A Runaway Black Hole in COSMOS: Gravitational Wave or Slingshot Recoil?
  We present a detailed study of a peculiar source in the COSMOS survey at
z=0.359. Source CXOCJ100043.1+020637 (CID-42) presents two compact optical
sources embedded in the same galaxy. The distance between the 2, measured in
the HST/ACS image, is 0.495" that, at the redshift of the source, corresponds
to a projected separation of 2.46 kpc. A large (~1200 km/s) velocity offset
between the narrow and broad components of Hbeta has been measured in three
different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments.
CID-42 is also the only X-ray source having in its X-ray spectra a strong
redshifted broad absorption iron line, and an iron emission line, drawing an
inverted P-Cygni profile. The Chandra and XMM data show that the absorption
line is variable in energy by 500 eV over 4 years and that the absorber has to
be highly ionized, in order not to leave a signature in the soft X-ray
spectrum. That these features occur in the same source is unlikely to be a
coincidence. We envisage two possible explanations: (1) a gravitational wave
recoiling black hole (BH), caught 1-10 Myr after merging, (2) a Type 1/ Type 2
system in the same galaxy where the Type 1 is recoiling due to slingshot effect
produced by a triple BH system. The first possibility gives us a candidate
gravitational waves recoiling BH with both spectroscopic and imaging
signatures. In the second case, the X-ray absorption line can be explained as a
BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one
(a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us
the first opportunity to show that fast winds are present in obscured AGN.
