  We present Chandra High Resolution Camera observations of CID-42, a candidate
recoiling supermassive black hole (SMBH) at z=0.359 in the COSMOS survey.
CID-42 shows two optical compact sources resolved in the HST/ACS image embedded
in the same galaxy structure and a velocity offset of ~1300 km/s between the
H\beta\ broad and narrow emission line, as presented by Civano et al. (2010).
Two scenarios have been proposed to explain the properties of CID-42: a GW
recoiling SMBH and a double Type 1/ Type 2 AGN system, where one of the two is
recoiling because of slingshot effect. In both scenario, one of the optical
nuclei hosts an unobscured AGN, while the other one, either an obscured AGN or
a star forming compact region. The X-ray Chandra data allow to unambiguously
resolve the X-ray emission, and unveil the nature, of the two optical sources
in CID-42. We find that only one of the optical nuclei is responsible for the
whole X-ray unobscured emission observed and a 3sigma upper limit on the flux
of the second optical nucleus is measured. The upper limit on the X-ray
luminosity plus the analysis of the multiwavelength spectral energy
distribution indicate the presence of a starforming region in the second source
rather than an obscured SMBH, thus favoring the GW recoil scenario. However the
presence of a very obscured SMBH cannot be fully ruled-out. A new X-ray
feature, in a SW direction with respect to the main source, is discovered and
discussed.
