  The Fourier harmonics, $v_2$ and $v_3$ of negative pions are measured at
center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}$= 17.3 GeV
around midrapidity by the CERES/NA45 experiment at the CERN SPS in 0--30\%
central PbAu collisions with a mean centrality of 5.5\%. The analysis is
performed in two centrality bins as a function of the transverse momentum
$\mathrm{p_{\mathrm{T}}}$ from 0.05 GeV/$c$ to more than 2 GeV/$c$. This is the
first measurement of the $v^{1/3}_{3}/v^{1/2}_{2}$ ratio as a function of
transverse momentum at SPS energies, that reveals, independently of the
hydrodynamic models, hydrodynamic behavior of the formed system. For
$\mathrm{p_{\mathrm{T}}}$ above 0.5 GeV/$c$, the ratio is nearly flat in
accordance with the hydrodynamic prediction and as previously observed by the
ATLAS and ALICE experiments at the much higher LHC energies. The results are
also compared with the SMASH-vHLLE hybrid model predictions.
