We report on the serendipitous discovery of a z = 4.0 , M _ { 1500 } = -22.20 star-forming galaxy ( Ion3 ) showing copious Lyman continuum ( LyC ) leakage ( \sim 60 % escaping ) , a remarkable multiple peaked Ly \alpha emission , and significant Ly \alpha radiation directly emerging at the resonance frequency .
This is the highest redshift confirmed LyC emitter in which the ionising and Ly \alpha radiation possibly share a common ionised channel ( with N _ { HI } < 10 ^ { 17.2 } cm ^ { -2 } ) . Ion3 is spatially resolved , it shows clear stellar winds signatures like the P-Cygni N v \lambda 1240 profile , and has blue ultraviolet continuum ( \beta = -2.5 \pm 0.25 , F _ { \lambda } \sim \lambda ^ { \beta } ) with weak low-ionisation interstellar metal lines .
Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6 \mu m and 4.5 \mu m imaging show a clear photometric signature of the H \alpha line with equivalent width of 1000Å rest-frame emerging over a flat continuum ( Ks -4.5 \mu m \simeq 0 ) .
From the SED fitting we derive a stellar mass of 1.5 \times 10 ^ { 9 } M _ { \odot } , SFR of 140 \mathrm { M } _ { \odot } yr ^ { -1 } and age of \sim 10 Myr , with a low dust extinction , E ( B-V ) \lesssim 0.1 , placing the source in the starburst region of the SFR - M ^ { * } plane . Ion3 shows similar properties of another LyC emitter previously discovered ( z = 3.21 , Ion2 , Vanzella et al .
2016 ) . Ion3 ( and Ion2 ) represents ideal high-redshift reference cases to guide the search for reionising sources at z > 6.5 with JWST .