letter. However, In [@Li:2008ge] we have found that the calculated $\lambda$ factor with UrQMD can be comparable with experimental data at RHIC energies (although somewhat larger than data). At SPS energies, $\lambda_{\rm
UrQMD}\approx 0.8-0.9$, is also compatible with experimental data which $\lambda_{\rm data}\approx 0.6-0.8$. The $R_{OL}^{2}$ represents the cross-term and plays a role at large rapidity. To fit the correlator with Eq. (\[fit1\]), we use ROOT [@roothomepage] software and minimize $\chi^2$.

HBT results
===========

![Transverse momentum $k_T$ dependence of the HBT radii $R_L$, $R_O$, and $R_S$ (at midrapidity) of $\pi^-$ source from central HICs at SPS energies ($E_{\rm lab}=20A$, $30A$, $40A$, $80A$, and $158A$ GeV). The NA49 data are indicated by solid stars [@Alt:2007uj]. The pure cascade calculation is depicted by lines while the hybrid model calculations with different EoS (HG, BM and CH) are depicted by dashed lines with open symbols. []{data-label="HBT-fig1"}](HBT-fig1.eps){width="48.00000%"}

![Freeze-out time dependence of the $\pi^-$ emission in central Pb+Pb at $E_{\rm lab}=20A$ GeV (left plot) and $158A$ GeV (right). Calculations with the UrQMD cascade are compared with the hybrid model calculations with the EoS of HG, BM, and CH. []{data-label="HBT-fig2"}](HBT-fig2.eps){width="48.00000%"}

Fig. \[HBT-fig1\] shows the transverse momentum $k_T$ ($\textbf{k}_T=(\textbf{p}_{1T}+\textbf{p}_{2T})/2$) dependence of the HBT radii $R_L$, $R_O$, and 