Gravitational waves may have just delivered the first sighting of a black hole devouring a neutron star. If confirmed, it would be the first evidence of the existence of such binary systems. The news comes just a day after astronomers had 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 analyzing the data and doing computer simulations to interpret them.
But they are already considering the tantalizing prospect that they have made a long-hoped-for detection that could produce 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 also racing to observe the phenomenon 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.
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 had previously caught gravitational waves — faint ripples in the fabric of space-time — from two types of cataclysmic event: the mergers of two black holes, and of two neutron stars. The latter are small but ultra-dense objects formed after the collapse of stars more massive than the Sun.
The latest event, provisionally labeled #S190426c, appears to have occurred around 375 megaparsecs (1.2 billion light-years) away, the LIGO–Virgo team calculated. The researchers have drawn a ‘sky map’, showing where the gravitational waves are most likely to have 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 in India, where it was night time when the gravitational waves arrived. “If the 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’s 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 are expected to be neutron stars. Based on the waves’ loudness, the researchers also 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 the 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.
But in this case, unlike many others, LIGO and Virgo were unable to significantly narrow down the direction in the sky 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 had 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 had detected a burst of gamma rays — 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.
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. To begin with, it 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 seen with pairs of black holes. In the neutron star-black hole case, the more-massive black hole would twist 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 the merger. As tidal forces tear the neutron star apart, they could help astrophysicist 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 the 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.”