The Transverse Proximity Effect: A Probe to the Environment, Anisotropy, and Megayear Variability of QSOs

The transverse proximity effect is the expected decrease in the strength of the Lyα forest absorption in a QSO spectrum when another QSO lying close to the line of sight enhances the photoionization rate above that due to the average cosmic ionizing background. We select three QSOs from the Early Data Release of the Sloan Digital Sky Survey that have nearby foreground QSOs, with proper line-of-sight tangential separations of 0.50, 0.82, and 1.10 h-1 Mpc. We estimate that the ionizing flux from the foreground QSO should increase the photoionization rate by a factor (94, 13, 13) in these three cases, which would be clearly detectable in the first QSO and marginally so in the other two. We do not detect the transverse proximity effect. Three possible explanations are provided: an increase of the gas density in the vicinity of QSOs, time variability, and anisotropy of the QSO emission. We find that the increase of gas density near QSOs can be important if they are located in the most massive halos present at high redshift, but it is not enough to fully explain the absence of the transverse proximity effect. Anisotropy requires an unrealistically small opening angle of the QSO emission. Variability demands that the luminosity of the QSO with the largest predicted effect was much lower 106 yr ago, whereas the transverse proximity effect observed in the He II Lyα absorption in QSO 0302-003 by Jakobsen et al. implies a lifetime longer than 107 yr. A combination of all three effects may better explain the lack of Lyα absorption reduction. A larger sample of QSO pairs may be used to diagnose the environment, anisotropy, and lifetime distribution of QSOs.