  For a class of analytical parametrizations of the freeze-out state of
relativistic heavy ion collisions, we perform a simultaneous analysis of the
single-particle m_t-spectra and two-particle Bose-Einstein correlations
measured in central Pb+Pb collisions at the CERN SPS. The analysis includes a
full model parameter scan with chi^2 confidence levels. A comparison of
different transverse density profiles for the particle emission region allows
for a quantitative discussion of possible model dependencies of the results.
Our fit results suggest a low thermal freeze-out temperature T approximately 95
+- 15 MeV and a large average transverse flow velocity of about 0.55c +- 0.07c.
Moreover, the fit favours a box-shaped transverse density profile over a
Gaussian one. We discuss the origins and the consequences of these results in
detail. In order to reproduce the measured pion multiplicity our model requires
a positive pion chemical potential. A study of the pion phase-space density
indicates \mu_\pi approximately 60 MeV for T = 100 MeV.
