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tokens: lines available in the MUSE spectral range can be seen, and our upper limit on the C iv /Ly flux ratio seems compelling enough to exclude photoionization by the QSO as the main driver of the LAB Ly luminosity. The upper limit on the He ii /Ly ratio is just below the border mentioned, so that based on this criterion alone, our object would be on the verge of being compatible with photoionization by the central source. Interestingly, only one among the B16 radio quiet targets lies in the photoionization region in Fig. 13 , but since its position corresponds to upper limits only, it remains impossible to tell how far its luminosity can really be explained by photoionization. All other objects have a too low line ratio along either one or both axes. Table 2 Upper limits to the SB of lines due to highly ionized species. Fig. 13 Ratios of high-ionization line fluxes for our object (red) and for those of B16 (Table 2) surrounding radio-quiet QSOs (blue). The He ii line is detected in only one object of the latter sample, while the C iv line is detected in three objects. The green hatched area represents the range of line ratios compatible with photoionization models ( Arrigoni Battaia et al. 2015 ). Fig. 14 Relation size–luminosity for our object (red) and for those of B16 that are radio quiet (blue). The magenta full dots represent the 14 giant LABs discovered by Matsuda et al. (2011) . 5.3.1. Two other probable Lyman alpha emitters at the same redshift We have detected two other possible Ly emission features in the neighborhood of the QSO; their coordinates are given in Table 3 . The brighter feature (LAE1) lies away from the QSO, which corresponds to a projected distance of about pkpc, and it emits within the range Å to Å. The vicinity to the QSO and the emission at wavelengths identical to the central LAB suggest that this object is emitting in the Ly line. Figure 15 (left panel) shows the extension of this emitter as found with CubEx, which is about or pkpc in diameter; if the eastern extension is real and physically belongs to the LAE, then the major axis of the LAE would reach or pkpc. This appears very large, but is consistent with the finding of Wisotzki et al. (2016) , who found many examples of LAEs with Ly emission extending to radius at similar redshift. The emission feature of this nebula consists of two peaks, as shown in the right panel of Fig. 15 . This is very typical of some Ly emitters (LAEs). LAE 221745.3+002006, for instance, shows a very similar profile ( Yamada et al. 2012 , Fig. 2). The latter authors concluded from their observation of LAEs that more than % of them show a double peak, and that in most cases the red component is stronger than the blue one, which is naturally explained by transfer effects in expanding media, even inhomogeneous ones ( Verhamme et al. 2006 , 2008 , 2012 ; Gronke et al. 2016 , 2017 ). Fig. 15 Left : CubEx optimally extracted map of the probable Ly emitter LAE1 located away from the QSO. The key