Gravitational waves could deliver the first observation of a black hole swallowed by the neutron star. If confirmed, it will be the first proof of the existence of such binary systems. The news comes just one day after astronomers discovered gravitational waves from the merging of two neutron stars only for the second time.
At 15:22:17 UTC on April 26, double detectors at the GIGIT Observatory in the United States and the Virgin Observatory of the Virgin in Italy reported a wave of unusual waves. Astronomers still analyze data and perform computer simulations to interpret them. But they are already looking at the remarkable prospect that they have made extensive discoveries that can produce a wealth of cosmic information, from precise tests of the general theory of relativity to measuring the rate of expansion of the universe. Astronomers from all over the world also run to observe this phenomenon with the help of many telescopes.
"I think the classification relies on a neutron star and black hole coupling," said Chad Hanna, a senior member of the Ligo data analysis team and a physicist at Pennsylvania State University at the University Park.
But the signal was not very strong, which means it can be unusual. "I think people should get excited about it, but they also need to be aware that the meaning is much lower" than in many previous events, he says. LIGO and the Virgin have previously attracted gravity waves – weak ripples in the structure of space-time – from two types of cataclysmic events: the coupling of two black holes and two neutron stars. The latter are small, but ultra-dense objects formed after the collapse of stars that are massive from the Sun.
The latest event, temporarily labeled # S190426c, appears to have appeared around 375 megaparks (1.2 billion light-years), the LIGO-Virgo team has been calculated. The researchers made a "map of the sky," showing where gravity waves were likely to originate, and sent this information as a public warning, so that astronomers around the world could begin looking for the sky for light from the event. Appropriate gravitational waves with other forms of radiation in this way can produce much more information about the event than only one type of data can occur.
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 Observatories that monitor transients that occur (RAST). Her team can command robotic telescopes that spread around the world. In this case, the researchers immediately started working in India, where it was a night time when gravitational waves arrived. "If time co-operates, I think that in less than 24 hours we have to have coverage on almost the entire map of the sky," she says.
Two at once
Astronomers have already worked in overdrive when they noticed a potential neutron star stellion. At 08:18:26 UTC on April 25th, another train wound struck the Ligo Detector in Livingston, Louisiana and Virgo. (At that time, the second Ligo machine in Hanford, Washington, was short of commission.)
This event was a clear case of two neutron stars that merge, says Hannah – almost two years after the first historical discovery of such an event was made in August 2017.
Researchers can usually make such a call, because the waves reveal the masses of objects in question; objects that are approximately twice the size of the sun are expected to be neutron stars. Based on the wavelengths of the waves, the researchers also estimated that the collision occurred around 150 megacities (500 million light-years), says Hannah. It was about three times beyond the merger in 2017.
Iair Arcavi, an astrophysicist at the Tel Aviv University, who works at Las Cumbres Observatory, one of the RAST competitors, was in Baltimore, Maryland, to attend a conference called Enabling Multimedia Astrophysics (EMMA) – a practice of observing these events in more wavelengths. The alarm for the April 25th event came at 5:01 am. "I asked him to send me a text message and wake me up," he says.
Astronomers who competed normally shared information while sitting with their laptops around the tables, hit the storm of activity. "We are losing our mind here on # EMMA2019," a tweeted astronomer Andy Howell.
But scientists received bad news about this event. In this case, unlike many others, the LIGO and the Virgo were unable to significantly narrow the direction of the sky from which the waves come. The researchers could only say that the waves were from a wide region that covered about a quarter of the sky. They narrowed the region a day after.
However, astronomers had well-equipped machines for doing just that kind of search, and the data they gathered next night should finally discover the source, Kisliv said. "If it existed in that region, there is no way to miss it."
In the neutron star merger in 2017, the combination of observations at different wavelengths creates a fantastic source of science. Two seconds after the event, an orbiting telescope discovered a burst of gamma rays – probably released when the tagged star collapsed into a black hole. And some 70 other observatories were busy for months, watching the event that goes through the electromagnetic spectrum, from radio waves to X-rays.
If the event of April 26th is not a blending of black holes-neutron stars, it is likely that the neutron stars will join, which will bring total detection of this type to three.
But seeing a black hole blurring a neutron star can create a wealth of information that has no other kind of events, says B. S. SatyaPrakash, a theoretical physicist of LIGO in the state of Pennsylvania. For starters, it confirms that these long-term systems exist, originating from binary stars with very different masses.
And the orbits of the two objects that are tracked in the final stages of their approach could be different from those seen by pairs of black holes. In the case of a neutron star-black hole, a more massive black hole will twist space around it when it turns. "The neutron star will be rolled around in a spherical orbit, not a quasi-circular orbit," says Satilprakash. For this reason, "neutron star-black hole systems can be more powerful testers for general relativity," he says.
In addition, gravitational waves and accompanying observations from astronomers could show what happens in the final stages before the merger. As the tidal forces break down the neutron star, they can help the astrophysicist solve a long-standing mystery: what kind of condition is inside these ultra-compact objects.
Co-operation with the LIGO-Virgo began observing on April 1, and it was expected to see roughly one black hole coupling per week and one of the neutron stars per month. So far, those predictions have been met – observatories also noted several black hole mergers this month. "This is incredible," Kislival said. "The universe is fantastic".