Top: Swift/XRT and Swift/BAT fluxes of 4U 1820−30 before, during and after the ALMA and ATCA observations. Bottom: Hardness ratio (HR) between the Swift/BAT and Swift/XRT fluxes. Note that the last HR point was calculated between the BAT and the XRT fluxes reported for two consecutive days since after the day of the ALMA observation no measurements were available within the same day. The Swift/XRT fluxes are calculated using a model of two components: a disc blackbody and a power law. The Swift/BAT fluxes are the average between the fluxes obtained assuming a power law of index 1 and a power law of index 2 and the error corresponds to the difference between these fluxes (note that the power-law indices of the fits to the Swift/XRT spectra range between 1 and 2 except for the last observation, for which the index is 0.6 ± 0.3, and a fit with two thermal components has been judged as being more scientifically meaningful).