109 research outputs found
Towards micro-arcsecond spatial resolution with Air Cherenkov Telescope arrays as optical intensity interferometers
In this poster contribution we highlight the equivalence between an Imaging
Air Cherenkov Telescope (IACT) array and an Intensity Interferometer for a
range of technical requirements. We touch on the differences between a
Michelson and an Intensity Interferometer and give a brief overview of the
current IACT arrays, their upgrades and next generation concepts (CTA, AGIS,
completion 2015). The latter are foreseen to include 30-90 telescopes that will
provide 400-4000 different baselines that range in length between 50m and a
kilometre. Intensity interferometry with such arrays of telescopes attains 50
micro-arcseconds resolution for a limiting V magnitude of ~8.5. This technique
opens the possibility of a wide range of studies, amongst others, probing the
stellar surface activity and the dynamic AU scale circumstellar environment of
stars in various crucial evolutionary stages. Here we discuss possibilities for
using IACT arrays as optical Intensity Interferometers.Comment: Appeared in the proceedings of "The Universe under the Microscope -
Astrophysics at High Angular Resolution", Journal of Physics:Conference
Series (IOP; http://www.iop.org/EJ/toc/1742-6596/131/1
Selection and 3D-Reconstruction of Gamma-Ray-induced Air Showers with a Stereoscopic System of Atmospheric Cherenkov Telescopes
A simple 3D-reconstruction method for gamma-ray induced air showers is
presented, which takes full advantage of the assets of a system of Atmospheric
Cherenkov Telescopes combining stereoscopy and fine-grain imaging like the High
Energy Stereoscopic System (H.E.S.S.). The rich information collected by the
cameras allows to select electromagnetic showers on the basis of their
rotational symmetry with respect to the incident direction, as well as of their
relatively small lateral spread. In the framework of a 3D-model of the shower,
its main parameters -- incident direction, shower core position on the ground,
slant depth of shower maximum, average lateral spread of Cherenkov photon
origins (or ``photosphere 3D-width'') and primary energy -- are fitted to the
pixel contents of the different images. For gamma-ray showers, the photosphere
3D-width is found to scale with the slant depth of shower maximum, an effect
related to the variation of the Cherenkov threshold with the altitude; this
property allows to define a dimensionless quantity omega (the ``reduced
3D-width''), which turns out to be an efficient and robust variable to
discriminate gamma-rays from primary hadrons. In addition, the omega
distribution varies only slowly with the gamma-ray energy and is practically
independent of the zenith angle. The performance of the method as applied to
H.E.S.S. is presented. Depending on the requirements imposed to reconstructed
showers, the angular resolution at zenith varies from 0.04 to 0.1 degrees and
the spectral resolution in the same conditions from 15% to 20%.Comment: 32 pages including 38 figures. Accepted by AstroParticle Physic
Ornamental colors reveal age in the king penguin
We investigated whether delayed plumage maturation occurred in king penguins (Aptenodytes patagonicus). Therefore we examined the relationships between age and sex on spectral properties and size of two colored plumage patches and a UV-reflective beak spot, using known-age cohorts. Unlike the colored patch on the breast, we found age differences in ear and beak coloration. These results suggest that head ornaments in king penguins could signal sexual maturity or social status. No sex differences were found in the intensity of colored ornaments, which can result from mutual mate choice or genetic correlation between sexes. Size of colored patches did not relate to age or sex
Comparison of simulated longitudinal profiles of hadronic air showers with MASS2 balloon data
The KASKADE and CORSIKA air shower generators are compared to the data
collected by MASS2 balloon experiment in 1991. The test of longitudinal profile
for proton, helium and muon flux production provide good constraints on these
air shower generators. KASKADE and CORSIKA especially with the new simulator
UrQMD for low energies are found to fit these data well. This study is limited
to a comparison of longitudinal profiles and therefore does not provide
constraints on the overall shower development.Comment: to be published in Astroparticle Physic
Detection Techniques of Microsecond Gamma-Ray Bursts using Ground-Based Telescopes
Gamma-ray observations above 200 MeV are conventionally made by
satellite-based detectors. The EGRET detector on the Compton Gamma Ray
Observatory (CGRO) has provided good sensitivity for the detection of bursts
lasting for more than 200 ms. Theoretical predictions of high-energy gamma-ray
bursts produced by quantum-mechanical decay of primordial black holes (Hawking
1971) suggest the emission of bursts on shorter time scales. The final stage of
a primordial black hole results in a burst of gamma-rays, peaking around 250
MeV and lasting for a tenth of a microsecond or longer depending on particle
physics. In this work we show that there is an observational window using
ground-based imaging Cherenkov detectors to measure gamma-ray burst emission at
energies E greater than 200 MeV. This technique, with a sensitivity for bursts
lasting nanoseconds to several microseconds, is based on the detection of
multi-photon-initiated air showers.Comment: accepted for publication in the Astrophysical Journa
New Astrophysical Opportunities Exploiting Spatio-Temporal Optical Correlations
The space-time correlations of streams of photons can provide fundamentally
new channels of information about the Universe. Today's astronomical
observations essentially measure certain amplitude coherence functions produced
by a source. The spatial correlations of wave fields has traditionally been
exploited in Michelson-style amplitude interferometry. However the technology
of the past was largely incapable of fine timing resolution and recording
multiple beams. When time and space correlations are combined it is possible to
achieve spectacular measurements that are impossible by any other means.
