The proximity effect in a close group of QSOs

We present an analysis of the proximity effect in a sample of 10 2-Angstrom -resolution QSO spectra of the Ly alpha forest at [z] = 2.9. Rather than investigating variations in the number density of individual absorption lines, we employ a novel technique that is based on the statistics of the transmitted flux itself. We confirm the existence of the proximity effect at the > 99 per cent confidence level. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of J = 3.5(-1.3)(+3.5) x 10(-22) erg s(-1) cm(-2) Hz(-1) sr(-1). This value assumes that QSO redshifts measured from high-ionization lines differ from the true systemic redshifts by Deltav approximate to 800 km s(-1). We find evidence at a level of 2.6 sigma that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. Allowing for known QSO variability, the significance of the correlation reduces to 2.1 sigma. The QSOs form a close group on the sky and the sample is thus well suited for an investigation of the foreground proximity effect, where the Lya forest of a background QSO is influenced by the UV radiation from a nearby foreground QSO. From the complete sample we find no evidence for the existence of this effect, implying either that J > 20 x 10(-22) erg s(-1) cm(-2) Hz(-1) sr(-1) or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example. Our sample includes the fortunate constellation of a foreground QSO surrounded by four nearby background QSOs. These four spectra all show underdense absorption within +/- 3000 km s(-1) of the redshift of the foreground QSO.