Full Table 1, full Fig. 2 and Appendix A are only available in electronic form at http://www.edpsciences.org

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http://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/w3browse.pl

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... site

http://www.merate.mi.astro.it:8081/interroga/dbServer?cmd=bmw2

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... LEDA

Several mirrors exist to access LEDA; we used the OAB, http://www.brera.mi.astro.it/hypercat/

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... galaxies

Two distinct sources but of different extent are associated with both NGC 1399 and M 86; we list them in Table 1, but we consider only the largest one for computing fluxes and luminosities.

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... archives

http://wave.xray.mpe.mpg.de/ and http://heasarc.gsfc.nasa.gov/

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... archive

http://archive.eso.org/dss/dss

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... (IRAF)

IRAF is a general purpose software system for the reduction and analysis of astronomical data. IRAF is written and supported by the IRAF programming group at the National Optical Astronomy Observatories (NOAO) in Tucson, Arizona. NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation.

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