Context :

Aims : We report on high resolution spectra of the bright QSO HS 0747+4259 ( z _ { em } = 1.90 , V = 15.8 ) observed to search for intermediate redshift O vi absorption systems .

Methods : The spectra were obtained by means of the Space Telescope Imaging Spectrograph ( STIS ) at the Hubble Space Telescope ( HST ) and the High Resolution Echelle Spectrometer ( HIRES ) at the W. M. Keck telescope .

Results : We identify 16 O vi systems in the range 1.07 \leq z \leq 1.87 .
Among them , six systems with z _ { abs } = 1.46 –1.8 exhibit a sufficient number of lines of different ionic transitions to estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10 Ryd .
All recovered UV ionizing spectra are characterized by the enhanced intensity at E > 3 Ryd compared to the model spectrum of Haardt & Madau ( 1996 ) .
This is in line with the observational evidence of a deficiency of strong Ly \alpha absorbers with N ( H i ) > 10 ^ { 15 } cm ^ { -2 } at z < 2 .
The UV background shows significant local variations : the spectral shape estimated at z = 1.59 differs from that obtained at z = 1.81 and 1.73 .
A possible cause of these variations is the presence of a QSO/AGN at z \simeq 1.54 - 1.59 close to the line of sight .
No features favoring the input of stellar radiation to the ionizing background are detected , limiting the escape fraction of the galactic UV photons to f _ { esc } < 0.05 .

Conclusions :