Evidence of a neutron star smashing into a black hole

This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. New gravitational wave observations may point to a different sort of collision.

Detecting gravitational waves doesn’t have the sort of bombshell panache it did when it first happened three years ago, but that doesn’t make it any less remarkable. These observatories—arguably the most sensitive instruments humans have ever built—keep teaching us about events in the universe that were hidden until very recently. We’ve studied pairs of black holes merging, neutron stars colliding into one-another, and now, we may have finally witnessed signs of a black hole slamming into a neutron star. That’s something scientists were never really sure was even possible.

“It is a discovery of a novel astrophysical system whose existence was not certain,” says Katerina Chatziioannou, a member of the LIGO team at the Perimeter Institute in Waterloo, Canada, and a soon-to-be professor at Caltech. “These systems have been theorized to form in various astrophysical environments, so proving that they exist and estimating how common they are will tell us about the environments in which they are formed.

To sum up: two new sets of gravitational wave observations were detected on April 25 and April 26, by the pair of interferometers run by the Laser Interferometer Gravitational-Wave Observatory (LIGO) based in Livingston, Louisiana, and Hanford, Washington, as well as the Virgo interferometer based in Italy. While the former signals seem to originate from a pair of neutron stars (ultra-dense bodies composed of closely-packed neutrons, formed by the collapse of massive stars after a supernova) crashing into one-another, the latter seem to come from a rare black hole-neutron star merger.

These newest detections come hot off the heels of major upgrades to the LIGO and Virgo observatories. The power of their lasers have been doubled, reducing the effect of “noise” and increasing the sensitivity of the detectors by nearly 40 percent. “These detections could have been made previously, but the improved sensitivity allows us to get a more accurate picture,” says Rana Adhikari, a LIGO team member and professor of physics at Caltech. “Like having a conversation in a quiet room instead of a busy coffee shop.”

The April 25 detection of two neutron stars smashing, which scientists are calling S190425z, is thought to have happened about 500 million light-years away from Earth (two to three times farther than the first neutron star merger observed). Only the LIGO Livingston and Virgo observatories picked up the gravitational waves of this event (Hanford’s observatory was offline at the time), and the lack of a complete detection means we’re still unclear on the precise origin of the event (it took place in a rage that covers one-fourth of the sky).

Meanwhile, the April 26 neutron star-black hole crash, called S190426c, probably happened about 1.2 billion light-years away. All three observatories caught its much-weaker signal, so scientists have narrowed the location down to a 3 percent area of the sky.

How do we know this is a mash-up of a neutron star and a black hole and not just pairs of either type? According to Chatziioannou, it comes down to mass. Neutron stars typically have a lower mass than black holes, and estimates made from the gravitational wave signals that were measured fall into a middling, Goldilocks-like range that’s not too light and not too heavy.

Unfortunately, that’s about all we really do know about the origin of the April 26 signals. We need to confirm that they indeed originate from a neutron star colliding with a black hole before we can figure out what those objects looked like before, and what the resulting cosmic chimera looks like now. Chatziioannou explains that she and her team need some time to sift through the gravitational wave data as well as other measurements like gamma rays and x rays. It would also help to discover more events like this in the coming months to ensure it isn’t a false alarm. For now, the probability that this is neutron star-black hole merger is four times higher than the odds it is simply a binary neutron star.

But there is certainly no lack of ideas for how such an event occurred. One theory, says Shaon Ghosh, a postdoctoral research associate at the University of Wisconsin Milwaukee and a member of the LIGO team, is a “co-evolving system, where two massive stars spend their lifetime in a binary system, evolve and then form a neutron star and a black hole. These two compact objects emit gravitational waves, losing energy and angular momentum and shrinking the separation between them and eventually coalescing.” Another theory involves something called dynamical capture, where an unrelated neutron star and a black hole accidentally get too close, and start to gravitationally interact with each other, until they eventually merge.

Ghosh emphasizes that since black holes don’t actually have a surface, a neutron star and a black hole merger isn’t really a collision that might spew matter out in every direction, but rather a soft smash-up of the two bodies. “If the event was indeed from a coalescence of a neutron star and a black hole,” says Ghosh, “then the gravity of the black hole may sufficiently deform the neutron star,” ripping it up into bits. It could form a cosmic orbit of matter hanging around the black hole’s point-of-no-return.

If we get confirmation, however, it would be an astounding discovery. The confirmation of gravitational waves has consistently been touted as proof of a major part of Einstein’s theory of general relativity, but many have hoped our detection of these signals could help us glimpse a whole new world of astrophysics. It looks like these could be one of the first examples of that realized potential.

“Any system that involves a neutron star carries information about dense matter at extreme densities,” says Chatziioannou. “So by studying the gravitational wave data we might be able to infer something about the properties of the neutron star matter.” Ghosh adds that follow-up observations ought to help us understand the disruptive physical effects that occur under the extreme gravity exerted by black holes.

It’s going to take time for the LIGO and Virgo teams to sit down with the results and make more sense of them, but if these first round of hypotheses hold true, we’re on the cusp of a major paradigm shift in our understanding of the astrophysics of the known universe.

Gravitational Waves may help in detection of star colliding with BlackHole.

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 analysing 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 labelled #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 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 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.

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. 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 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.”