Spectral shape of the UV ionizing background and HeII absorption at redshifts 1.8<z<2.9

Aims. The shape of the UV ionizing background is reconstructed from optically thin metal absorption-line systems identified in spectra of HE 2347 - 4342, Q 1157+3143, and HS 1700+6416 in the redshift interval 1.8 < z < 2.9. Methods. The systems are analyzed by means of the Monte Carlo Inversion method completed with the spectral shape recovering procedure. Results. The UVB spectral shape fluctuates at 2.4 < z < 2.9 mostly due to radiative transfer processes in the clumpy IGM. At z less than or similar to 1.8, the IGM becomes almost transparent both in the H I and He II Lyman continua and the variability of the spectral shape comes from diversity of spectral indices describing the QSO/AGN intrinsic radiation. At z > 2.4, the recovered spectral shapes show intensity depression between 3 and 4 Ryd due to He II Ly alpha absorption in the IGM clouds (line blanketing) and continuous medium (true Gunn-Petersen effect). The mean He ii Lya opacity estimated from the depth of this depression corresponds within 1-2 sigma to the values directly measured from the H I/He II Ly alpha forest towards the quasars studied. The observed scatter eta = N(He II)/N(H I) and anti-correlation between N(H I) and. can be explained by the combined action of variable spectral softness and differences in the mean gas density between the absorbing clouds. Neither of the recovered spectral shapes show features which can be attributed to the putative input of radiation from soft sources like starburst galaxies.