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There is a hope that looking into the early Universe with next-generationtelescopes, one will be able to observe the early accretion growth ofsupermassive black holes (BHs) when their masses were $\sim 10^4$-$10^6M_\odot$. According to the standard theory, the bulk of the gravitationalpotential energy released by radiatively efficient accretion of matter onto aBH in this mass range is expected to be emitted in the extreme UV-ultrasoftX-ray bands. We demonstrate that such a 'miniquasar' at $z\sim 15$ should leavea specific, localized imprint on the 21 cm cosmological signal. Namely, itsposition on the sky will be surrounded by a region with a fairly sharp boundaryof several arcmin radius, within which the 21 cm brightness temperature quicklygrows inwards from the background value of $\sim -200$ mK to $\sim +30$ mK. Thesize of this region is only weakly sensitive to the BH mass, so that the fluxdensity of the excess 21 cm signal is expected to take a nearly universal valueof $\sim 0.2[(1+z)/16]^{-2}$~mJy and should be detectable by the SquareKilometer Array. We argue that an optimal strategy would be to search for suchsignals from high-$z$ miniquasar candidates that can be found and localizedwith a next-generation X-ray mission such as Lynx. A detection of the predicted21 cm signal would provide a measurement of the growing BH's redshift to within$\Delta z/(1+z)<0.01$.

A universal 21 cm signature of growing massive black holes in the early Universe