518 research outputs found
Detecting stars, galaxies, and asteroids with Gaia
(Abridged) Gaia aims to make a 3-dimensional map of 1,000 million stars in
our Milky Way to unravel its kinematical, dynamical, and chemical structure and
evolution. Gaia's on-board detection software discriminates stars from spurious
objects like cosmic rays and Solar protons. For this, parametrised
point-spread-function-shape criteria are used. This study aims to provide an
optimum set of parameters for these filters. We developed an emulation of the
on-board detection software, which has 20 free, so-called rejection parameters
which govern the boundaries between stars on the one hand and sharp or extended
events on the other hand. We evaluate the detection and rejection performance
of the algorithm using catalogues of simulated single stars, double stars,
cosmic rays, Solar protons, unresolved galaxies, and asteroids. We optimised
the rejection parameters, improving - with respect to the functional baseline -
the detection performance of single and double stars, while, at the same time,
improving the rejection performance of cosmic rays and of Solar protons. We
find that the minimum separation to resolve a close, equal-brightness double
star is 0.23 arcsec in the along-scan and 0.70 arcsec in the across-scan
direction, independent of the brightness of the primary. We find that, whereas
the optimised rejection parameters have no significant impact on the
detectability of de Vaucouleurs profiles, they do significantly improve the
detection of exponential-disk profiles. We also find that the optimised
rejection parameters provide detection gains for asteroids fainter than 20 mag
and for fast-moving near-Earth objects fainter than 18 mag, albeit this gain
comes at the expense of a modest detection-probability loss for bright,
fast-moving near-Earth objects. The major side effect of the optimised
parameters is that spurious ghosts in the wings of bright stars essentially
pass unfiltered.Comment: Accepted for publication in A&
First results of a cryogenic optical photon counting imaging spectrometer using a DROID array
Context. In this paper we present the first system test in which we
demonstrate the concept of using an array of Distributed Read Out Imaging
Devices (DROIDs) for optical photon detection. Aims. After the successful S-Cam
3 detector the next step in the development of a cryogenic optical photon
counting imaging spectrometer under the S-Cam project is to increase the field
of view using DROIDs. With this modification the field of view of the camera
has been increased by a factor of 5 in area, while keeping the number of
readout channels the same. Methods. The test has been performed using the
flexible S-Cam 3 system and exchanging the 10x12 Superconducting Tunnel
Junction array for a 3x20 DROID array. The extra data reduction needed with
DROIDs is performed offline. Results. We show that, although the responsivity
(number of tunnelled quasiparticles per unit of absorbed photon energy, e- /eV)
of the current array is too low for direct astronomical applications, the
imaging quality is already good enough for pattern detection, and will improve
further with increasing responsivity. Conclusions. The obtained knowledge can
be used to optimise the system for the use of DROIDs.Comment: 7 pages, 9 figures, accepted for publicaiton in A&
Origin Of The Far Off-Axis GRB171205A
We show that observed properties of the low luminosity GRB171205A and its
afterglow, like those of most other low-luminosity (LL) gamma ray bursts (GRBs)
associate with a supernova (SN), indicate that it is an ordinary SN-GRB, which
was produced by inverse Compton scattering of glory light by a highly
relativistic narrowly collimated jet ejected in a supernova explosion and
viewed from a far off-axis angle. As such, VLA/VLBI follow-up radio
observations of a superluminal displacement of its bright radio afterglow from
its parent supernova, will be able to test clearly whether it is an ordinary
SN-GRB viewed from far off-axis or it belongs to a distinct class of GRBs,
which are different from ordinary GRBs, and cannot be explained by standard
fireball models of GRBs as ordinary GRBsComment: 5 pages, 6 figures, updated data in Fig. 3, Corrected GRB angular
distance used in Fig.
Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets
(ABRIDGED) We present a new Schwarzschild orbit-superposition code designed
to model discrete datasets composed of velocities of individual kinematic
tracers in a dynamical system. This constitutes an extension of previous
implementations that can only address continuous data in the form of (the
moments of) velocity distributions, thus avoiding potentially important losses
of information due to data binning. Furthermore, the code can handle any
combination of available velocity components, i.e., only line-of-sight
velocities, only proper motions, or a combination of both. It can also handle a
combination of discrete and continuous data. The code finds the distribution
function (DF, a function of the three integrals of motion E, Lz, and I3) that
best reproduces the available kinematic and photometric observations in a given
axisymmetric gravitational potential. The fully numerical approach ensures
considerable freedom on the form of the DF f(E,Lz,I3). This allows a very
general modeling of the orbital structure, thus avoiding restrictive
assumptions about the degree of (an)isotropy of the orbits. We describe the
implementation of the discrete code and present a series of tests of its
performance based on the modeling of simulated datasets generated from a known
DF. We find that the discrete Schwarzschild code recovers the original orbital
structure, M/L ratios, and inclination of the input datasets to satisfactory
accuracy, as quantified by various statistics. The code will be valuable, e.g.,
for modeling stellar motions in Galactic globular clusters, and those of
individual stars, planetary nebulae, or globular clusters in nearby galaxies.
