SPICES II. Optical and Near-Infrared Identifications of Faint X-Ray   Sources from Deep Chandra Observations of Lynx
  We present our first results on field X-ray sources detected in a deep, 184.7 ks observation with the ACIS-I camera on Chandra. The observations target the Lynx field of SPICES, and contains three known X-ray-emitting clusters out to z=1.27. Not including the known clusters, in the 17'x17' ACIS-I field we detect 132 sources in the 0.5-2 keV (soft) X-ray band down to a limiting flux of \~1.7e-16 erg/cm2/s and 111 sources in the 2-10 keV (hard) X-ray band down to a limiting flux of ~1.3e-15 erg/cm2/s. The combined catalog contains a total of 153 sources, of which 42 are detected only in the soft band and 21 are detected only in the hard band. Confirming previous Chandra results, we find that the fainter sources have harder X-ray spectra, providing a consistent solution to the long-standing `spectral paradox'. From deep optical and near-infrared follow-up data, 77% of the X-ray sources have optical counterparts to I=24 and 71% of the X-ray sources have near-infrared counterparts to K=20. Four of the 24 sources in the near-IR field are associated with extremely red objects (EROs; I-K>4). We have obtained spectroscopic redshifts with the Keck telescopes of 18 of the Lynx Chandra sources. These sources comprise a mix of broad-lined active galaxies, apparently normal galaxies, and two late-type Galactic dwarfs. Intriguingly, one Galactic source is identified with an M7 dwarf exhibiting non-transient, hard X-ray emission. We review non-AGN mechanisms to produce X-ray emission and discuss properties of the Lynx Chandra sample in relation to other samples of X-ray and non-X-ray sources. 
Daniel Stern (1), Paolo Tozzi (2), S.A. Stanford (3), Piero Rosati   (4), Brad Holden (3), Peter Eisenhardt (1), Richard Els...