X-Ray Redshifts for Obscured AGN: A Case Study in the J1030 Deep Field

We present a procedure to constrain the redshifts of obscured ( ${N}_{H}\gt {10}^{22}\,{\mathrm{cm}}^{-2}$ ) 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, ${\rm{HR}}\gt -0.1$ , 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 ${\rm{K}}\alpha $ 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 ( $0.1\lesssim z\lesssim 4$ ), we derived absorbing column densities in the range $\sim {10}^{22}\mbox{--}{10}^{24}\,{\mathrm{cm}}^{-2}$ and absorption-corrected, 2-10 keV rest-frame luminosities between $\sim {10}^{42}$ and 10⁴⁵ erg s^-1, with median values of ${N}_{H}=1.7\times {10}^{23}\,{\mathrm{cm}}^{-2}$ and ${L}_{2-10\mathrm{keV}}=8.3\times {10}^{43}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ , 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.