The Transverse Proximity Effect in Spectral Hardness

We report on our recent detection of the transverse proximity effect of 4 foreground quasars near Q0302-003 (z=3.285) as a local hardness fluctuation in the spectral shape of the intergalactic UV radiation field, found by correlating the HI Ly\alpha absorption with the corresponding HeII Ly\alpha absorption. We argue that the spectral hardness is a sensitive physical measure to reveal the influence of quasars onto the UV background over scales of several Mpc, and that it breaks the density degeneracy hampering the search for the elusive transverse proximity effect. From the transverse proximity effect on scales of several Mpc we obtain minimum quasar lifetimes of ~10-30Myr.