Summary of the Ion1 source. In the top-left panel the HST/ACS, VLT/U, and B-band filters are shown, superimposed to a BC galaxy template placed at the same redshift of Ion1. In particular, the solid blue and red dotted lines represent the HST/B₄₃₅ and VLT/B band filters. The shaded transparent blue region marks the LyC region. More than half of the B₄₃₅ and VIMOS/B bands probe the ionizing domain, while the U-band probes λ< 830 Å rest-frame. In the top right panel the simulated distribution of the (B₄₃₅ − V₆₀₆) colors are calculated assuming fesc = 0/1 and compared to the observed color of the source Ion1. The probability of a null fesc is very low. Similarly, the simulated U-band magnitude reproduces the observed one only marginally (middle right panel). The bottom panel shows the thumbnails for Ion1. The blue circle in the B₄₃₅-band has a diameter of 1′′.