The UV background photoionization rate at 2.3 < z < 4.6 as measured from
  the Sloan Digital Sky Survey
  (Abridged) We present the results from the largest investigation to date of
the proximity effect in the HI Lyalpha forest, using the fifth SDSS data
release. The sample consists of 1733 QSOs at redshifts z>2.3 and S/N>10. We
adopted the flux statistic to infer the evolution of the HI effective optical
depth in the Lyalpha forest between 2<z<4.5, finding very good agreement with
results from high-resolution QSO samples. We compared the average opacity close
to the quasar emission with its expected behavior in the Lyalpha forest and
estimated the signature of the proximity effect towards individual objects at
high significance in about 98% of the QSOs. Dividing the whole sample of
objects in eight subsets according to their emission redshift, we inferred the
proximity effect strength distribution (PESD) on each of them finding in all
cases a prominent peak and an extending tail towards values associated to a
weak effect. We provide for the first time observational evidence for an
evolution in the asymmetry of the PESD with redshift. Adopting the modal values
of the PESDs as our best and unbiased estimates of the UV background
photoionization rate (Gamma_HI), we determine its evolution within the range
2.3<z<4.6. Our measurements do not show any significant decline towards high
redshift and are located at Log(Gamma_HI)=-11.78\pm0.07 in units of s^-1. We
decompose the observed photoionization rate into two major contributors: quasar
and star-forming galaxies. By modeling the quasar contribution with different
luminosity functions we estimated their contribution to Gamma_HI, thus putting
a constraint on the residuals. We conclude that independently of the assumed
luminosity function, stars are dominating the UVB at z>3.
