The SWIRE ELAIS N1 field was imaged by the IRAC multiband camera on Spitzer in 2004 January and with MIPS in early 2004 February. The observations were centered at the position (16h 00m, +59d 01'). The X-ray observations were taken as part of the ELAIS Deep X-ray Survey (EDXS) and are described in detail in Manners et al. (2003, MNRAS, 343, 293). For this analysis, the Chandra Advanced CCD Imaging Spectrometer (ACIS) observation of 75 ks centered on (16h 10m 20.11s, +54d 33' 22.3") (J2000.0) in the ELAIS N1 region. The aim point was focused on the ACIS-I chips, which consist of four CCDs arranged in a 2 x 2 array covering an area of 16.9' x 16.9' (286 square arcmin). Bad pixels and columns were removed, and data were filtered to eliminate high background times (due to strong solar flares), leaving 71.5 ks of good data after filtering. Counts-to-photon calibration assumed a standard power-law model spectrum with photon index Gamma = 1.7. Sources were detected to flux levels of 2.3 x 10^-15 erg s^-1 cm^-2 in the 0.5 - 8 keV band, 9.4 x 10^-16 erg s^-1 cm^-2 in the 0.5 - 2 keV band, and 5.2 x 10^-15 erg s^-1 cm^-2 in the 2 - 8 keV band. Sources are detectable to these flux limits over 90% of the nominal survey area. For this analysis, the authors used sources detected in the full band of ACIS-I only, of which there are 122 in the N1 region. Of the 102 sources in common between Chandra and SWIRE, 83 have significant detections in the separate soft X-ray band (0.5 - 2 keV) and 64 are detected in the hard (2 - 8 keV) band.
A simple near-neighbor search was performed to cross-correlate the Spitzer and Chandra source catalogs within the Chandra ACIS-I chip image, using a d = 5" search radius (roughly the quadratic sum of the astrometric errors). All together, the authors found reliably associated counterparts for 102 of the 122 Chandra sources (84% in total). The vast majority of these were detected with the IRAC channels 1 and 2 (3.6 and 4.5 um): 100 of the 122 Chandra sources in each case. As many as 59 Chandra objects are reliably associated with MIPS 24 um sources (all of them having IRAC counterparts), and just 1 had a MIPS 70 um counterpart. Of the 102 Spitzer-identified Chandra sources, three turned out to correspond to Galactic stars on the basis of their position on color-magnitude plots and optical morphology and were excluded from the subsequent analysis (and this table).
A complete multiwavelength characterization of faint Chandra X-ray sources seen in the Spitzer wide-area infrared extragalactic (SWIRE) survey. Franceschini A., Manners J., Polletta M.D.C., Lonsdale C., Gonzalez-Solares E., Surace J., Shupe D., Fang F., Xu C.K., Farrah D., Berta S., Rodighiero G., Perez-Fournon I., Hatziminaoglou E., Smith H.E., Siana B., Rowan-Robinson M., Nandra K., Babbedge T., Vaccari M., Oliver S., Wilkes B., Owen F., Padgett D., Frayer D., Jarrett T., Masci F., Stacey G., Almaini O., McMahon R., Johnson O., Lawrence A., Willott C. <Astron. J., 129, 2074-2101 (2005)> =2005AJ....129.2074F
1 = type-1 AGN; 2 = type-2 AGN; 3 = galaxy; 4 = early-type galaxy.
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