881 research outputs found
High resolution images at 11 and 20 microns of the Active Galactic Nucleus in NGC 1068
We present diffraction-limited IR images at 11.2 and 20.5 microns of the
central 6''x6'' region in NGC 1068, collected with the CAMIRAS instrument
mounted at the f/36 IR focus of the CFHT/Hawaii 3.6m telescope and at the f/35
IR focus of the ESO/La Silla 3.6m telescope, respectively. After deconvolution,
the achieved resolution (0.6'') reveals a prominent central core emitting about
95 % of the total flux at these wavelengths, as well as extended emission, to
the South-West and to the North-East, broken into patchy components which are
particularly conspicuous at 20.5 microns and can be isolated as individual
clouds. The central core shows an East-West FWHM of 0.6'' (hence unresolved)
and a North-South FWHM of 0.9'' corresponding to a resolved full size extension
of abound 100 pc. Such an elongated shape is in agreement with model
predictions of a dusty/molecular torus surrounding the central engine in NGC
1068, observed under an inclination angle around 65 degrees.Comment: 7 pages, 2 figures To appear in Astronomy and Astrophysic
Signatures of the transition from galactic to extragalactic cosmic rays
We discuss the signatures of the transition from galactic to extragalactic
cosmic rays in different scenarios, giving most attention to the dip scenario.
The dip is a feature in the diffuse spectrum of ultra-high energy (UHE) protons
in the energy range eV, which is caused by
electron-positron pair production on the cosmic microwave background (CMB)
radiation. The dip scenario provides a simple physical description of the
transition from galactic to extragalactic cosmic rays. Here we summarize the
signatures of the pair production dip model for the transition, most notably
the spectrum, the anisotropy and the chemical composition. The main focus of
our work is however on the description of the features that arise in the
elongation rate and in the distribution of the depths of shower maximum in the dip scenario. We find that the curve for shows a
sharp increase with energy, which reflects a sharp transition from an iron
dominated flux at low energies to a proton dominated flux at
eV. We also discuss in detail the shape of the distributions for
cosmic rays of given energy and demonstrate that this represents a powerful
tool to discriminate between the dip scenario and other possible models of the
transition.Comment: Version accepted for publication in Physical Review
Antimatter production in supernova remnants
We calculate the energy spectra of cosmic rays (CR) and their secondaries
produced in a supernova remnant (SNR), taking into account the time-dependence
of the SNR shock. We model the trajectories of charged particles as a random
walk with a prescribed diffusion coefficient, accelerating the particles at
each shock crossing. Secondary production by CRs colliding with gas is included
as a Monte Carlo process. We find that SNRs produce less antimatter than
suggested previously: The positron/electron ratio and the antiproton/proton
ratio are a few percent and few , respectively. Moreover, the
obtained positron/electron ratio decreases with energy, while the
antiproton/proton ratio rises at most by a factor of two above 10 GeV.Comment: 8 pages, 8 eps figures; extended version of arXiv:1004.1118; v2:
minor corrections, matches published versio
Searching for galactic cosmic ray pevatrons with multi-TeV gamma rays and neutrinos
The recent HESS detections of supernova remnant shells in TeV gamma-rays
confirm the theoretical predictions that supernova remnants can operate as
powerful cosmic ray accelerators. If these objects are responsible for the bulk
of galactic cosmic rays, then they should accelerate protons and nuclei to
10^15 eV and beyond, i.e. act as cosmic PeVatrons. The model of diffusive shock
acceleration allows, under certain conditions, acceleration of particles to
such high energies and their gradual injection into the interstellar medium,
mainly during the Sedov phase of the remnant evolution. The most energetic
particles are released first, while particles of lower energies are more
effectively confined in the shell, and are released at later epochs. Thus the
spectrum of nonthermal paticles inside the shell extends to PeV energies only
during a relatively short period of the evolution of the remnant. For this
reason one may expect spectra of secondary gamma-rays and neutrinos extending
to energies beyond 10 TeV only from T \lesssim 1000 yr old supernova remnants.
