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

There is a hope that looking into the early Universe with next-generation telescopes, one will be able to observe the early accretion growth of supermassive black holes (BHs) when their masses were ∼10⁴-10⁶ M_⊙. According to the standard accretion theory, the bulk of the gravitational potential energy released by radiatively efficient accretion of matter on to a BH in this mass range is expected to be emitted in the extreme UV-ultrasoft X-ray bands. We demonstrate that such a `miniquasar' at z ∼ 15 should leave a specific, localized imprint on the 21 cm cosmological signal. Namely, its position on the sky will be surrounded by a region with a fairly sharp boundary of several arcmin radius, within which the 21 cm brightness temperature quickly grows inwards from a background value of (∼-250) mK to (∼+30) mK. The size of this region is only weakly sensitive to the BH mass, so that the flux density of the excess 21 cm signal is expected to be ∼0.1-0.2 mJy at z ∼ 15 and should be detectable by the Square Kilometer Array. We argue that an optimal strategy would be to search for such signals from high-z miniquasar candidates that can be found and localized with a next-generation X-ray mission such as Lynx. A detection of the predicted 21 cm signal would provide a measurement of the growing BH's redshift to within Δz/(1 + z) ≲ 0.01.