  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.
