  We reexamine likelihood analyzes of the Local Group (LG) acceleration, paying
particular attention to nonlinear effects. Under the approximation that the
joint distribution of the LG acceleration and velocity is Gaussian, two
quantities describing nonlinear effects enter these analyzes. The first one is
the coherence function, i.e. the cross-correlation coefficient of the Fourier
modes of gravity and velocity fields. The second one is the ratio of velocity
power spectrum to gravity power spectrum. To date, in all analyzes of the LG
acceleration the second quantity was not accounted for. Extending our previous
work, we study both the coherence function and the ratio of the power spectra.
With the aid of numerical simulations we obtain expressions for the two as
functions of wavevector and \sigma_8. Adopting WMAP's best determination of
\sigma_8, we estimate the most likely value of the parameter \beta and its
errors. As the observed values of the LG velocity and gravity, we adopt
respectively a CMB-based estimate of the LG velocity, and Schmoldt et al.'s
(1999) estimate of the LG acceleration from the PSCz catalog. We obtain \beta =
0.66^{+0.21}_{-0.07}; thus our errorbars are significantly smaller than those
of Schmoldt et al. This is not surprising, because the coherence function they
used greatly overestimates actual decoherence between nonlinear gravity and
velocity.
