SED for 4U 1820−30. We fit the broad-band spectrum with a power law of index α = 0.13 ± 0.04. The upper limit at 9 GHz was not considered to perform the fit. The Swift/XRT point corresponds to the (almost identical) fluxes on the days before and after the ALMA and ATCA observations (see Table 1). The Swift/BAT points correspond to the flux densities derived in Sect. 3.2 for observations on the days before and after the ALMA and ATCA observations (see Table 1).