EDP Sciences
Free Access
Volume 397, Number 2, January II 2003
Page(s) 445 - 462
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20021514
Published online 17 December 2002

A&A 397, 445-462 (2003)
DOI: 10.1051/0004-6361:20021514

XMM-Newton confirmation of soft X-ray excess emission in clusters of galaxies - The discovery of O VII emission from an extended warm baryonic component

J. S. Kaastra1, R. Lieu2, T. Tamura1, F. B. S. Paerels3 and J. W. den Herder1

1  SRON National Institute for Space Research Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
2  Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899
3  Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA

(Received 3 June 2002 / Accepted 14 October 2002 )

We investigate a sample of 14 clusters of galaxies observed with XMM-Newton in a search for soft X-ray excess emission. In five of these clusters a significant soft excess is evident. This soft X-ray excess is compared with the thermal emission from both the hot intracluster gas and any cooling (flow) gas that may be present. A warm ( kT=0.2 keV), extended (several Mpc), plasma component is particularly clear in the outer parts of the cluster, where the normal cluster X-ray emission is weak. This warm component causes both a thermal soft X-ray excess at low energies (below 0.4-0.5 keV), as well as $\ion{O}{vii}$ line emission with a redshift consistent with a cluster origin, and not easily interpreted as Galactic foreground emission. The intensity of this component is commensurate with what has been measured before with the ROSAT PSPC in the 1/4 keV band. We attribute this component to emission from intercluster filaments of the Warm-Hot Intergalactic Medium in the vicinity of these clusters. For the central regions of clusters the detection of lines in the soft X-ray spectrum is more difficult, due to the predominance of the X-ray emitting hot plasma there, hence we cannot discriminate between the thermal and nonthermal origin of the soft excess, leaving several options open. These include thermal emission from warm filaments seen in projection in front of or behind the cluster center, thermal or nonthermal emission in the cluster core itself related to magnetic reconnection, or Inverse Compton emission from the cosmic microwave background on relativistic electrons.

Key words: galaxies: clusters: general -- galaxies: cooling flows -- X-rays: galaxies: clusters

Offprint request: J. S. Kaastra, J.Kaastra@sron.nl

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© ESO 2003

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