The discovery: Gravitational waves from neutron stars
On Aug. 17, advanced LIGO and advanced Virgo (the current upgraded versions of both observatories) detected a gravitational-wave signal possessing an extraordinary amount of energy — "something like a billion times the energy of the luminosity of the Milky Way," said Mansi Kasliwal, of the California Institute of Technology in Pasadena. Kasliwal is principal investigator of Global Relay of Observatories Watching Transients Happen (GROWTH), an international collaboration focused on cosmic transient events such as neutron-star mergers .
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"Its energy was enough to outshine the 100 billion stars in our galaxy by about a billion-fold for the 50 or so seconds it took place," said Kasliwal, one of many scientists who took part in this discovery.
This event is the first time scientists have witnessed two neutron stars merging. One main clue that the signal came from such a merger was its duration, the longest gravitational-wave signal detected to date, Kasliwal said.
Black holes are denser than neutron stars, so the signals from their mergers are relatively brief. "Previously detected black-hole mergers lasted for a second, maybe two seconds," Kasliwal told Space.com. "This latest event lasted nearly a whole minute."
There was another main clue that this new signal came from a neutron-star merger: the masses of the objects generating these gravitational waves. The frequency of gravitational waves depends on the mass of the objects that generates them — the higher the frequency, the lower the mass, Kasliwal said. The two merging objects that generated this new signal were about 1.3 and 1.5 times the mass of the sun, respectively, which is typical of neutron stars, Kasliwal said. In comparison, "the first black-hole merger that LIGO detectedinvolves black holes each about 30 times the mass of the sun," Kasliwal said.
As powerful as this new signal was, it was also much less powerful than ones seen from black-hole mergers. This neutron-star merger converted about 0.025 times the mass of the sun into energy, "which is a stupendous amount of energy," Kasliwal said. However, the first black-hole merger LIGO detected converted three solar masses into energy, "which outshone everything we had ever seen until then," Kasliwal said.
So far, LIGO has detected four black-hole mergers and one neutron-star merger. Some researchers had predicted neutron-star mergers would be more common than black-hole mergers, whereas others had predicted the opposite, Kasliwal said. She explained that while neutron-star mergers are more common in any given volume, black-hole mergers are more energetic "and so can get detected from much farther out."