SDSS J0909$+$4449: A Large-separation Strongly Lensed Quasar at $z \sim
  2.8$ with Three Images
  We report the discovery of SDSS J0909$+$4449, an exceptional system
consisting of a quasar at $z=2.788$ strongly lensed by a group of galaxies at
$z \sim 0.9$ into three images separated by up to 14\arcsec\, based on archival
data collected by the Sloan Digital Sky Survey, extended Baryon Oscillation
Spectroscopic Survey, Beijing--Arizona Sky Survey, the Mayall z-band Legacy
Survey, and the Gemini Telescope. We discuss two hypotheses on the nature of
SDSS J0909$+$4449, i.e. a rare triply-imaged quasar in the naked cusp
configuration and a typical quadruply-imaged quasar with the fourth image
undetected in the current data. We find that simple lens models can provide
excellent fits to the observed image positions and the non-detection under
either hypothesis. Deeper imaging data, spectroscopic observations, and
follow-up light curve measurements will be helpful in determining which
hypothesis is correct and provide better constraints on the lens mass
distribution. Nevertheless, given its unusually large image separations, SDSS
J0909$+$4449 will be a unique probe for the mass structure and the underlying
cooling and stellar feedback processes on group or cluster scales.
