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

Abstract

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 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 10 44 erg s −1 , or 10 −3 of its Eddington luminosity, 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 Chandra 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.

Excess ionization and soft X-ray emission from cooling flow clusters

“…Let us first consider the effects of free–free emission from hot cluster gas. We estimate the internal radiation field of a cluster following Sazonov, Sunyaev & Cramphorn (2002), who did so to constrain resonant X‐ray scattering. Consider a point some distance r from the centre of a cluster.…”

“…It is also interesting to note that because F AGN ∝ r −2 and F crit ∝ n ∝ r −2 , outside the central density core the AGN flux is also always some constant multiple of the critical flux, similar to the free–free radiation case. The invocation of AGN to provide ionizing radiation rather than the ambient hot cluster gas is somewhat less desirable, because such sources have a short duty cycle and are not seen in all cooling flow clusters [although Thompson and resonant line scattering could trap their radiation for some time and boost the X‐ray surface brightness by a factor of ∼3–10 over the free–free continuum (Sazonov et al 2002)]. Furthermore, the universal scaling of the cut‐off temperature with the ambient plasma temperature is lost, although one could argue that F AGN ∝ M BH ∝σ α CD ∝ T α/2 cluster , where σ CD is the velocity dispersion of the CD galaxy and α is the slope of the M BH –σ relation.…”

Resonant Scattering of X-ray Emission Lines in the Hot Intergalactic Medium

“…The AGN luminosity was L X = 0.01 L Edd before switch-off. Adapted from Sazonov, Sunyaev, & Cramphorn (2002).…”

Scattering of emission lines in galaxy cluster cores: measuring electron temperature

“…However, subsequent works (see e.g., Dey et al 1996;Zakamska et al 2005) ruled out scattering by hot (Te 10 4 K) electrons, as constrained by the observed lines widths, instead preferring the scattering by relatively cool gas and dust. Sazonov et al (2002) considered resonant scattering of X-ray emission from a central AGN by the ambient intracluster gas, proposing it as a method to indicate the parameters of the surrounding hot gas. Also, Holder & Loeb (2004) proposed that joint observations of scattered radio emission along with the SZ effect could be used to probe the temperature profiles of clusters.…”