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

We report the discovery of a quasar at , closely coincident in redshift with the isolated low-opacity feature seen near in the otherwise black portion of the Gunn-Peterson absorption trough seen toward the background quasar Q0302-003, located away on the sky. We explore plausible models for the ionization zone created by this neighboring quasar and its interception with the line of sight toward Q0302-003. At its present brightness of and separation of Mpc, the quasar can readily account for the opacity gap in the absorption spectrum of Q0302-003, provided it has been active for 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.