----- Attempting Submission 1 (Tue Jan 25 22:59:56 GMT 2011) ----- We ask for 80ks of HRC imaging observation to unambiguously resolve the X-ray emission, and unveil the nature, of two optical sources hosted by a galaxy in the COSMOS field (CID-42) and separated by only 0.495’’. One of the two sources is the best candidate to date for being a recoiling super-massive black hole (SMBH) with both spectroscopic and imaging signatures, in a recently merged system. CID-42 is a possible ‘‘Rosetta stone’’ for the study of SMBH mergers that are believed to occur during galaxy-galaxy mergers. Is CID-42 (1) a GW recoiling SMBH from a recent merger or (2) a slingshot recoiling SMBH in a triple SMBH system? HRC imaging will decide clearly.

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