Radio transients from newborn black holes

We consider radio emission from a newborn black hole (BH), which is accompanied by a mini-disc with a mass of ≲M_⊙. Such a disc can be formed from an outer edge of the progenitor's envelope, especially for metal-poor massive stars and/or massive stars in close binaries. The disc accretion rate is typically super-Eddington and an ultrafast outflow with a velocity of ~0.1-0.3c will be launched into the circumstellar medium. The outflow forms a collisionless shock, and electrons are accelerated and emit synchrotron emission in radio bands with a flux of ~10^26-30 erg s^-1 Hz^-1 days to decades after the BH formation. The model predicts not only a fast UV/optical transient but also quasi-simultaneous inverse-Compton X-ray emission approximately a few days after the BH formation, and the discovery of the radio counterpart with coordinated searches will enable us to identify this type of transients. The occurrence rate can be 0.1-10 per cent of the core-collapse supernova rate, which makes them a promising target of dedicated radio observations such as the Jansky VLA Sky Survey.