# Outflow-Driven Transients from the Birth of Binary Black Holes I: Tidally-Locked Secondary Supernovae

## TL;DR

This paper proposes a new electromagnetic transient signal from the birth of binary black holes, caused by accretion disk outflows following the core-collapse of a tidally-locked Wolf-Rayet star, with potential optical and radio observables.

## Contribution

It introduces a novel transient type linked to binary black hole formation, highlighting the role of tidally synchronized Wolf-Rayet stars and their accretion disks in producing observable signals.

## Key findings

- Optical transients with magnitude -14 to -17 lasting about a day.
- Radio signals detectable 1-10 years after black hole formation.
- Distinct light curve features enable differentiation from ordinary supernovae.

## Abstract

We propose a new type of electromagnetic transients associated with the birth of binary black holes (BBHs), which may lead to merger events accompanied by gravitational waves in $\sim0.1-1$ Gyr. We consider the newborn BBHs formed through the evolution of isolated massive stellar binaries. For a close massive binary, consisting of a primary black hole (BH) and a secondary Wolf-Rayet (WR) star that are orbiting around each other, the spin period of the WR star can be tidally synchronized to its orbital period. Then, the angular momentum of the outer material of the WR star is large enough to form an accretion disk around a newborn, secondary BH, following its core-collapse. This disk may produce an energetic outflow with a kinetic energy of $\sim10^{50}-10^{52}$ erg and an outflow velocity of $\sim10^{10}\rm~cm~s^{-1}$, resulting in an optical transient with an absolute magnitude from $\sim -14$ to $\sim-17$ with a duration of around a day. This type of transient also produces detectable radio signals $\sim1-10$ years after the birth of BBHs, via synchrotron emission from non-thermal electrons accelerated at external shocks. The predicted optical transients have a shorter duration than ordinary core-collapse supernovae. Dedicated optical transient surveys can detect them, and distinguish it from ordinary SNe using the different features of its light curve and late-time spectrum. In the paper I, we investigate disk-driven outflows from the secondary BH, whereas possible signals from the primary BH will be discussed in the paper II.