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The ionising continuum from active galactic nuclei (AGN) is fundamental forinterpreting their broad emission lines and understanding their impact on thesurrounding gas. Furthermore, it provides hints on how matter accretes ontosupermassive black holes. Using HST's Wide Field Camera 3 we have constructedthe first stacked ultraviolet (rest-frame wavelengths 600-2500\AA) spectrum of53 luminous quasars at z=2.4, with a state-of-the-art correction for theintervening Lyman forest and Lyman continuum absorption. The continuum slope($f_\nu \propto \nu^{\alpha_\nu}$) of the full sample shows a break at ~912\AA\with spectral index $\alpha_\nu=-0.61\pm0.01$ at $\lambda>912$\AA\ and asoftening at shorter wavelengths ($\alpha_\nu=-1.70 \pm 0.61$ at $\lambda\leq912$\AA). Our analysis proves that a proper intergalactic medium absorptioncorrection is required to establish the intrinsic continuum emission ofquasars. We interpret our average ultraviolet spectrum in the context ofphotoionisation, accretion disk models, and quasar contribution to theultraviolet background. We find that observed broad line ratios are consistentwith those predicted assuming an ionising slope of $\alpha_\mathrm{ion}=$-2.0,similar to the observed ionising spectrum in the same wavelength range. Thecontinuum break and softening are consistent with accretion disk plus X-raycorona models when black hole spin is taken into account. Our spectral energydistribution yields a 30% increase to previous estimates of the specific quasaremissivity, such that quasars may contribute significantly to the totalspecific Lyman limit emissivity estimated from the Ly$\alpha$ forest at z<3.2.

The first ultraviolet quasar stacked spectrum at z=2.4 from WFC3