Starts With A Bang

Astronomy’s ‘Rosetta Stone’: Merging Neutron Stars Seen With Both Gravitational Waves And Light (Synopsis)

3D rendering of the gravitational waves emitted from a binary neutron star system at merger. The central region (in density) is stretched by a factor of ~5 for better visibility. Image credit: AEI Potsdam-Golm.

“It’s becoming clear that in a sense the cosmos provides the only laboratory where sufficiently extreme conditions are ever achieved to test new ideas on particle physics. The energies in the Big Bang were far higher than we can ever achieve on Earth. So by looking at evidence for the Big Bang, and by studying things like neutron stars, we are in effect learning something about fundamental physics.” -Martin Rees

When the Advanced LIGO detectors turned on in 2015, it shook up the world when they detected their first event: the merger of two quite massive black holes. Since that time, they’ve observed black hole-black hole mergers multiple times, with the VIRGO detector in Italy joining them for the fourth event. But this wasn’t what LIGO/VIRGO expected to see; rather, they were built to hunt for merging neutron stars that were much closer by.

Two merging neutron stars, as illustrated here, do spiral in and emit gravitational waves, but are much more difficult to detect than black holes. Hence, they can only be seen if they’re close by. However, unlike black holes, they should eject a fraction of their mass back into the Universe, where it composes most of the heaviest elements we know of, and emits an electromagnetic counterpart. Image credit: Dana Berry / Skyworks Digital, Inc..

Neutron star mergers would be superior to black hole mergers in an extraordinary way: it would enable other astronomers to get in on the action. Unlike black holes, merging neutron stars should emit radiation across the electromagnetic spectrum, from gamma-rays to UV/optical afterglows. On August 17th, LIGO and VIRGO saw their very first neutron star merger, pinpointing its location to galaxy NGC 4993, just 120 million light years away.

As soon as the location had been pinpointed, many of Earth’s greatest observatories, including the space-based Hubble, turned towards NGC 4993 to observe it. The telltale sign of a neutron star-neutron star merger, shown above, represented the first cross-correlation between the gravitational wave and electromagnetic sky. Image credit: P.K. Blanchard / E. Berger / Harvard-CfA / HST.

For the first time, we’ve joined the gravitational wave and light-based skies together with an incredible event. It’s a glorious step forward. And it’s just the beginning.