The Chandra Multiwavelength Project: Optical Follow‐up of Serendipitous Chandra Sources

We present followup optical g', r', and i', imaging and spectroscopy ofserendipitous X-ray sources detected in 6 archival Chandra, images included inthe Chandra, Multiwavelength Project (ChaMP). Of the 486 X-ray sources detectedbetween 3e-16 and 2e-13 (with a median flux of 3e-15 erg cm-2 s-1, we findoptical counterparts for 377 (78%), or 335 (68%) counting only uniquecounterparts. We present spectroscopic classifications for 125 objects,representing 75% of sources with r<21 optical counterparts (63% to r=22). Ofall classified objects, 63 (50%) are broad line AGN, which tend to be blue ing-r colors. X-ray information efficiently segregates these quasars from stars,which otherwise strongly overlap in these SDSS colors until z>3.5. We identify28 sources (22%) as galaxies that show narrow emission lines, while 22 (18%)are absorption line galaxies. Eight galaxies lacking broad line emission haveX-ray luminosities that require they host an AGN (logL_X>43). Half of thesehave hard X-ray emission suggesting that high gas columns obscure both theX-ray continuum and the broad emission line regions. We find objects in oursample that show signs of X-ray or optical absorption, or both, but with nostrong evidence that these properties are coupled. ChaMP's deep X-ray andoptical imaging enable multiband selection of small and/or high-redshift groupsand 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 similarredshift.