On the other hand, if by a chance a massive gas cloud appears in the \lesssim
100 pc vicinity of the supernova remnant, ``delayed'' multi-TeV signals of
gamma-rays and neutrinos arise when the most energetic partices emerged from
the supernova shell reach the cloud. The detection of such delayed emission of
multi-TeV gamma-rays and neutrinos allows indirect identification of the
supernova remnant as a particle PeVatron.Comment: ApJ Letters, in press. Reference to recent MILAGRO results adde
Submillimetre/TeraHertz Astronomy at Dome C with CEA filled bolometer array
Submillimetre/TeraHertz (e.g. 200, 350, 450 microns) astronomy is the prime
technique to unveil the birth and early evolution of a broad range of
astrophysical objects. A major obstacle to carry out submm observations from
ground is the atmosphere. Preliminary site testing and atmospheric transmission
models tend to demonstrate that Dome C could offer the best conditions on Earth
for submm/THz astronomy. The CAMISTIC project aims to install a filled
bolometer-array camera with 16x16 pixels on IRAIT at Dome C and explore the
200-m windows for potential ground-based observations.Comment: 6 page
Observing transiting planets with JWST -- Prime targets and their synthetic spectral observations
The James Webb Space Telescope will enable astronomers to obtain exoplanet
spectra of unprecedented precision. Especially the MIRI instrument may shed
light on the nature of the cloud particles obscuring planetary transmission
spectra in the optical and near-infrared. We provide self-consistent
atmospheric models and synthetic JWST observations for prime exoplanet targets
in order to identify spectral regions of interest and estimate the number of
transits needed to distinguish between model setups. We select targets which
span a wide range in planetary temperature and surface gravity, ranging from
super-Earths to giant planets, and have a high expected SNR. For all targets we
vary the enrichment, C/O ratio, presence of optical absorbers (TiO/VO) and
cloud treatment. We calculate atmospheric structures and emission and
transmission spectra for all targets and use a radiometric model to obtain
simulated observations. We analyze JWST's ability to distinguish between
various scenarios. We find that in very cloudy planets such as GJ 1214b less
than 10 transits with NIRSpec may be enough to reveal molecular features.
Further, the presence of small silicate grains in atmospheres of hot Jupiters
may be detectable with a single JWST MIRI transit. For a more detailed
characterization of such particles less than 10 transits are necessary.
Finally, we find that some of the hottest hot Jupiters are well fitted by
models which neglect the redistribution of the insolation and harbor
inversions, and that 1-4 eclipse measurements with NIRSpec are needed to
distinguish between the inversion models. Wet thus demonstrate the capabilities
of JWST for solving some of the most intriguing puzzles in current exoplanet
atmospheric research. Further, by publishing all models calculated for this
study we enable the community to carry out similar or retrieval analyses for
all planets included in our target list.Comment: 24 pages, 7 figures, accepted for publication in A&
Cosmic ray composition at high energies: Results from the TRACER project
The TRACER instrument Transition Radiation Array for Cosmic Energetic
Radiation is designed to measure the individual energy spectra of cosmic-ray
nuclei in long-duration balloon flights The large geometric factor of TRACER 5
m 2 sr permits statistically significant measurements at particle energies well
beyond 10 14 eV TRACER identifies individual cosmic-ray nuclei with
single-element resolution and measures their energy over a very wide range from
about 0 5 to 10 000 GeV nucleon This is accomplished with a gas detector system
of 1600 single-wire proportional tubes and plastic fiber radiators that measure
specific ionization and transition radiation signals combined with plastic
scintillators and acrylic Cherenkov counters A two-week flight in Antarctica in
December 2003 has led to a measurement of the nuclear species oxygen to iron O
Ne Mg Si S Ar Ca and Fe up to about 3 000 GeV nucleon We shall present the
energy spectra and relative abundances for these elements and discuss the
implication of the results in the context of current models of acceleration and
propagation of galactic cosmic rays The instrument has been refurbished for a
second long-duration flight in the Northern hemisphere scheduled for summer
2006 For this flight the dynamic range of TRACER has been extended to permit
inclusion of the lighter elements B C and N in the measurement.Comment: 36th COSPAR Scientific Assembly. Held 16 - 23 July 2006, in Beijing,
China., p.251
VISIR-VLT Images of the Water Maser Emitting Planetary Nebula K 3-35
K3-35 is an extremely young bipolar planetary nebula that contains a
precessing bipolar jet and a small (radius 80 AU) water maser equatorial ring.
We have obtained VISIR- VLT images of K 3-35 in the PAH1 ({\lambda}=8.6
{\mu}m), [S iv] ({\lambda}=10.6 {\mu}m), and SiC ({\lambda}=11.85 {\mu}m)
filters to analize the mid-IR morphology and the temperature structure of its
dust emission. The images show the innermost nebular regions undetected at
optical wavelegths and the precessing bipolar jets. The temperature map shows
variations in the temperature in the equatorial zone and in regions associated
to its jets.Comment: 2 pages, 2 figures, 283 IAU Symp. Planetary Nebulae an Eye to the
Futur
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