Robert Roy Britt in Space.com:
A faint visible-light flash moments after a high-energy gamma-ray burst likely heralds the merger of two dense neutron stars to create a relatively low-mass black hole, said Neil Gehrels of NASA’s Goddard Space Flight Center. It is the first time an optical counterpart to a very short-duration gamma-ray burst has ever been detected.
Gamma rays are the most energetic form of radiation on the electromagnetic spectrum, which also includes X-rays, light and radio waves.
The merger occurred 2.2 billion light-years away, so it actually took place 2.2 billion years ago and the light just reached Earth this morning.