Introduction Scientific Objective Satellite and Payload

The advantage of an all-sky survey consists of its unbiased view providing the potential for new discoveries and yielding large samples of objects for statistical studies. Ultimately, the unlimited field of view of telecopic sky scans uncovers extended objects and large-scale structures of the universe.

The Röntgensatellit ROSAT has demonstrated this advantage very convincingly in the soft X-ray band between 0.1 keV and 2.4 keV. The objective of the ABRIXAS mission (A Broad Band Imaging X-ray All-Sky Survey) will be to perform a sky survey at higher energies from ~0.5 keV to ~12 keV. But this mission will not simply complement the ROSAT All-Sky Survey to an higher energy range, but give a substantial approvement in spectral and angular resolution by using a charge coupled device (pn-CCD) detector camera, built in a complete new technology.

Some ten thousand X-ray sources will be detected with the ABRIXAS mission, among them at least 10,000 new discoveries, obscured sources, which are too absorbed to be detected in the ROSAT All-Sky Survey. Thus, ABRIXAS will have a pathfinder role for the large X-ray missions like the American AXAF, the European XMM, and Japanese ASTRO-E.

With its comparatively high spectral and spatial resolution ABRIXAS will carry out detailed spectroscopy of diffuse large-scale sources which are too extended for the relatively small field of view of pointed X-ray telescopes. These investigations will complement our knowledge of nearby supernova remnants, the diffuse emission region of the galactic ridge, and cluster of galaxies from the ROSAT survey to study the complex states of hot interstellar and intergalactic plasmas.

In additon, ABRIXAS will obtain the spatial distribution and high-quality spectra of the diffuse extragalactic background, which can be studied in detail after the identification of the thermal galactic contributions through their line emission. It will allow to search for fluctuations generated by the large-scale structure of the universe and for features of the red shifted iron line expected from superposition of many active galactic nuclei.

After all, since ABRIXAS's compound eye telescope scans the sky in a regular pattern the time variability of bright X-ray sources can be monitored in a wide range between seconds and years.

The joint scientific satellite project ABRIXAS of the Astrophysikalisches Institut Potsdam (AIP), the Max-Planck-Institut für extraterrestrische Physik (MPE), and the Institut für Astronomie and Astrophysik Tübingen (IAAT) aims at to survey the whole sky similar like ROSAT by scanning it in great circles. The three-years survey will provide a full sky coverage in the energy band between 0.4 and 12 keV with an angular resolution of better than 1 arcmin. Its sensitivity in the energy band around 1 keV is comparable to that of ROSAT.

The optical system consists of seven identical Wolter I telescopes of 160  cm focal length, each with 27 nested mirror shells of 300 mm lengths and diameters ranging from 163 mm to 76 mm. All seven focal planes share one common 6 cm × 6 cm XMM pn-CCD array. Consequently, their optical axes diverge and their fields of view, each about 40' in diameter, are ~7° apart of each other and form a hexagonal pattern on the sky. The shift of 4' in ecliptic longitude between successive orbits provides a contiguous coverage of the sky.

The imaging pn-CCD detector used for ABRIXAS including electronics is identical with the EPIC-Maxi detector developed by MPE/IAAT for XMM. However, because ABRIXAS (in contrast to XMM) is circulating the earth in a low orbit the CCD chip has to be operated at a somewhat higher temperature. Accordingly, the lowest energy will be between 0.3 keV and 0.5 keV. Unique features of the detector are its high sensitivity (95% at 10 keV) and its ~70 ms time resolution in the full frame parallel readout. This high time resolution (70 ms correspond to a scan path of 17") is important to avoid source smearing along the scan direction.

The small and low cost satellite with a mass of 460 kg, which will be launched by a KOSMOS rocket purchased from Polyot, is devloped and built by Orbital- und Hydrotechnologie Bremen (OHB) with the Zentrum für angewandte Raumfahrttechnologie und Mikrogravitation (ZARM) as the subcontractor for the attitude measurement and control system. The X-ray mirrors are manufactured by Carl Zeiss and tested in MPE's PANTER facility. The imaging CCD detector is developed and built at the MPI/MPE Semiconductor Laboratory (HLL) with parts of the readout electronics being provided by IAAT.

The overall project management is subject to the German Aerospace Center (DLR) as the successor of DARA and the ground station will be operated by its Space Operations Center (DLR/GSOC).

ABRIXAS is expected to be launched with a KOSMOS rocket in April 1999 from the Russian Kapustyn Yar located about 150 km east of Volgograd.