On Aug. 17, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected something new. Some 130 million light-years away, two super-dense neutron stars, each as small as a city but heavier than the sun, had crashed into each other, producing a colossal convulsion called a kilonova and sending a telltale ripple through space-time to Earth.
When LIGO picked up the signal, the astronomer Edo Berger was in his office at Harvard University suffering through a committee meeting. Berger leads an effort to search for the afterglow of collisions detected by LIGO. But when his office phone rang, he ignored it. Shortly afterward, his cellphone rang. He glanced at the display to discover a flurry of missed text messages:
Edo, check your email!
Pick up your phone!
“I kicked everybody out that very moment and jumped into action,” Berger said. “I had not expected this.”
LIGO’s pair of ultrasensitive detectors in Louisiana and Washington state made history two years ago by recording the gravitational waves coming from the collision of two black holes — a discovery that earned the experiment’s architects the Nobel Prize in Physicsthis month. Three more signals from black hole collisions followed the initial discovery.
Yet black holes don’t give off light, so making any observations of these faraway cataclysms beyond the gravitational waves themselves was unlikely. Colliding neutron stars, on the other hand, produce fireworks. Astronomers had never seen such a show before, but now LIGO was telling them where to look, which sent teams of researchers like Berger’s scurrying to capture the immediate aftermath of the collision across the full range of electromagnetic signals. In total, more than 70 telescopes swiveled toward the same location in the sky.
They struck the motherlode. In the days after Aug. 17, astronomers made successful observations of the colliding neutron stars with optical, radio, X-ray, gamma-ray, infrared and ultraviolet telescopes. The enormous collaborative effort, detailed today in dozens of papers appearing simultaneously in Physical Review Letters, Nature, Science, Astrophysical Journal Letters and other journals, has not only allowed astrophysicists to piece together a coherent account of the event, but also to answer longstanding questions in astrophysics.