On why the iron K-shell absorption in AGN is not a signature of the local warm/hot intergalactic medium

James Reeves,Martin Turner,Yuichi Terashima,Chris Done,Ken Pounds,Masahiro Uchino,Kiyoshi Hayashida,Naohisa Anabuki

doi:10.1111/j.1745-3933.2008.00443.x

Abstract

Abstract We present a comparison between the 2001 XMM–Newton and 2005 Suzaku observations of the quasar, PG 1211+143, at z = 0.0809. Variability is observed in the 7 keV iron K-shell absorption line (at 7.6 keV in the quasar frame), which is significantly weaker in 2005 than during the 2001 XMM–Newton observation. From a recombination time-scale of &lt;4 yr, this implies an absorber density n &gt; 4 × 103 cm−3, while the absorber column is 5 × 1022 &lt; NH &lt; 1 × 1024 cm−2. Thus, the size scale of the absorber is too compact (pc scale) and the surface brightness of the dense gas too high (by 9–10 orders of magnitude) to arise from local hot gas, such as the local bubble, group or warm/hot intergalactic medium (WHIM), as suggested by McKernan, Yaqoob & Reynolds. Instead, the iron K-shell absorption must be associated with an active galactic nucleus (AGN) outflow with mildly relativistic velocities. Finally, we show that the association of the absorption in PG 1211+143 with local hot gas is simply a coincidence, and the comparison between the recession and iron K absorber outflow velocities in other AGN does not reveal a one-to-one kinematic correlation.

Highlights

The cosmological requirement that half the baryons in the Universe are in a warm/hot intergalactic medium (WHIM) has motivated ultraviolet (UV) and X-ray absorption-line studies to detect this otherwise invisible gas, using absorption against a bright active galactic nucleus (AGN) to probe the line of sight material (Bregman 2007)
We show that the observed variability in the absorption line in PG 1211+143 between the XMM–Newton and Suzaku data taken 4 yr apart conclusively rules out a diffuse gas origin such as the local Galactic halo or WHIM
The gas must be associated with the AGN, as is further evidenced by its large column density in the XMM– Newton 2001 data which are far too high for local or intergalactic gas

Summary

INTRODUCTION

The cosmological requirement that half the baryons in the Universe are in a warm/hot intergalactic medium (WHIM) has motivated ultraviolet (UV) and X-ray absorption-line studies to detect this otherwise invisible gas, using absorption against a bright active galactic nucleus (AGN) to probe the line of sight material (Bregman 2007). Kaspi & Behar (2006) identified the S and Mg K-shell lines with weaker He-like Kβ lines (with little velocity offset), instead of the blueshifted H-like Kα transitions, which begs the question as to why the corresponding stronger Kα lines are not seen These authors model the Fe K absorption in the 2001 XMM–Newton data with an edge at 7.27 keV (rest frame) of optical depth τ ∼ 0.5, corresponding to a large column (NH > 1023 cm−2) of very low ionization state matter (Fe VII/VIII); this should have substantial low energy absorption which is clearly not present in the data. This requires a large blueshift (0.08c or 0.14c, respectively) if the absorption is intrinsic to the AGN, with the implication being that the kinetic energy associated with this material can dominate the AGN energetics (Pounds & Reeves 2007)

Suzaku analysis

XMM–Newton analysis

A COMPARISON BETWEEN SUZAKU AND X M M – NEWTONOBSE RVAT I O N S

THE ORIGIN OF THE IRON K-SHELL ABSORPTION

Findings

CONCLUSIONS