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.
