2002A&A...393..793S

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 ∼10⁶ 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 10⁴⁴erg/s, 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>1) if they have cluster-like gaseous coronae, as suggested by Rosat and Chandra observations of active galaxies at z≲1. Their surface brightness profiles in the X-ray continuum above ∼10 keV G 266.2-1.2kT/(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.