We exploit a set of high signal-to-noise ( \sim 70 ) , low-resolution ( R \sim 800 ) quasar spectra to search for the signature of the so-called proximity effect in the H i Ly \alpha forest .
Our sample consists of 17 bright quasars in the redshift range 2.7 < z < 4.1 .
Analysing the spectra with the flux transmission technique , we detect the proximity effect in the sample at high significance .
We use this to estimate the average intensity of the metagalactic UV background , assuming it to be constant over this redshift range .
We obtain a value of J = ( 9 \pm 4 ) \times 10 ^ { -22 } erg cm ^ { -2 } s ^ { -1 } Hz ^ { -1 } sr ^ { -1 } , in good agreement with previous measurements at similar z .
We then apply the same procedure to individual lines of sight , finding a significant deficit in the effective optical depth close to the emission redshift in every single object except one ( which by a different line of evidence does nevertheless show a noticeable proximity effect ) .
Thus , we clearly see the proximity effect as a universal phenomenon associated with individual quasars .
Using extensive Monte-Carlo simulations to quantify the error budget , we assess the expected statistical scatter in the strength of the proximity effect due to shot noise ( cosmic variance ) .
The observed scatter is larger than the predicted one , so that additional sources of scatter are required .
We rule out a dispersion of spectral slopes as a significant contributor .
Possible effects are long time-scale variability of the quasars and/or gravitational clustering of Ly \alpha forest lines .
We speculate on the possibility of using the proximity effect as a tool to constrain individual quasar ages , finding that ages between \sim 10 ^ { 6 } and \sim 10 ^ { 8 } yrs might produce a characteristic signature in the optical depth profile towards the QSO .
We identify one possible candidate for this effect in our sample .