  Differential triangular flow, $v_3(p_T)$, of negative pions is measured at
$\sqrt{s_{NN}}$= 17.3 GeV around midrapidity by the CERES/NA45 experiment at
CERN in central PbAu collisions in the range 0-30\% with a mean centrality of
5.5\%. This is the first measurement as a function of transverse momentum of
the triangular flow at SPS energies. The $p_T$ range extends from about 0.05
GeV/c to more than 2 GeV/c. The triangular flow magnitude, corrected for the
HBT effects, is smaller by a factor of about 2 than the one measured by the
PHENIX experiment at RHIC and the ALICE experiment at the LHC. Within the
analyzed range of central collisions no significant centrality dependence is
observed. The data are found to be well described by a viscous hydrodynamic
calculation combined with an UrQMD cascade model for the late stages.
