On why the Iron K-shell absorption in AGN is not a signature of the local Warm/Hot Intergalactic Medium

TL;DR

This study compares X-ray observations of quasar PG1211+143 to demonstrate that iron K-shell absorption features are caused by AGN outflows, not local intergalactic hot gas, challenging previous interpretations.

Contribution

It provides evidence that iron K-shell absorption in AGN is due to outflows rather than local hot intergalactic medium, based on variability and physical constraints.

Findings

Absorption variability indicates a compact, dense absorber.

Absorber properties are inconsistent with local hot gas origins.

Absorption is linked to AGN outflows, not the local intergalactic medium.

Abstract

We present a comparison between the 2001 XMM-Newton and 2005 Suzaku observations of the quasar, PG1211+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 timescale of <4 years, this implies an absorber density n>0.004 particles/cm3, while the absorber column is 5e22<N_H <1 1e24 particles/cm2. Thus the sizescale 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 et al. (2004, 2005). Instead the iron K-shell absorption must be associated with an AGN outflow with mildly relativistic velocities. Finally we show that the the association of the absorption in PG1211+143 with local hot gas is simply a coincidence, the comparison between the recession and iron K absorber outflow velocities in other AGN does not reveal a one to one kinematic correlation.