On the nature of the absorber in IRAS 09104+4109: the X-ray and mid-infrared view

TL;DR

This study uses X-ray and mid-infrared observations to investigate the obscuration and nature of the active nucleus in IRAS 09104+4109, revealing it is heavily obscured but not Compton-thick, with contamination from a nearby AGN.

Contribution

The paper provides a comprehensive multi-wavelength analysis clarifying the obscuration level of IRAS 09104+4109, challenging previous claims of its Compton-thick nature.

Findings

IRAS 09104+4109 is heavily obscured but not Compton-thick.

Contamination from a nearby AGN affects earlier X-ray measurements.

Mid-IR and optical data suggest different levels of obscuration.

Abstract

We present a long (~76 ks) Chandra observation of IRAS 09104+4109, a hyper-luminous galaxy, optically classified as a Type 2 AGN hosted in a cD galaxy in a cluster at z=0.442. We also report on the results obtained by fitting its broad-band spectral energy distribution. The Compton-thick nature of this source (which has been often referred to as an "archetype" of Compton-thick Type 2 quasars) was formerly claimed on the basis of its marginal detection in the PDS instrument onboard BeppoSAX, being then disputed using XMM-Newton data. Both Chandra analysis and optical/mid-IR spectral fitting are consistent with the presence of heavy (~1-5 10^{23} cm^{-2}) but not extreme (Compton-thick) obscuration. However, using the mid-IR and the [OIII] emission as proxies of the nuclear hard X-ray luminosity suggests the presence of heavier obscuration. The 54-month Swift BAT map shows excess hard X-ray emission likely related to a nearby (z=0.009) Type 2 AGN, close enough to IRAS 09104+4109 to significantly enhance and contaminate its emission in the early BeppoSAX PDS data.