Radio Transients from Newborn Black Holes

We consider radio emission from a newborn black hole (BH), which is accompanied by a mini-disk with a mass of $\lesssim M_{\odot }$. Such a disk 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 disk accretion rate is typically super-Eddington and an ultrafast outflow with a velocity of $\sim 0.1-0.3c$ 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 $\sim 10^{26-30}
\rm erg \ s^{-1} \ Hz^{-1}$ 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 $\sim$ 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 $0.1-10\mathrm{\%}$ of the core-collapse supernova rate, which makes them a promising target of dedicated radio observations such as the Jansky VLA Sky Survey.