3-mm – 3-cm spectral index image. The spectral index α is defined as S_ν ∝ ν^α and was determined from the ratio of the 3-cm image from observations performed on 2011 January 25 (Ng. et al., in prep.) and the 3-mm image from observations around July 2011. Both images were restored on a 0.′′7 circular beam and centred on the VLBI position of SN 1987A determined by Reynolds et al. (1995) (blue square). The images are overlaid with contours representing the 70% and 90% flux density levels (white: 3 mm; blue: 3 cm).