Measurements of the UV background at 4.6 < z < 6.4 using the quasar proximity effect

We present measurements of the ionising ultraviolet background (UVB) at z ~ 5-6 using the quasar proximity effect. The fifteen quasars in our sample cover the range 4.6 < z_q < 6.4, enabling the first proximity effect measurements of the UVB at z > 5. The metagalactic hydrogen ionisation rate, Gamma_bkg, was determined by modelling the combined ionisation field from the quasar and the UVB in the proximity zone on a pixel-by-pixel basis. The optical depths in the spectra were corrected for the expected effect of the quasar until the mean flux in the proximity region equalled that in the average Ly-alpha forest, and from this we make a measurement of Gamma_bkg. A number of systematic effects were tested using synthetic spectra. Noise in the flux was found to be the largest source of bias at z ~ 5, while uncertainties in the mean transmitted Ly-alpha flux are responsible for the largest bias at z ~ 6. The impacts of large-scale overdensities and Lyman limit systems on Gamma_bkg were also investigated, but found to be small at z > 5. We find a decline in Gamma_bkg with redshift, from log(Gamma_bkg) = -12.15 $\pm$ 0.16 at z ~ 5 to log(Gamma_bkg) = -12.84 $\pm$ 0.18 at z ~ 6 (1 sigma errors). Compared to UVB measurements at lower redshifts, our measurements suggest a drop of a factor of five in the HI photoionisation rate between z ~ 4 and z ~ 6. The decline of Gamma_bkg appears to be gradual, and we find no evidence for a sudden change in the UVB at any redshift that would indicate a rapid change in the attenuation length of ionising photons. Combined with recent measurements of the evolution of the mean free path of ionising photons, our results imply decline in the emissivity of ionising photons by roughly a factor of two from z ~ 5 to 6, albeit with significant uncertainty due to the measurement errors in both Gamma_bkg and the mean free path.