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We report on high resolution spectra of the bright QSO HS0747+4259 (zem =1.90, V = 15.8) observed to search for intermediate redshift OVI absorptionsystems. The spectra were obtained by means of the Space Telescope ImagingSpectrograph (STIS) at the Hubble Space Telescope (HST) and the High ResolutionEchelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVIsystems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs =1.46-1.8 exhibit a sufficient number of lines of different ionic transitions toestimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10Ryd. All recovered UV ionizing spectra are characterized by the enhancedintensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau(1996). This is in line with the observational evidence of a deficiency ofstrong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UVbackground shows significant local variations: the spectral shape estimated atz = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause ofthese variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to theline of sight. No features favoring the input of stellar radiation to theionizing background are detected, limiting the escape fraction of the galacticUV photons to f_esc < 0.05.Comment: 14 pages, 7 figures, 5 tables; accepted for publication in A&

Spectral shape of the UV ionizing background and O VI absorbers at z $\mathsf{\sim 1.5}$ towards HS 0747+4259