A unique window into the epoch of reionisation: A double-peaked Lyman-

We present a detailed study of a double-peaked Lyα emitter, named LAE-11, found in the proximity zone of quasar J0910-0414 at z ∼ 6.6 at a proper distance of dQSO ∼ 0.3 pMpc from the quasar. We use a combination of deep photometric data from the Subaru Telescope, Hubble Space Telescope, and James Webb Space Telescope with spectroscopic data from Keck/DEIMOS, JWST/NIRCam WFSS, and JWST/NIRSpec MSA to characterise the ionising and general properties of the galaxy, as well as the quasar environment surrounding it. Apart from Lyα, we detect Hβ, [OIII]λλ4960, 5008 doublet, and Hα emission lines in the various spectral datasets. The presence of a double-peaked Lyα profile in the galaxy spectrum allows us to characterise the opening angle and lifetime of the central quasar as θQ > 49.17° and tQ > 3.8 × 105 years, and probe the effect of the quasar’s environment on the star formation of the galaxy. LAE-11 is a fairly bright (M$ _{\mathrm{UV}} = -19.90^{+0.13}_{-0.12} $), blue galaxy with a UV slope of $ \beta = -2.61^{+0.06}_{-0.08} $ and a moderate ongoing star formation rate (SFRUV = 5.55 ± 0.65 M⊙ yr−1 and SFRHα = 12.93 ± 1.20 M⊙ yr−1). Since the galaxy is located in a quasar-ionised region, we have a unique opportunity to measure the escape fraction of Lyman continuum photons using the un-attenuated double-peaked Lyα emission profile and its equivalent width at such high redshift. Moreover, we employ diagnostics which do not rely on the detection of Lyα for comparison, and find that all tracers of ionising photon leakage agree within 1σ uncertainty. We measure a moderate escape of Lyman continuum photons from LAE-11 of fescLyC = (9 − 43)%. Detections of both Hα and Hβ emission lines allow for separate measurements of the ionising photon production efficiency, resulting in $ \log(\xi_{\mathrm{ion}}/\mathrm{Hz\,erg^{-1}}) = 25.57^{+0.16}_{-0.12} $ and $ 25.63^{+0.17}_{-0.11} $ for Hα and Hβ, respectively, when using the median fescLyC. The total ionising output of LAE-11, $ \log(f_{\mathrm{esc}}^{\mathrm{LyC}}\xi_{\mathrm{ion, H\alpha}}/\mathrm{Hz\,erg^{-1}}) = 24.85^{+0.26}_{-0.29} $, is higher than the value of 24.3 − 24.8 that is traditionally assumed to be needed to drive reionisation forward.