Grayscale 2MASS image (K_S band) of the region around the radio position of PMN J1603−4904, indicated by the black arrow. The dashed black circle indicates the 95%-confidence ellipse of Fermi/LAT, the dot-dashed red circle the Swift/XRT HPD. The close-up in the north western corner shows the higher resolved GMOS (r′ band) image revealing a faint elliptically shaped source. The red cross marks the position of the associated WISE/2MASS source, the blue one the closest source detected by WISE (see Sect. 3.4). No other known radio source is found in this region, indicating a high-confidence association with the high-energy counterpart.