X-ray redshifts for obscured AGN: A case study in the J1030 deep field

We present a procedure to constrain the redshifts of obscured (NH > 1022 cm-2) active galactic nuclei (AGN) based on low count statistics X-ray spectra, which can be adopted when photometric and/or spectroscopic redshifts are unavailable or difficult to obtain. We selected a sample of 54 obscured AGN candidates on the basis of their X-ray hardness ratio, HR > -0.1, in the Chandra deep field (∼479 ks, 335 arcmin2) around the z = 6.3 QSO SDSS J1030 +0524. The sample has a median value of ≈80 net counts in the 0.5-7 keV energy band. We estimate reliable X-ray redshift solutions taking advantage of the main features in obscured AGN spectra, like the Fe 6.4 keV Ka emission line, the 7.1 keV Fe absorption edge, and the photoelectric absorption cutoff. The significance of such features is investigated through spectral simulations, and the derived X-ray redshift solutions are then compared with photometric redshifts. Both photometric and X-ray redshifts are derived for 33 sources. When multiple solutions are derived by any method, we find that combining the redshift solutions of the two techniques improves the rms by a factor of 2. Using our redshift estimates (0.1 ≲ z ≲ 4), we derived absorbing column densities in the range ~1022-1024 cm-2 and absorption-corrected, 2-10 keV rest-frame luminosities between ~1042 and 1045 erg s−1, with median values of NH = 1.7 ´ 1023 cm-2 and L2 -10 keV = 8.3 ´ 1043 erg s-1, respectively. Our results suggest that the adopted procedure can be applied to current and future X-ray surveys for sources detected only in X-rays or that have uncertain photometric or single-line spectroscopic redshifts.