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.
"I think that the classification is leaning towards a neutron star-black hole" merger, says Chad Hanna, a senior member of the LIGO data-analysis team and a physicist at Pennsylvania State University in the 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 space-time fabric – from two types of cataclysmic events: the merger of two black holes and two neutron stars.
The latest event, provisionally labeled # S190426c, seems to have occurred around 375 mega-parsecs (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 originated, and sent this information out as a public alert so that astronomers around the world could begin to search 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 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 the Global Relay of Observatories Watching Transients Happen (GROWTH). Her team can commandeer robotic telescopes that are spread around the world. In this case, the researchers immediately started up one in India, where it was a night time when the gravitational waves arrived. "If weather cooperates, I think in less than 24 hours we should have coverage on 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 April 25, 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.)
This event was a clear-cut case of two merging neutron stars, Hanna says – almost two years after the first historic discovery Such an event was made in August 2017.
Researchers can usually make such a call because the waves reveal the masses of the involved objects; Objects roughly twice as heavy as the Sun are expected to be neutron stars. Based on the wave's loudness, the researchers also estimated that the collision took about 150 megaparks (500 million light-years) away, says Hanna. That was about three times farther than the 2017 merger.
Iaar Arcavi, an astrophysicist at the 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 25th of 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 the scientists also received some bad news for this event. 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 only say that the waves were from a wide region that covers roughly one-quarter of the sky.
Still, astronomers had well-honed machines for doing just that type of search, and the data they collected the next 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, a 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 unfolding across the electromagnetic spectrum, from radio waves to rhythms.
If the 26 April event is not a black-hole-neutron star merger, it is probably also a merger of neutron stars that would bring total detections of this type up to three.
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And the orbits of the two objects trace in their final approaches to their approach could be quite different from those seen with pairs of black holes. In the neutron star-black hole case, the more massive black hole would twist the space around it as it spins. "The neutron star will be swirled around 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 companion observations from astronomers could reveal what happens in the final phases before the merger . As the tidal forces tear the neutron star apart, they could help the astrophysicist to solve a long-standing mystery: what state is matter inside these ultra-compact objects.
The LIGO-Virgo collaboration began its current observation run on April 1, and had expected to see roughly one merger of black holes per week and one of neutron stars per month. So far, these predictions have been met – Observations have seen this month also several black-and-dock mergers. "This is just amazing," says Kasliwal. The Universe is fantastic.