After reionization, the intergalactic hydrogen and helium is kept highly ionized by the ambient UV background generated by quasars and star-forming galaxies. A quasar can locally increase the UV radiation field in its vicinity. While this proximity effect has been found with high statistical significance near background quasars, it has been generally not revealed near foreground quasars close to the line of sight. By comparing the intergalactic Lyman alpha absorption of HI and HeII it is possible to estimate the spectral shape of the UV background. Analyzing the variations of the UV spectral shape on two lines of sight towards Q0302-003 and HE2347-4342, we have found evidence for a hard ionizing radiation field near the two background quasars and 7 foreground quasars. Since only quasars can produce the required hard photons we interpret this as a signature of the transverse proximity effect as a fluctuation in the spectral shape of the radiation field. Our findings point towards radiative feedback of quasars onto the intergalactic medium on scales of several Mpc and quasar lifetimes of 10-30Myr.
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