SDSS 0956+5128: A Broad-line Quasar with Extreme Velocity Offsets
  We report on the discovery of a Type 1 quasar, SDSS 0956+5128, with a
surprising combination of extreme velocity offsets. SDSS 0956+5128 is a
broad-lined quasar exhibiting emission lines at three substantially different
redshifts: a systemic redshift of z ~ 0.714 based on narrow emission lines, a
broad MgII emission line centered 1200 km/s bluer than the systemic velocity,
at z ~ 0.707, and broad H\alpha and H\beta emission lines centered at z ~
0.690. The Balmer line peaks are 4100 km/s bluer than the systemic redshift.
There are no previously known objects with such an extreme difference between
broad MgII and broad Balmer emission. The two most promising explanations are
either an extreme disk emitter or a high-velocity black hole recoil. However,
neither explanation appears able to explain all of the observed features of
SDSS 0956+5128, so the object may provide a challenge to our general
understanding of quasar physics.
