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The Chandra Multiwavelength Project: Optical Followup of Serendipitous Chandra Sources

We present followup optical g', r', and i', imaging and spectroscopy of serendipitous X-ray sources detected in 6 archival Chandra, images included in the Chandra, Multiwavelength Project (ChaMP). Of the 486 X-ray sources detected between 3e-16 and 2e-13 (with a median flux of 3e-15 erg cm-2 s-1, we find optical counterparts for 377 (78%), or 335 (68%) counting only unique counterparts. We present spectroscopic classifications for 125 objects, representing 75% of sources with r<21 optical counterparts (63% to r=22). Of all classified objects, 63 (50%) are broad line AGN, which tend to be blue in g-r colors. X-ray information efficiently segregates these quasars from stars, which otherwise strongly overlap in these SDSS colors until z>3.5. We identify 28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%) are absorption line galaxies. Eight galaxies lacking broad line emission have X-ray luminosities that require they host an AGN (logL_X>43). Half of these have hard X-ray emission suggesting that high gas columns obscure both the X-ray continuum and the broad emission line regions. We find objects in our sample that show signs of X-ray or optical absorption, or both, but with no strong evidence that these properties are coupled. ChaMP's deep X-ray and optical imaging enable multiband selection of small and/or high-redshift groups and clusters. In these 6 fields we have discovered 3 new clusters of galaxies, two with z>0.4, and one with photometric evidence that it is at a similar redshift.