The UV-brightest Lyman continuum emitting star-forming galaxy

We report the discovery of J0121+0025, an extremely luminous and young star-forming galaxy (M_UV = -24.11, log[L_Lya / erg s^-1] 43.8) at z 3.244 showing copious Lyman continuum (LyC) leakage (f_esc,abs ~ 40%). High signal-to-noise ratio rest-frame UV spectroscopy with Gran Telescopio Canarias reveals a high significance (7.9 sigma) emission below limit (< 912A), flux density level f_900A 0.78 +/- 0.10 uJy, strong P-Cygni in wind lines OVI 1033A, NV 1240A CIV 1550A that are indicative age starburst (<10 Myr). The spectrum is rich stellar photospheric features, for which significant contribution AGN these wavelengths ruled out. Low-ionization ISM absorption also detected, but weak (EW0 1A) show large residual intensities, suggesting clumpy geometry gas non-unity covering fraction or highly ionized ISM. foreground contamination to LyC signal unlikely. Deep optical Spitzer/IRAC 4.5um imaging spectral energy distribution J0121+0025 dominated by starburst, log(M*/Msun) 9.9 0.1 SFR 981 232 Msun yr^-1. most powerful emitter known among population. such leaking radiation suggests photons can escape sources wide range luminosities not restricted faintest ones as previously thought. These findings might shed further light on role starbursts cosmic reionization.