Astronomy and Astrophysics Constraining the past X-ray Luminosity of Agn in Clusters of Galaxies: the Role of Resonant Scattering

Your thesaurus codes are: missing; you have not inserted them Abstract. 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 ∼ 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 M87 and Cygnus A active galaxies. For instance , scattered line radiation should be detectable from the Virgo cluster if the X-ray luminosity of M87 was a few times 10 44 erg s −1 , or 10 −3 of its Eddington lumi-nosity, until a few times 10 5 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 Chan-dra observations of active galaxies at z ∼ < 1. Their surface brightness profiles in the X-ray continuum above ∼ 10 keV ≫ 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.