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We study the proximity effect in the Ly-a forest around high redshift quasarsas a function of redshift and environment employing a set of 3D radiativetransfer simulations.  The analysis is based on dark matter only simulations at redshifts 3, 4, and4.9 and, adopting an effective equation of state for the baryonic matter, weinfer the HI densities and temperatures in the cosmological box. The UVbackground (UVB) and additional QSO radiation with Lyman limit flux of L_{\nuLL} = 1e31 and 1e32 erg / Hz s are implemented with a radiative transfer codeuntil an equilibrium configuration is reached. We analyse mock spectraoriginating at the QSO in the most massive halo, in a random filament and in avoid. The proximity effect is studied using flux transmission statistics, inparticular with the normalised optical depth.  Beyond a radius of r > 1 Mpc / h from the quasar, we measure a transmissionprofile consistent with geometric dilution of the QSO ionising radiation. Adeparture from geometric dilution is only seen, when strong absorbers intervenethe line-of-sight. The cosmic density distribution around the QSO causes alarge scatter in the normalised optical depth. The scatter decreases withincreasing redshift and increasing QSO luminosity. The mean proximity effectprovides an average signal that is biased by random large scale densityenhancements on scales up to r \approx 15 Mpc / h. The distribution of theproximity effect strength provides a measure of the proximity effect alongindividual lines of sight. It shows a clear maximum almost without anenvironmental bias. Therefore it can be used for an unbiased estimate of theUVB. Differing spectral energy distributions between the QSO and the UVB modifythe profile which can be reasonably well corrected analytically. A few Lymanlimit systems have been identified that prevent the detection of the proximityeffect due to shadowing.

Cosmological radiative transfer for the line-of-sight proximity effect