123 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
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
The properties of the peculiar Type Ia SN~1991bg. Analysis and discussion of two years of observations
Observations of the peculiar type Ia SN~1991bg in NGC~4374 collected at
ESO--La Silla and Asiago are presented and discussed. The broad--band light
curves in the early months have a narrower peak and a luminosity decline faster
than other SNIa while the R and I light curves do not show the secondary peak.
The SN is intrinsically very red (\bv) and faint
(). The bolometric light curve of SN~1991bg is fainter
with a steeper decline than that of the normal SNIa (e.g. 1992A). This object
enhances the correlation which exists between the peak luminosity of SNIa, the
decline rate and the kinetic energy. Although a number of spectral
peculiarities are present the spectral evolution retains a general resemblance
to that of other SNIa until the latest available observation (day 203). This
work suggests that the explosion energy was probably a factor 3 to 5 lower than
in normal SNIa.Comment: 17pages, Latex, MNRAS in press, postscript with full resolution
figures available at http://www.pd.astro.it/supern/preprints.htm
Helium Emission in the Type Ic SN 1999cq
We present the first unambiguous detection of helium emission lines in
spectra of Type Ic supernovae (SNe Ic). The presence of He I lines, with full
width at half maximum ~ 2000 km/s, and the distinct absence of any other
intermediate-width emission (e.g., Halpha), implies that the ejecta of SN Ic
1999cq are interacting with dense circumstellar material composed of almost
pure helium. This strengthens the argument that the progenitors of SNe Ic are
core-collapse events in stars that have lost both their hydrogen and helium
envelopes, either through a dense wind or mass-transfer to a companion. In this
way, SN 1999cq is similar to supernovae such as SN 1987K and SN 1993J that
helped firmly establish a physical connection between Type Ib and Type II
supernovae. The light curve of SN 1999cq is very fast, with an extremely rapid
rise followed by a quick decline. SN 1999cq is also found to exhibit a high
level of emission at blue wavelengths (< 5500 A), likely resulting from either
an unusually large amount of iron and iron-group element emission or
uncharacteristically low reddening compared with other SNe Ic.Comment: 17 pages (AASTeX V5.0), 4 figures, accepted for publication in the
Astronomical Journa
The bright linear type II SN 1990K
We obtained extensive photometric and spectroscopic observations of the Linear type II SN 1990K. The photometry suggests that the SN suffered a relatively high extinction (A_B_=~2mag) and was therefore very bright at maximum (M^0^_V_sun_). The spectral evolution is typical for SNII, very similar for instance to that of the SN 1987A (neglecting the fact that the latter SN showed unusually strong BaII lines). The line flux evolution of SN 1990K compared with that of other SNe and with theoretical models indicates that the envelope mass of SN 1990K was relatively low (~5Msun_). Also, the envelope density is probably low (i.e. a factor 3 lower than in SN 1987A). Two months after discovery a bump appeared in the blue side of the Hα line which has a strong similarity to the same feature observed in SN 1987A (Bochum event) and in SN 1988A and has been related to the emergence of radioactive material into the outer hydrogen-rich envelope
The study of neutron star magnetospheres with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of magnetospheres of isolated
neutron stars. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
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