring reliable redshifts from their nearly featureless, continuum-dominated
optical spectra. The knowledge of the redshift is fundamental for understanding
the emission from blazars, for population studies and also for indirect studies
of the extragalactic background light and searches for Lorentz invariance
violation and axion-like particles using blazars. Aims. This paper is the first
of a series of papers which aim to measure the redshift of a sample of blazars
likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a
ground based gamma-ray observatory. Methods. Monte Carlo simulations were
performed to select those hard spectrum gamma-ray blazars detected with the
Fermi-LAT telescope still lacking redshift measurements but likely to be
detected by CTA in 30 hours of observing time or less. Optical observing
campaigns involving deep imaging and spectroscopic observations were organised
to efficiently constrain their redshifts. We performed deep medium to high
resolution spectroscopy of nineteen blazar optical counterparts using the ESI
spectrograph at Keck, the RSS spectrograph at the SALT telescope, and the
EFOSC2 spectrograph at the ESO NTT. We searched systematically for spectral
features and, when possible, we estimated the contribution of the host galaxy
to the total flux. Results. We measured eleven firm spectroscopic redshifts
with values ranging from 0.1116 to 0.482. one tentative redshift, three
redshift lower limits including one at z > 0.449 and another at z > 0.868.
There were four objects found to have featureless spectra.Comment: Accepted by Astronomy & Astrophysic
Dark Matter Constraints from a Joint Analysis of Dwarf Spheroidal Galaxy Observations with VERITAS
We present constraints on the annihilation cross section of WIMP dark matter
based on the joint statistical analysis of four dwarf galaxies with VERITAS.
These results are derived from an optimized photon weighting statistical
technique that improves on standard imaging atmospheric Cherenkov telescope
(IACT) analyses by utilizing the spectral and spatial properties of individual
photon events. We report on the results of 230 hours of observations of
five dwarf galaxies and the joint statistical analysis of four of the dwarf
galaxies. We find no evidence of gamma-ray emission from any individual dwarf
nor in the joint analysis. The derived upper limit on the dark matter
annihilation cross section from the joint analysis is at 1 TeV for the bottom quark () final state,
at 1 TeV for the tau lepton
() final state and at 1 TeV for the gauge boson () final state.Comment: 14 pages, 9 figures, published in PRD, Ascii tables containing
annihilation cross sections limits are available for download as ancillary
files with readme.txt file description of limit
Resistance to a Rhabdovirus (VHSV) in Rainbow Trout: Identification of a Major QTL Related to Innate Mechanisms
Chantier qualité GAHealth control is a major issue in animal breeding and a better knowledge of the genetic bases of resistance to diseases is needed in farm animals including fish. The detection of quantitative trait loci (QTL) will help uncovering the genetic architecture of important traits and understanding the mechanisms involved in resistance to pathogens. We report here the detection of QTL for resistance to Viral Haemorrhagic Septicaemia Virus (VHSV), a major threat for European aquaculture industry. Two induced mitogynogenetic doubled haploid F2 rainbow trout (Oncorhynchus mykiss) families were used. These families combined the genome of susceptible and resistant F0 breeders and contained only fully homozygous individuals. For phenotyping, fish survival after an immersion challenge with the virus was recorded, as well as in vitro virus replication on fin explants. A bidirectional selective genotyping strategy identified seven QTL associated to survival. One of those QTL was significant at the genome-wide level and largely explained both survival and viral replication in fin explants in the different families of the design (up to 65% and 49% of phenotypic variance explained respectively). These results evidence the key role of innate defence in resistance to the virus and pave the way for the identification of the gene(s) responsible for resistance. The identification of a major QTL also opens appealing perspectives for selective breeding of fish with improved resistance
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