Caught in the act : a helium - reionizing quasar near the line of sight to Q 0302 − 003 ?

We report the discovery of a quasar at z = 3.050 ± 0.003, closely coincident in redshift with the isolated low-opacity feature seen near z 3.056 in the otherwise black portion of the He  Gunn-Peterson absorption trough seen toward the z = 3.286 background quasar Q0302−003, located 6. 5 away on the sky. We explore plausible models for the He  ionization zone created by this neighboring quasar and its interception with the line of sight toward Q0302−003. At its present brightness of V 20.5 and separation of D ⊥ 3.2 Mpc, the quasar can readily account for the opacity gap in the He  absorption spectrum of Q0302−003, provided it has been active for t Q > ∼ 10 7 y. This is the first clear example of the 'transverse' proximity effect and the association of a quasar with its imprint on the intervening absorption detected along an adjacent line of sight.