  The $\Lambda$ ($\bar{\Lambda}$) hyperon polarization along the beam direction
has been measured for the first time in Au+Au collisions at $\sqrt{s_{_{NN}}}$
= 200 GeV. The polarization dependence on the hyperons' emission angle relative
to the second-order event plane exhibits a sine modulation, indicating a
quadrupole pattern of the vorticity component along the beam direction. The
polarization is found to increase in more peripheral collisions, and shows no
strong transverse momentum ($p_T$) dependence at $p_T>1$ GeV/$c$. The magnitude
of the signal is about five times smaller than those predicted by hydrodynamic
and multiphase transport models; the observed phase of the emission angle
dependence is also opposite to these model predictions. In contrast, blast-wave
model calculations reproduce the modulation phase measured in the data and
capture the centrality and transverse momentum dependence of the signal once
the model is required to reproduce the azimuthal dependence of the Gaussian
source radii measured via the Hanbury-Brown and Twiss intensity interferometry
technique.
