XZ: Deriving redshifts from X-ray spectra of obscured AGN

Context. Redshifts are fundamental for our understanding of extragalactic X-ray sources. Ambiguous counterpart associations, expensive optical spectroscopy, and/or multimission multiwavelength coverage to resolve degeneracies often make estimation difficult in practice. Aims. We attempt to constrain redshifts of obscured active galactic nuclei (AGN) using only low-resolution X-ray spectra. Methods. Our method for determining redshifts from the X-ray spectrum (XZ) fits AGN X-ray spectra with a moderately complex spectral model incorporating a corona, a torus obscurer, and a warm mirror. Using the Bayesian X-ray Astronomy (BXA) package, we constrain redshift, column density, photon index, and luminosity simultaneously. The redshift information primarily comes from absorption edges in Compton-thin AGN, and from the Fe K alpha fluorescent line in heavily obscured AGN. A new generic background fitting method allows us to extract more information from limited numbers of source counts. Results. We derive redshift constraints for 74/321 hard-band detected sources in the Chandra deep field South. Comparing with spectroscopic redshifts, we find an outlier fraction of 8%, indicating that our model assumptions are valid. For three Chandra deep fields, we release our XZ redshift estimates. Conclusions. The independent XZ estimate is easy to apply and effective for a large fraction of obscured AGN in today's deep surveys without the need for any additional data. Compared to different redshift estimation methods, XZ can resolve degeneracies in photometric redshifts, help detect potential association problems, and confirm uncertain single-line spectroscopic redshifts. With high spectral resolution and a large collecting area, this technique will be highly effective for Athena/WFI observations.