This merger approximately happen at 5 gigaparsecs (1 gigaparsecs = 19 trillion miles) from our planet. This caused the huge gravitational waves in cosmic space and this gravitational waves are detected by two detector Ligo and Virgo . According to research this event is caused by merger of two intermediate black hole and their mass are between the 100 to 1000 times of of our SUN. This was named as GW190521 the name given based on the gravitational waves detection on May 21 ,2019.
info: gravitational waves means massive ripple effect in space time caused by violent explosion like supernova explosion or merger of black holes or merger of neutron stars.
Ligo : This is the 4km long detector in United states .
Virgo: This is the 3km long detector in Italy.
These two detector used to listen the gravitational waves that passes the earth.
According to Ligo this merger creates the new mega black hole which is 142 times massive than our sun and also released energy which massive as 8 times of our sun, this energy spread across the space in the form of gravitational waves. This event is happen in just 1/10 of second.
Official press release by LIGO: https://www.ligo.caltech.edu/news/ligo20200902