Integrated flux density of the vibrationally excited HCN 4−3 (in contours) superimposed on the 347.9 GHz continuum emission (grayscale). Integrated velocities are 40 to 240 km s^-1 for the eastern nucleus, covering the whole line, and from 20 to 170 km s^-1 (the redshifted region is not affected by blending with HCO⁺) for the western nucleus. Contours are 10, 20, 40, 60, 80,and 90% of the peak-integrated flux density. The black cross represents the fitted center to the continuum emission, while the black, red, and blue plus signs represent the fitted centers of the vibrational line for the integrated, redshifted, and blueshifted parts of the line profile (see text in Sect. 5.2), respectively. For the western nucleus only the redshifted emission was available because of blending with HCO⁺ on the blueshifted side.