Normally, no light should be produced when two black holes merge,
because no light can escape them, obviously. However, in a bizarre
coincidence, two stellar mass BH's collided with each other, while they
were both also in orbit around a supermassive BH. the supermassive had
an accretion disk surrounding it, and the newly merged small BH crashed
through the accretion disk of the supermassive and sent a flare out of
the accretion disk.

https://www.ctvnews.ca/mobile/sci-te...B8pyZD2XWuKwDQ