2003A&A...397..891J

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 HeII 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 HeIII 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_{{perpendicular}}≃3.2Mpc, the quasar can readily account for the opacity gap in the HeII absorption spectrum of Q0302-003, provided it has been active for t_Q>10⁷y. 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.