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 |~ 10²⁶⁻³⁰ \, 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.