Frontiers Of Space And Ground-Based Astronomy
Frontiers Of Space And Ground-Based Astronomy

Author: Willem Wamsteker

Publisher: Springer Science & Business Media

Published: 2012-12-06

Total Pages: 721

ISBN-13: 9401107947

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ROSAT Observations G. HASINGER Max-Planck-Institut flir extraterrestrische Physik, D-85740 Garching, Germany Abstract. This review describes the most recent advances in the study of the extragalactic soft X-ray background and what we can learn about its constituents. The deepest pointed observations with the ROSAT PSPC are discussed. The logN-logS relation is presented, which reaches to the faintest X-ray fluxes and to the highest AGN surface densities ever achieved. The N(>S) relation shows a 2 density in excess of 400 deg- at the faintest fluxes and a flattening below the Einstein Deep Survey limit. About 60% of the extragalactic background has been resolved in the deepest field. Detailed source spectra and first optical and radio identifications will be discussed. The results are put into perspective of the higher energy X -ray background. Key words: X-rays, background radiations, active galactic nuclei. 1. Introduction The extragalactic X-ray background (XRB), discovered about 30 years ago, has been studied extensively with many X-ray experiments, in particular with the satel lites HEAO I and II (see ego Boldt 1987) and with ROSAT (e. g. Hasinger et aI. , 1993). Figure 1 shows a compilation of some of the most recent spectral measure ments for the X-ray background. Over the energy range from 3 to about 100 keY its spectrum can be well approximated by an optically thin thermal bremsstrahlung model with kT ~ 40 keY, while at lower X-ray energies a steepening into a new component has been observed observed (e. g.

Astronomical and Astrophysical Objectives of Sub-Milliarcsecond Optical Astrometry
Astronomical and Astrophysical Objectives of Sub-Milliarcsecond Optical Astrometry

Author: Erik Høg

Publisher: Springer Science & Business Media

Published: 2013-12-01

Total Pages: 427

ISBN-13: 9401100691

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Astrometry is on the threshold of great changes due to the fact that this decade, alone, is witnessing an improvement of stellar positions equivalent to the total improvement of the previous two centuries. The Hipparcos Satellite has concluded its observations, and the catalog is in preparation. Preliminary results assure that the Hipparcos catalog will provide positions, parallaxes and annual proper motions for over 100,000 stars with accuracies of 1.5 milliarcseconds. In addition, the Tycho catalog will provide positions of about 30 milliarcseconds accuracy for over 1 million stars, and annual proper motions with 3 milliarcsecond accuracy will subsequently be ob tained by means of first epoch positions from the Astrographic Catalog. Optical interferometers on the ground are beginning operation, and these instruments can provide observational accuracies of approximately one milliarcsecond. Also, the traditional reference frame based on the Fun damental Catalog of bright stars is being replaced by the extragalactic ref erence frame, based on radio sources with accuracies of one milliarcsecond. Thus, astrometry will change from a fundamental reference frame defined in terms of the dynamical reference frame of the solar system with accuracies of 100 milliarcseconds to a space-fixed, extragalactic reference frame with accuracies of one milliarcsecond. Future astrometric observations should be in the 1 -100 milliarcsecond accuracy range. There are a number of concepts for future astrometric instruments in space. Most of these can provide sub-milliarcsecond astrometric accuracies.