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=3.2 Mpc, 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 > 10^7 y. This is the first clear detection of the `transverse proximity effect and imprint of a quasar on the intervening absorption detected along an adjacent line of sight.
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