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Astron. Astrophys. 338, 843-855 (1998)

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X-ray observations of the rich cluster CL 0939+4713 and discovery of the strongly variable source RXJ0943.0+4701

Sabine Schindler 1, 2, 3, Paola Belloni 4, Yasushi Ikebe 1, Makoto Hattori 5, Joachim Wambsganss 6 and Yasuo Tanaka 1, 7

1 MPI für Extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany (e-mail: sas@staru1.livjm.ac.uk)
2 MPI für Astrophysik, Karl-Schwarzschild-Strasse 1, D-85748 Garching, Germany
3 Astrophysics Research Institute, Liverpool John-Moores University, Byrom Street, Liverpool L3 3AF, UK
4 Universitätssternwarte München, Scheinerstrasse 1, D-81689 München, Germany
5 Astronomical Institute, Tôhoku University, Aoba Aramaki, Sendai 980, Japan (e-mail: hattori@astroa.astr.tohoku.ac.jp)
6 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany (e-mail: jwambsganss@aip.de)
7 Institute of Space and Astronautical Science, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229, Japan

Received 17 February 1998 / Accepted 29 July 1998

Abstract

Recent X-ray observations of the rich galaxy cluster CL 0939+4713 (or Abell 851) at [FORMULA] with the ROSAT/HRI as well as with the ASCA/GIS and ASCA/SIS instruments are presented and analysed. With the high resolution imaging data (ROSAT/HRI) we confirm and extend the earlier impression that the cluster has a lot of substructure. Two maxima of the cluster emission are obvious in the images, each of them shows even some internal structure. One of the subclusters can be nicely modeled with an elliptical model. For the total luminosity of the cluster in the ROSAT band we obtain [FORMULA] erg/s, for the bolometric luminosity [FORMULA] erg/s. We perform spectral fits for the two ASCA instruments and for the ROSAT/PSPC simultaneously. The most reliable numbers for the temperature and metallicity for the intracluster gas are [FORMULA] keV and [FORMULA], respectively. We find a relatively small total mass, a small gas mass ratio and a small iron mass to light ratio. These numbers together with relatively low luminosity for such an optically rich cluster and the pronounced substructure confirm that CL 0939+4713 is a young cluster still far away from a virialised state.

In the same ROSAT/HRI image the X-ray emission of a background quasar ([FORMULA]) can clearly be identified. With a luminosity of [FORMULA] erg/s (which is necessarily affected by gravitational lensing) it belongs to the X-ray brightest quasars. A striking difference between the ROSAT/PSPC and the ROSAT/HRI observations (taken almost five years apart) is the fainting of one X-ray source (RXJ0943.0+4701) by at least a factor of ten. We try to identify this source from deep optical data. Our best candidate is a blue compact object, possibly an AGN.

Key words: galaxies: clusters: general – galaxies: clusters: individual: CL0939+4713 – galaxies: clusters: individual: Abell 851 – intergalactic medium – X-rays: galaxies

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998
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