  A data-driven method was applied to measurements of Au+Au collisions at
$\sqrt{s_{_{\rm NN}}} =$ 200 GeV made with the STAR detector at RHIC to isolate
pseudorapidity distance $\Delta\eta$-dependent and $\Delta\eta$-independent
correlations by using two- and four-particle azimuthal cumulant measurements.
We identified a component of the correlation that is $\Delta\eta$-independent,
which is likely dominated by anisotropic flow and flow fluctuations. It was
also found to be independent of $\eta$ within the measured range of
pseudorapidity $|\eta|<1$. The relative flow fluctuation was found to be $34\%
\pm 2\% (stat.) \pm 3\% (sys.)$ for particles of transverse momentum $p_{T}$
less than $2$ GeV/$c$. The $\Delta\eta$-dependent part may be attributed to
nonflow correlations, and is found to be $5\% \pm 2\% (sys.)$ relative to the
flow of the measured second harmonic cumulant at $|\Delta\eta| > 0.7$.
