Scientific simulation of a black hole consuming a neutron star.Credit: A. Tonita, L. Rezzolla, F. Pannarale

Gravitational waves might have just delivered the first sighting of a black hole devouring a neutron star. If confirmed, this would be the first evidence of the existence of such binary systems. The news came just a day after astronomers detected gravitational waves from a merger of two neutron stars for only the second time.

At 15:22:17 UTC on 26 April, the twin detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States and the Virgo observatory in Italy reported a burst of waves of an unusual type. Astronomers are still analysing the data and performing computer simulations to interpret them.

But researchers are already considering the tantalizing prospect that they have made a long-hoped-for detection that could lead to a wealth of cosmic information, from precise tests of the general theory of relativity to measuring the Universe’s rate of expansion. Astronomers around the world are racing to observe the phenomenon themselves using different types of telescope.

“I think that the classification is leaning towards neutron star–black hole” merger, says Chad Hanna, a senior member of LIGO’s data-analysis team and a physicist at Pennsylvania State University in University Park.

Gravitational waves: How LIGO forged the path to victory

But the signal was not very strong, which means that it could be a fluke. “I think people should get excited about it, but they should also be aware that the significance is much lower” than in many previous events, he says. LIGO and Virgo have previously caught gravitational waves — faint ripples in the fabric of space-time — from two types of cataclysmic event: the merger of two black holes, and the collision of two neutron stars. The latter are small, ultra-dense objects formed after the collapse of stars more massive than the Sun.

The latest event, provisionally labelled #S190426c, seems to have occurred around 375 megaparsecs (1.2 billion light-years) away, the LIGO–Virgo team calculated. The researchers have drawn a ‘sky map’ that shows where the gravitational waves most likely originated, and sent this information out as a public alert, so that astronomers around the world could begin searching the sky for light from the event. Matching gravitational waves to other forms of radiation in this way can produce much more information about the event than either type of data can alone.

Mansi Kasliwal, an astrophysicist at the California Institute of Technology in Pasadena, leads one of several projects designed to do this type of follow-up work, called Global Relay of Observatories Watching Transients Happen (GROWTH). Her team can commandeer robotic telescopes around the world. In this case, the researchers immediately started up one such telescope in India, where it was night time when the gravitational waves arrived. “If weather cooperates, I think in less than 24 hours we should have coverage in almost the entire sky map,” she says.

Two at once

Astronomers were already working in overdrive when they spotted the potential black hole–neutron star merger. At 08:18:26 UTC on 25 April, another train of waves hit the LIGO detector in Livingston, Louisiana, and Virgo. (At the time, LIGO’s second machine, in Hanford, Washington, was briefly out of commission.)

That event was a clear-cut case of two merging neutron stars, Hanna says — nearly two years after ﻿the first historic discovery of such an event was made in August 2017.

Researchers can usually make such a call because the waves reveal the masses of the objects involved; objects roughly twice as heavy as the Sun or less are expected to be neutron stars. Based on the waves’ loudness, the researchers estimated that the collision occurred some 150 megaparsecs (500 million light years) away, says Hanna. That was around three times farther than the 2017 merger.

Iair Arcavi, an astrophysicist at Tel Aviv University who works on the Las Cumbres Observatory, one of GROWTH’s competitors, was in Baltimore, Maryland, to attend a conference called Enabling Multi-Messenger Astrophysics (EMMA) — the practice of observing these events in multiple wavelengths. The alert of the 25 April event came at 5:01 a.m. “I set it up to send me a text message, and it woke me up,” he says.

A storm of activity swept the meeting, with astronomers who would normally compete with each other exchanging information as they sat with their laptops around coffee tables. “We’re losing our minds over here at #EMMA2019”, tweeted astronomer Andy Howell.

The black-hole collision that reshaped physics

But in this case, unlike many others, LIGO and Virgo were unable to determine the direction that the waves came from. The researchers could say only that the signal was from a wide region that covers roughly one-quarter of the sky. They narrowed down the region slightly the day after.

Still, astronomers have well-honed machines for doing just this type of search, and the data they collected the following night should ultimately reveal the source, Kasliwal says. “If it existed in that region, there’s no way we would have missed it.”

In the 2017 neutron-star merger, the combination of observations in different wavelengths produced a stupendous amount of science. Two seconds after the event, an orbiting telescope detected a burst of gamma rays — which were presumably released when the merged star collapsed into a black hole. And some 70 other observatories were busy for months, watching the event unfold across the electromagnetic spectrum, from radio waves to X-rays.

If the 26 April event is not a black hole–neutron star merger, it is probably also a collision of neutron stars, which would bring the total detections of this type up to three.

Long-sought system

But seeing a black hole sweep up a neutron star could produce a wealth of information that no other type of event can provide, says B. S. Sathyaprakash, a LIGO theoretical physicist at Pennsylvania State University. To begin with, the observation confirms that these long-sought systems do exist, originating from binary stars of very different masses.

And the orbits the two objects trace in the final phases of their approach could be rather different from those followed by pairs of black holes. In the neutron star–black hole case, the more-massive black hole twists space around it as it spins. “The neutron star will be swirled around in a spherical orbit rather than a quasi-circular orbit,” Sathyaprakash says. For this reason, “neutron star–black hole systems can be more powerful test beds for general relativity”, he says.

Moreover, the gravitational waves and the companion observations from astronomers could reveal what happens in the final phases before such a merger. As tidal forces tear the neutron star apart, they could help astrophysicists to solve a long-standing mystery: what state is matter in inside these ultra-compact objects?

The LIGO–Virgo collaboration began its current observing run on 1 April, and had expected to see roughly one merger of black holes per week and one of neutron stars per month. So far, those predictions have been met — the observatories have also seen several black-hole mergers this month. “This is just amazing,” says Kasliwal. “The Universe is fantastic.”