Title: 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
survey running in parallel to the eBOSS cosmology project. SPIDERS will obtain
optical spectroscopy for large numbers of X-ray-selected AGN and galaxy cluster
members detected in wide area eROSITA, XMM-Newton and ROSAT surveys. We
describe the methods used to choose spectroscopic targets for two
sub-programmes of SPIDERS: X-ray selected AGN candidates detected in the ROSAT
All Sky and the XMM-Newton Slew surveys. We have exploited a Bayesian
cross-matching algorithm, guided by priors based on mid-IR colour-magnitude
information from the WISE survey, to select the most probable optical
counterpart to each X-ray detection. We empirically demonstrate the high
fidelity of our counterpart selection method using a reference sample of bright
well-localised X-ray sources collated from XMM-Newton, Chandra and Swift-XRT
serendipitous catalogues, and also by examining blank-sky locations. We
describe the down-selection steps which resulted in the final set of
SPIDERS-AGN targets put forward for spectroscopy within the eBOSS/TDSS/SPIDERS
survey, and present catalogues of these targets. We also present catalogues of
~12000 ROSAT and ~1500 XMM-Newton Slew survey sources which have existing
optical spectroscopy from SDSS-DR12, including the results of our visual
inspections. On completion of the SPIDERS program, we expect to have collected
homogeneous spectroscopic redshift information over a footprint of ~7500
deg$^2$ for >85 percent of the ROSAT and XMM-Newton Slew survey sources having
optical counterparts in the magnitude range 17<r<22.5, producing a large and
highly complete sample of bright X-ray-selected AGN suitable for statistical
studies of AGN evolution and clustering.