Upper limit of 0.38 mJy extended to the radio wavelengths considering α  = − 0.5 (black lines), α  = − 1.0 (green lines) and α  = − 2.0 (blue lines). Dashed lines do not consider any absorption, while solid lines are affected by free-free absorption using Eqs. (1) and (2) and considering the orbital position at which the absorption is lowest (phase 0.61). Note that no intrinsic low-frequency cutoff has been introduced. Ten-hour potential sensitivities are plotted for the LOFAR (pentagon), GBT (triangles), Effelsberg (stars), EVLA (circles) and ALMA (diamonds) observatories. For the two latter, empty symbols are plotted using a reduced bandwidth of Δν  =  800 MHz.