Stellar intensity interferometry is ripe for development and is one of the few
unexploited mechanisms to obtain potentially revolutionary new information in
astronomy. As we discuss below, the modern use of stellar intensity
interferometry can yield unprecedented measures of stellar diameters, binary
stars, distance measures including Cepheids, rapidly rotating stars, pulsating
stars, and short-time scale fluctuations that have never been measured before.Comment: Science white paper prepared for the Astro2010 Decadal Revie
Bidirectional reflectance measurement of tungsten samples to assess reflection model in WEST tokamak
This paper presents the measurement of the bidirectional reflectance distribution function of tungsten (W) samples and the resulting reflection models in the nuclear fusion device WEST (tokamak). For this, an experimental gonio-spectrophotometer was developed to fully characterize the material’s optical and thermal-radiative properties of metallic samples with different roughnesses. Ray-tracing photonic simulation was then carried out to predict the photon behavior in a fully metallic environment as a function of reflectance measurement. Low emissivity (0.1 at 4 μm) and highly specular reflectance (fitting with a Gaussian distribution around the specular direction with a small width lower than 10°) are found for W samples. These measurements have been used as input for the photonic simulation, and the resulting synthetic image reproduced the reflection features well on the upper divertor, detected in WEST infrared experimental images
Search for TeV Gamma-Rays from Shell-Type Supernova Remnants
If cosmic rays with energies <100 TeV originate in the galaxy and are
accelerated in shock waves in shell-type supernova remnants (SNRs), gamma-rays
will be produced as the result of proton and electron interactions with the
local interstellar medium, and by inverse Compton emission from electrons
scattering soft photon fields. We report on observations of two supernova
remnants with the Whipple Observatory's 10 m gamma-ray telescope. No
significant detections have been made and upper limits on the >500 GeV flux are
reported. Non-thermal X-ray emission detected from one of these remnants
(Cassiopeia A) has been interpreted as synchrotron emission from electrons in
the ambient magnetic fields. Gamma-ray emission detected from the
Monoceros/Rosette Nebula region has been interpreted as evidence of cosmic-ray
acceleration. We interpret our results in the context of these observations.Comment: 4 pages, 2 figures, to appear in the proceedings of 26th
International Cosmic Ray Conference (Salt Lake City, 1999
Very High Energy Gamma-ray spectral properties of Mrk 501 from CAT Cerenkov telescope observations in 1997
The BL Lac object Mrk 501 went into a very high state of activity during
1997, both in VHE gamma-rays and X-rays. We present here results from
observations at energies above 250 GeV carried out between March and October
1997 with the CAT Cerenkov imaging Telescope. The average differential spectrum
between 30 GeV and 13 TeV shows significant curvature and is well represented
by phi_0 * E_TeV^{-(alpha + beta*log10(E_TeV))}, with: phi_0 = 5.19 +/- 0.13
{stat} +/- 0.12 {sys-MC} +1.66/-1.04 {sys-atm} * 10^-11 /cm^2/s/TeV alpha =
2.24 +/- 0.04 {stat} +/- 0.05 {sys} beta = 0.50 +/- 0.07 {stat} (negligible
systematics). The TeV spectral energy distribution of Mrk 501 clearly peaks in
the range 500 GeV-1 TeV. Investigation of spectral variations shows a
significant hardness-intensity correlation with no measurable effect on the
curvature. This can be described as an increase of the peak TeV emission energy
with intensity. Simultaneous and quasi-simultaneous CAT VHE gamma-ray and
BeppoSAX hard X-ray detections for the highest recorded flare on 16th April and
for lower-activity states of the same period show correlated variability with a
higher luminosity in X-rays than in gamma-rays. The observed spectral energy
distribution and the correlated variability between X-rays and gamma-rays, both
in amplitude and in hardening of spectra, favour a two-component emission
scheme where the low and high energy components are attributed to synchrotron
and inverse Compton (IC) radiation, respectively.Comment: Submitted to Astronomy and Astrophysics, 8 pages including 6 figures.
Published with minor change
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