Search Publications

The variable energy-distribution of the gamma-ray emission from Geminga

Grenier, I. A.; Hermsen, W.; Hote, C.

The gamma-ray emission of Geminga during five COS-B observations has been analysed to derive its spectral properties between 50 MeV and 5 GeV. In all observations but one, the spectrum presents a turnover between 100 and 200 MeV, the source becoming very weak at the lowest gamma- ray energies. During the remaining observation, the spectrum is cons…

Plasma parameters near the comet Halley bow shock

Coates, A. J.; Neubauer, F. M.; Rème, H. +5 more

We analyse the available plasma data from the JPA, MAG and RPA instruments on Giotto for the inbound bow shock crossing at comet Halley. Proton, cometary heavy ion, electron and magnetic field data are combined on the inbound pass to yield the pressures of all the plasma components, and hence the characteristic plasma speeds and the Mach number of…

Astrometry using the hubble space telescope fine guidance sensors

Benedict, G. F.; Hemenway, P. D.; Jefferys, W. H. +2 more

The U. S. National Aeronautics and Space Administration in a cooperative effort with the European Space Agency has developed a large optical telescope with an aperture of 2.4 meters and a length of 8.8 meters. This instrument, the Hubble Space Telescope was launched by the space shuttle (STS-31) on the morning of 24 April 1990 and was placed in ea…

Ultraviolet observations of cataclysmic variables

Verbunt, Frank

The ultraviolet spectra of cataclysmic variables provide us with information on the mass transfer in these systems, and on the variations therein on different time scales. The combined ultraviolet and optical spectra cannot be fitted with simple model spectra, and accordingly, our interpretations of the observed spectra are subject to uncertainty.…

Scientific requirements for very accurate astrometry

Turon, C.; Kovalevsky, J.

Two types of accurate astrometric data are considered:

1) Survey astrometry providing absolute parallaxes and annual proper motions with an accuracy of 2 milli-arcsec, to a few tenth of a milli-arcsec. Parallax determinations will advance our basic knowledge of the Universe by giving the necessary strong foundations for the first steps of the…

Hipparcos calibration

Schrijver, Hans

The methods and results of various Hipparcos in-flight calibrations are described. The geometrical calibrations allow the determination of the instrument distortions in the main field of view to better than a milli-arcsecond. A systematic decrease of the basic angle at a rate of two milli-arcseconds per month has been measured during the first 7 m…

A comparison between two strategies of data reduction for the Hipparcos project

Lattanzi, M. G.; Bucciarelli, B.; Sarasso, M. +2 more

Two different strategies for reducing HIPPARCOS data are directly compared and their respective impact on the final precision of the HIPPARCOS Catalog is evaluated. These strategies are the FAST (Fundamental Astronomy by Space Techniques) Consortium's Baseline and GLOBUS procedures. The Baseline is already operating on satellite data; GLOBUS was i…

Hipparcos: Revised mission overview

Perryman, M. A. C.

The Hipparcos astrometry satellite was launched on 8 August 1989, and after spacecraft and payload commissioning, commenced the routine data acquisition phase on 26 November 1989. Having failed to reach its planned geostationary orbit, major revisions in the mission operations had to be performed, although the impact on the final mission accuracie…

Groundbased optical and radio astrometry of Hipparcos extragalactic link objects

Zacharias, N.; Johnston, K. J.; de Vegt, Chr.

The construction of a new extragalactic all-sky reference frame based on groundbased high precision optical and radio astrometry of selected compact radio sources and radio stars has been started in 1987 as a joint longterm project involving several astrometric facilities in both hemispheres. At present this system is based on a masterlist of abou…

Search of the latest non-astrometric Hipparcos stars in the HST guide-star catalogue

Crifo, F.; Grenon, M.; Jahreiss, H. +1 more

During the preparation of the Hipparcos Input Catalogue, about 800 Hipparcos stars remained without reliable positions or with questionable identifications. The Hubble Space Telescope Guide Star Catalogue group at the STScI in Baltimore provided charts (10×10 or 16×16 arcmin) showing all stars in the field down to the limiting magnitude of the GSC…