SPIDERS: selection of spectroscopic targets using AGN candidates detected in all-sky X-ray surveys

Abstract

SPIDERS (SPectroscopic IDentification of eROSITA Sources) is an SDSS-IV\nsurvey running in parallel to the eBOSS cosmology project. SPIDERS will obtain\noptical spectroscopy for large numbers of X-ray-selected AGN and galaxy cluster\nmembers detected in wide area eROSITA, XMM-Newton and ROSAT surveys. We\ndescribe the methods used to choose spectroscopic targets for two\nsub-programmes of SPIDERS: X-ray selected AGN candidates detected in the ROSAT\nAll Sky and the XMM-Newton Slew surveys. We have exploited a Bayesian\ncross-matching algorithm, guided by priors based on mid-IR colour-magnitude\ninformation from the WISE survey, to select the most probable optical\ncounterpart to each X-ray detection. We empirically demonstrate the high\nfidelity of our counterpart selection method using a reference sample of bright\nwell-localised X-ray sources collated from XMM-Newton, Chandra and Swift-XRT\nserendipitous catalogues, and also by examining blank-sky locations. We\ndescribe the down-selection steps which resulted in the final set of\nSPIDERS-AGN targets put forward for spectroscopy within the eBOSS/TDSS/SPIDERS\nsurvey, and present catalogues of these targets. We also present catalogues of\n~12000 ROSAT and ~1500 XMM-Newton Slew survey sources which have existing\noptical spectroscopy from SDSS-DR12, including the results of our visual\ninspections. On completion of the SPIDERS program, we expect to have collected\nhomogeneous spectroscopic redshift information over a footprint of ~7500\ndeg$^2$ for >85 percent of the ROSAT and XMM-Newton Slew survey sources having\noptical counterparts in the magnitude range 17<r<22.5, producing a large and\nhighly complete sample of bright X-ray-selected AGN suitable for statistical\nstudies of AGN evolution and clustering.\n