161 research outputs found
An improved time of flight gamma-ray telescope to monitor diffuse gamma-ray in the energy range 5 MeV - 50 MeV
A time of flight measuring device is the basic triggering system of most of medium and high energy gamma-ray telescopes. A simple gamma-ray telescope has been built in order to check in flight conditions the functioning of an advanced time of flight system. The technical ratings of the system are described. This telescope has been flown twice with stratospheric balloons, its axis being oriented at various Zenital directions. Flight results are presented for diffuse gamma-rays, atmospheric secondaries, and various causes of noise in the 5 MeV-50 MeV energy range
Suivi dans des séquences d'images par coopération luminance/couleur
Cet article propose une technique de suivi de points différentielle robuste aux changements d'illumination par la coopération d'attributs couleur invariants et d'une normalisation photométrique en fonction du contexte. En effet, la plupart des invariants couleur s'avèrent bruités ou peu pertinents dans le cas d'une faible saturation et/ou d'une faible intensité, mettant le suivi en échec. Les combiner alors avec une information de luminance aboutit à un suivi plus performant quelles que soient les conditions d'éclairage. Quelques expérimentations prouvent la robustesse et la précision de cette approche
Secondary gamma-ray production in a coded aperture mask
The application of the coded aperture mask to high energy gamma-ray astronomy will provide the capability of locating a cosmic gamma-ray point source with a precision of a few arc-minutes above 20 MeV. Recent tests using a mask in conjunction with drift chamber detectors have shown that the expected point spread function is achieved over an acceptance cone of 25 deg. A telescope employing this technique differs from a conventional telescope only in that the presence of the mask modifies the radiation field in the vicinity of the detection plane. In addition to reducing the primary photon flux incident on the detector by absorption in the mask elements, the mask will also be a secondary radiator of gamma-rays. The various background components in a CAMTRAC (Coded Aperture Mask Track Chamber) telescope are considered. Monte-Carlo calculations are compared with recent measurements obtained using a prototype instrument in a tagged photon beam line
The ESO Key-Programme ``A Homogeneous Bright QSO Survey'' - I The Methods and the ``Deep'' Fields
This is the first paper in a series aimed at defining a statistically
significant sample of QSOs in the range and .
The selection is carried out using direct plates obtained at the ESO and UK
Schmidt Telescopes, scanned with the COSMOS facility and searched for objects
with an ultraviolet excess. Follow-up spectroscopy, carried out at ESO La
Silla, is used to classify each candidate. In this initial paper, we describe
the scientific objectives of the survey; the selection and observing techniques
used. We present the first sample of 285 QSOs () in a 153 deg
area, covered by the six ``deep'' fields, intended to obtain significant
statistics down with unprecedented photometric accuracy. From
this database, QSO counts are determined in the magnitude range .Comment: 21 pages uuencoded compressed postscript, to appear in Astronomy and
Astrophysics Supplements, 199
Light Echoes of Transients and Variables in the Local Universe
Astronomical light echoes, the time-dependent light scattered by dust in the
vicinity of varying objects, have been recognized for over a century.
Initially, their utility was thought to be confined to mapping out the
three-dimensional distribution of interstellar dust. Recently, the discovery of
spectroscopically-useful light echoes around centuries-old supernovae in the
Milky Way and the Large Magellanic Cloud has opened up new scientific
opportunities to exploit light echoes.
In this review, we describe the history of light echoes in the local Universe
and cover the many new developments in both the observation of light echoes and
the interpretation of the light scattered from them. Among other benefits, we
highlight our new ability to spectroscopically classify outbursting objects, to
view them from multiple perspectives, to obtain a spectroscopic time series of
the outburst, and to establish accurate distances to the source event. We also
describe the broader range of variable objects whose properties may be better
understood from light echo observations. Finally, we discuss the prospects of
new light echo techniques not yet realized in practice.Comment: 18 pages, 7 figures, 1 table. Accepted for publication in PAS
The Optical Polarisation of the Vela Pulsar revisited
In this work we present a revised measurement of the phase-averaged optical
polarisation of the Vela pulsar (PSR B0833-45), for which only one value has
been published so far (Wagner & Seifert 2000). Our measurement has been
obtained through an accurate reanalysis of archival polarisation observations
obtained with the FORS instrument at the VLT. We have measured a phase-averaged
linear polarisation degree P=9.4% +/- 4% and a position angle 146 +/- 11 deg,
very close to the ones of the axis of symmetry of the X-ray arcs and jets
detected by Chandra and of the pulsar proper motion.We have compared the
measured phase-averaged optical polarisation with the expectations of different
pulsars' magnetosphere models. We have found that all models consistently
predict too large values of the phase-averaged linear polarization with respect
to the observed one. This is probably a consequence of present models'
limitations which neglect the contributions of various depolarisation effects.
Interestingly, for the outer gap model we have found that, assuming synchrotron
radiation for the optical emission, the observed polarisation position angle
also implies an alignment between the pulsar rotational axis and the axis of
symmetry of the X-ray arcs and jets.Comment: 8 pages, 4 figure
- …