X-ray redshifts with the International X-ray Observatory (IXO)

We explore the capabilities of the future space science mission IXO (International X-ray Observatory) for obtaining cosmological redshifts of distant Active Galactic Nuclei (AGNs) using the X-ray data only. We first find in which regions of the X-ray luminosity (L-X) versus redshift (z) plane the weak but ubiquitous Fe K alpha narrow emission line can deliver an accurate redshift (delta(Z) < 5%) as a function of exposure time, using a CCD-based Wide Field Imager (IXO/WFI) as the one baselined for IXO. Down to a 2-10 keV X-ray flux of 10(-14) erg cm(-2) s(-1) IXO/WFI exposures of 100 ks, 300 ks and 1 Ms will deliver 20%, 40% and 60% of the redshifts. This means that in a typical 18' x 18' IXO/WFI field of view, 4, 10 and 25 redshifts will be obtained for free from the X-ray data alone, spanning a wide range up to z 2-3 and fairly sampling the real distribution. Measuring redshifts of fainter sources will indeed need spectroscopy at other wavebands. (C) 2011 COSPAR. Published by Elsevier Ltd. All rights reserved.