EDP Sciences
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Volume 393, Number 3, October III 2002
Page(s) 793 - 807
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20021033

A&A 393, 793-807 (2002)
DOI: 10.1051/0004-6361:20021033

Constraining the past X-ray luminosity of AGN in clusters of galaxies: The role of resonant scattering

S. Yu. Sazonov1, 2, R. A. Sunyaev1, 2 and C. K. Cramphorn1

1  Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany
2  Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

(Received 12 April 2002 / Accepted 28 June 2002 )

Only a small fraction of galactic nuclei in the nearby universe are luminous; most of them are currently dim. We investigate the feasibility of constraining the X-ray luminosity in the recent past (up to $\sim $10 6 years ago) of the nucleus of a cluster dominant galaxy by measuring the contribution of scattered radiation from the central source to the surface brightness of the intracluster gas dominated by thermal emission. We show that resonance X-ray lines present an advantage over the adjacent continuum, because the relative contribution of the scattered component is typically larger in the line case by a significant factor of 3-10. As an example, we estimate the level of constraint that could be derived from future fine spectroscopic observations on the past X-ray luminosity of the nearby M 87 and Cygnus A active galaxies. For instance, scattered line radiation should be detectable from the Virgo cluster if the X-ray luminosity of M 87 was a few times 1044 erg s -1, or 10-3 of its Eddington luminosity, until a few times 105 years ago. For comparison, upper limits inferred from the available XMM-Newton and Chandra X-ray continuum data are typically 1-2 orders of magnitude weaker.

The same method can be applied to distant powerful quasars (at redshifts $z\ga 1$) if they have cluster-like gaseous coronae, as suggested by Rosat and Chandra observations of active galaxies at $z\la
1$ . Their surface brightness profiles in the X-ray continuum above $\sim $10 keV $\gg kT/(1+z)$ (where T is the gas temperature) should be dominated by redshifted scattered radiation from the quasar. Therefore, measurements with forthcoming mirror-optics hard X-ray telescopes could give information on the lifetime of quasars and parameters of the hot gas around them.

Key words: galaxies: active -- galaxies: quasars: general -- scattering -- X-rays: galaxies: clusters

Offprint request: S. Yu. Sazonov, sazonov@mpa-garching.mpg.de

© ESO 2002