Absorption by associated ionised gas in the SNR W49B. Panel a: [FeII] line emission at 1.644 μm towards W49B (image kindly provided by Dr. Lee, Y.-H.) overlaid with contours (levels: 3, 10, 20, 30, 40, 50, and 60 mJy beam⁻¹) of the 1.4 GHz continuum radiation detected in the MAGPIS survey. The colour representation is linear in units of MJy sr⁻¹. Panel b: local distribution of the optical depth computed towards W49B at 74 MHz. Cyan contours of the near-IR [FeII] line emission matched at the 45′′ resolution are superimposed for comparison, with levels at 12, 47, 70, and 105 MJy sr⁻¹.