Does the X-ray emission of the luminous quasar RBS 1124 originate in a
  mildly relativistic outflowing corona?
  We have observed the luminous (L_x ~ 6x10^44 erg/s) radio-quiet quasar RBS
1124 (z=0.208) with Suzaku. We report the detection of a moderately broad iron
(Fe) line and of a weak soft X-ray excess. The X-ray data are very well
described by a simple model comprising a power law X-ray continuum plus its
reflection off the accretion disc. If the inner disc radius we measure (<=3.8
gravitational radii) is identified with the innermost stable circular orbit of
the black hole spacetime, we infer that the black hole powering RBS 1124 is
rotating rapidly with spin a>= 0.6. The soft excess contribution in the 0.5-2
keV band is ~15%, about half than that typically observed in unobscured Seyfert
1 galaxies and quasars, in line with the low disc reflection fraction we
measure (R_disc ~0.4). The low reflection fraction cannot be driven by disc
truncation which is at odds not only with the small inner disc radius we infer
but, most importantly, with the radiatively efficient nature of the source
(L_Bol / L_Edd ~1). A plausible explanation is that the X-ray corona is the
base of a failed jet (RBS 1124 being radio-quiet) and actually outflowing at
mildly relativistic speeds. Aberration reduces the irradiation of the disc,
thus forcing a lower than standard reflection fraction, and halves the inferred
source intrinsic luminosity, reducing the derived Eddington ratio from ~1to
~0.5. [abridged]