This can shed new light on the total mass distributions of these systems, with
central black holes and dark matter halos being of particular interest.Comment: ApJ, in press; 51 pages, 11 figures; manuscript revised following
comments by refere
The development of bronchiectasis on chest computed tomography in children with cystic fibrosis: can pre-stages be identified?
Objective: Bronchiectasis is an important component of cystic fibrosis (CF) lung disease but little is known about its development. We aimed to study the development of bronchiectasis and identify determinants for rapid progression of bronchiectasis on chest CT. Methods: Forty-three patients with CF with at least four consecutive biennial volumetric CTs were included. Areas with bronchiectasis on the most recent CT were marked as regions of interest (ROIs). These ROIs were generated on all preceding CTs using deformable image registration. Observers indicated whether: bronchiectasis, mucus plugging, airway wall thickening, atelectasis/consolidation or normal airways were present in the ROIs. Results: We identified 362 ROIs on the most recent CT. In 187 (51.7 %) ROIs bronchiectasis was present on all preceding CTs, while 175 ROIs showed development of bronchiectasis. In 139/175 (79.4 %) no pre-stages of bronchiectasis were identified. In 36/175 (20.6 %) bronchiectatic airways the following pre-stages were identified: mucus plugging (17.7 %), airway wall thickening (1.7 %) or atelectasis/consolidation (1.1 %). Pancreatic insufficiency was more prevalent in the rapid progressors compared to the slow progressors (p = 0.05). Conclusion: Most bronchiectatic airways developed within 2 years without visible pre-stages, underlining the treacherous nature of CF lung disease. Mucus plugging was the most frequent pre-stage. Key Points: âą Development of bronchiectasis in cystic fibrosis lung disease on CT.âą Most bronchiectatic airways developed within 2 years without pre-stages.âą The most frequently identified pre-stage was mucus plugging.âą This study underlines the treacherous nature of CF lung disease
High-speed energy-resolved STJ photometry of the eclipsing dwarf nova IY UMa
We present high time-resolution photometry of the dwarf nova IY UMa using the
S-Cam2 super-conducting tunnel junction device attached to the 4.2m William
Herschel Telescope on La Palma. Exploiting the well-defined white dwarf and hot
spot eclipse features, we derive an updated orbital ephemeris for IY UMa and an
orbital period of 0.07390897(5) days. A white dwarf ingress/egress duration of
31 +/- 2s along with the contact phases of the bright spot gives M_1=0.79 +/-
0.04 M_sun and M_2=0.10 +/- 0.01 Msun, corresponding to a mass ratio of q=0.125
+/- 0.008. The white dwarf eclipse width of 0.0637 then implies i=86.0 +/- 1
degrees. A curious rise with a duration of 30 +/- 2s is observed in the orbital
lightcurves during all three observed eclipses. It occurs between the end of
white dwarf ingress and hot spot ingress and is blue in colour. We suggest that
the source of this light lies in the buried part of the gas stream, resulting
in a compact, hot impact cavity.Comment: 7 pages, 6 figures, accepted for publication in MNRA
A Concept for an STJ-based Spectrograph
We describe a multi-order spectrograph concept suitable for 8m-class
telescopes, using the intrinsic spectral resolution of Superconducting
Tunneling Junction detectors to sort the spectral orders. The spectrograph
works at low orders, 1-5 or 1-6, and provides spectral coverage with a
resolving power of R~8000 from the atmospheric cutoff at 320 nm to the long
wavelength end of the infrared H or K band at 1800 nm or 2400 nm. We calculate
that the spectrograph would provide substantial throughput and wavelength
coverage, together with high time resolution and sufficient dynamic range. The
concept uses currently available technology, or technologies with short
development horizons, restricting the spatial sampling to two linear arrays;
however an upgrade path to provide more spatial sampling is identified. All of
the other challenging aspects of the concept - the cryogenics, thermal baffling
and magnetic field biasing - are identified as being feasible.Comment: Accepted in Monthly Notices of the Royal Astronomical Society, 12
pages with 10 figure
Axisymmetric Three-Integral Models for Galaxies
We describe an improved, practical method for constructing galaxy models that
match an arbitrary set of observational constraints, without prior assumptions
about the phase-space distribution function (DF). Our method is an extension of
Schwarzschild's orbit superposition technique. As in Schwarzschild's original
implementation, we compute a representative library of orbits in a given
potential. We then project each orbit onto the space of observables, consisting
of position on the sky and line-of-sight velocity, while properly taking into
account seeing convolution and pixel binning. We find the combination of orbits
that produces a dynamical model that best fits the observed photometry and
kinematics of the galaxy. A key new element of this work is the ability to
predict and match to the data the full line-of-sight velocity profile shapes. A
dark component (such as a black hole and/or a dark halo) can easily be included
in the models.
We have tested our method, by using it to reconstruct the properties of a
two-integral model built with independent software. The test model is
reproduced satisfactorily, either with the regular orbits, or with the
two-integral components. This paper mainly deals with the technical aspects of
the method, while applications to the galaxies M32 and NGC 4342 are described
elsewhere (van der Marel et al., Cretton & van den Bosch). (abridged)Comment: minor changes, accepted for publication in the Astrophysical Journal
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