  Following Bi & Davidsen (1997), we perform one dimensional semi analytic
simulations along the lines of sight to model the intergalactic medium (IGM).
Since this procedure is computationally efficient in probing the parameter
space -- and reasonably accurate -- we use it to recover the values of various
parameters related to the IGM (for a fixed background cosmology) by comparing
the model predictions with different observations. For the currently favoured
LCDM model (\Omega_m=0.4, \Omega_{\Lambda}=0.6 and h=0.65), we obtain, using
statistics obtained from the transmitted flux, constraints on (i) the
combination f=(\Omega_B h^2)^2/J_{-12}, where \Omega_B is the baryonic density
parameter and J_{-12} is the total photoionisation rate in units of 10^{-12}
s^{-1}, (ii) temperature T_0 corresponding to the mean density and (iii) the
slope \gamma of the effective equation of state of the IGM at a mean redshift z
\simeq 2.5. We find that 0.8 <(T_0/10^4 K)< 2.5 and 1.3<\gamma<2.3. while the
constraint obtained on f is 0.020^2<f<0.032^2. A reliable lower bound on
J_{-12} can be used to put a lower bound on \Omega_B h^2, which can be compared
with similar constraints obtained from Big Bang Nucleosynthesis (BBN) and CMBR
studies. We find that if J_{-12}>1.2, the lower bound on \Omega_B h^2 is in
violation of the BBN value.

Astrophysics