An extreme EXO: a type 2 QSO at z=1.87

TL;DR

This paper investigates an extreme X-ray-to-optical flux ratio source, revealing a complex AGN with both obscured and broad emission features, challenging simple dust and gas absorption models.

Contribution

It provides a detailed multi-wavelength analysis of a unique high flux ratio AGN, combining optical, infrared, and X-ray data to constrain its properties.

Findings

The source is a bright X-ray emitter with no optical detection down to i' > 25.2.

Spectroscopy shows a broad H-alpha line at z=1.87, indicating a type 1 AGN.

X-ray data reveals significant absorption, but optical/IR modeling suggests less dust extinction than expected.

Abstract

We aim to understand the multi-wavelength properties of 2XMM J123204+215255, the source with the most extreme X-ray-to-optical flux ratio amongst a sample of bright X-ray selected EXOs drawn from a cross-correlation of the 2XMMp catalogue with the SDSS-DR5 catalogue. We use 2XMMp X-ray data, SDSS-DR5, NOT and UKIRT optical/NIR photometric data and Subaru MOIRCS IR spectroscopy to study the properties of 2XMM J123204+215255. We created a model SED including an obscured QSO and the host galaxy component to constrain the optical/IR extinction and the relative contribution of the AGN and the galaxy to the total emission. 2XMM J123204+215255 is a bright X-ray source with f_X~10^{-12} erg cm^{-2} s^{-1} (2-10 keV energy band) which has no detection down to a magnitude i' > 25.2. NIR imaging reveals a faint K-band counterpart and NIR spectroscopy shows a single broad (FWHM=5300 km/s) emission line, which is almost certainly H-alpha at z=1.87. The X-ray spectrum shows evidence of significant absorption (N_H > 10^{23} cm^{-2}), typical of type 2 AGN, but the broad H-alpha emission suggests a type 1 AGN classification. The very red optical/NIR colours (i'-K > 5.3) strongly suggest significant reddening however. We find that simple modelling can successfully reproduce the NIR continuum and strongly constrain the intrinsic nuclear optical/IR extinction to A_V~4, which turns out to be much smaller than the expected from the X-ray absorption (assuming Galactic gas-to-dust ratio).