X-ray colour-colour selection for heavily absorbed active galactic nuclei

We present a method for the identification of heavily absorbed active galactic nuclei (AGN) (N_H > 10²³ cm^-2) from X-ray photometric data. We do this using a set of XMM-Newton reference spectra of local galaxies for which we have accurate N_H information, as described in Brightman & Nandra. The technique uses two rest-frame hardness ratios which are optimized for this task, which we designate HR1 (2-4/1-2 keV) and HR2 (4-16/2-4 keV). The selection method exploits the fact that while obscured AGN appear hard in HR2 due to absorption of the intrinsic source flux below ∼4 keV, they appear soft in HR1 due to excess emission originating from scattered source light, thermal emission or host galaxy emission. Such emission is ubiquitous in low-redshift samples. The technique offers a very simple and straightforward way of estimating the fraction of obscured AGN in samples with relatively low signal-to-noise ratio in the X-ray band. We apply this technique to a moderate redshift (z∼ 1) sample of AGN from the Chandra Deep Field-North, finding that 61 per cent of this sample has N_H > 10²³ cm^-2. A clear and robust conclusion from our analysis is that in deep surveys the vast majority of sources do not show hardness ratios consistent with a simple absorbed power law. The ubiquity of complex spectra in turn shows that simple hardness ratio analysis will not yield reliable obscuration estimates, justifying the more complex colour-colour analysis described in this paper. While this method does very well at separating sources with N_H > 10²³ cm^-2 from sources with lower N_H, only X-ray spectroscopy can identify Compton-thick sources, through the detection of the Fe Kα line. This will be made possible with the high throughput X-ray spectral capabilities of Athena.