Caught in the act: A helium-reionizing quasar near the line of sight to Q0302-003

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 similar or equal to 3.056 in the otherwise black portion of the He II 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 III ionization zone created by this neighboring quasar and its interception with the line of sight toward Q0302-003. At its present brightness of V similar or equal to 20.5 and separation of D-perpendicular to similar or equal to 3.2 Mpc, the quasar can readily account for the opacity gap in the He II absorption spectrum of Q0302-003, provided it has been active for t(Q) greater than or similar to 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.