A medium-deep Chandra and Subaru survey of the 13hr XMM/Rosat deep survey area

We present the results of a Chandra ACIS-I survey of a high-latitude region at 13 h +38degrees which was earlier observed with ROSAT and which has recently been observed by XMM-Newton for 200 ks. XMM-Newton will provide good-quality X-ray spectra for over 200 sources with fluxes around the knee of the log N / log S, which are responsible for the bulk of the X-ray background. The main aim of the Chandra observations is to provide arcsecond, or better, positions, and hence reliable identifications, for the XMM-Newton sources. The ACIS-I observations were arranged in a mosaic of four 30-ks pointings, covering almost all of the 15-arcmin radius XMM-Newton /ROSAT field. We detect 214 Chandra sources above a Cash likelihood statistic of 25, which approximates to 5sigma significance, to a limiting flux of similar to1.3 x 10(-15) erg cm(-2) s(-1) (0.5-7 keV). Optical counterparts are derived from a Subaru SuprimeCam image reaching to R similar to 27. The very large majority of the Chandra sources have an optical counterpart, with the distribution peaking at 23 <R <24, although 14 have no counterpart to R = 27. The fraction of X-ray sources with no identification brighter than R = 27 is similar to that found in deeper Chandra surveys. The majority of the identifications are with galaxies. As found in other Chandra surveys, there is a very wide range of optical magnitudes for a given X-ray flux, implying a range of emission mechanisms, and many sources have high L-X/L-opt ratios, implying absorption at moderate redshift. Comparison with the earlier ROSAT survey shows that the accuracy of the ROSAT positions agrees very well with the predictions from simulations by M-c Hardy et al. and that the large majority of the identifications were correct.