Astronomers have probably seen a black hole swallow a neutron star

900 million years ago, a black hole triggered a terrible eructation that resonated in the cosmos. On August 14, the resulting ripples in the space-time structure crossed the Earth, giving us the best evidence to date of a type of cosmic collision ever before that could offer new perspectives on the functioning of the universe.

The detection, called S190814bv, was probably triggered by the fusion of a black hole and a neutron star, the ultra-dense remains of an exploded star. Although astronomers have long expected the existence of such binary systems, they have never been seen by telescopes sweeping the skies in search of different wavelengths of light. (See the first image of the silhouette of a black hole, made with the help of a gigantic network of radio telescopes.)

However, astronomers also expect these systems to create ripples called gravitational waves if and when the black hole and the neutron star merge. Einstein's theory of general relativity predicted these space-time ripples more than a hundred years ago, suggesting that the collision of two extremely massive bodies would cause wrinkles in the very fabric of the universe.

Gravitational waves were detected for the first time in 2015, when the LIGO observatory picked up the signal from two black holes doing more than one. Since then, LIGO and its European counterpart, the Observatory of the Virgin, have detected new black hole mergers, as well as the collision of two neutron stars. LIGO and Virgo both detected S190814bv, and if it is actually a star-black neutron fusion, it is the third type of distinct collision captured by gravitational waves.

Although the detectors have also detected a melting of the neutron and the black hole on April 26th, the researchers say that S190814bv is much more convincing. The April event has a one in seven chance of being the sound of the Earth, and false alarms resembling the April signal should appear once every 20 months. But S190814bv almost certainly came from beyond our planet and, to see a false alarm resembling S190814bv, the LIGO team believes that it would take longer than the age of the universe.

"This is something that is much more exciting," says Christopher Berry, a member of the LIGO team, a physicist at Northwestern University. "It is much more likely to find a real one, which means it's worth investing more time and effort."

LIGO and Virgo also tracked the origin of the S190814bv in an oval sky zone about 11 times wider than the full moon, allowing telescopes to follow unusual lightning bolts. Instruments from around the world and in orbit have suspended their regular observations to join the hunt, posting their first results in real time.

"It's very exciting," said Aaron Tohuvavohu, a scientist at NASA's Swift Telescope Observatory, who was looking for flashes of X-rays and ultraviolet light in the same part of the sky as the gravitational signal. "I have not slept all night and I am very happy to do it."

If Swift and other telescopes saw the brilliance of the collision felt by LIGO and Virgo, it was a huge problem for astronomy, because the light would allow scientists to see for the first time the bowels of a star to neutrons, and possibly test the limits. of relativity in new ways.

"It would be fantastic [and] a dream for a theoretician, "says Vicky Kalogera, a member of the LIGO team, a physicist at Northwestern University.

However, it is not accepted that telescopes will see anything. Current theory predicts that collisions of neutron stars and black holes do not always give off light, depending on the comparison of the masses of the two objects.

The closer the masses of the black hole and the neutron star are, the longer it takes for the star to spin into the black hole. This allows the pair to articulate more closely into orbit, giving the black hole more opportunity to gravitationally shred the neutron star. Before these brilliant confetti fall into the black hole, they can emit light that telescopes can capture.

But if the black hole is much larger than the neutron star, it can engulf the entire star with little muss or fuss, without giving off light. Kalogera says scientists are still studying S190814bv data to limit the black hole mass, which should clarify the situation for this event.

Another stranger possibility is that the smaller object of S190814bv is not a neutron star at all.

LIGO and Virgo classify the mergers they see according to the estimated mass of objects in each collision. Any mass less than three times the mass of our sun is considered a neutron star. Anything over five times the mass of our sun is considered a black hole. In this case, the smaller object in S190814bv is estimated to be less than three solar masses.

Although less massive black holes may theoretically exist, the X-ray measurements of the cosmos have not yet revealed any signs. Similarly, our best theories about neutron stars say that if they exceed much more than two solar masses, they will collapse into black holes. What happens if this space between three and five solar masses simply reflects a space in our observations and that the smallest object of S190814bv is a black hole the size of a pint?

"There are really two mysteries that this event could tell us about," says Berry. "What is the maximum mass of a neutron star and what is the minimum mass of a black hole?"

The subtle characteristics of gravitational waves could allow scientists to determine the identity of the smallest object in S190814bv. And if the tracking measures detect a remanence (which, according to Kalogera, could take weeks), it would almost confirm that the smaller object is a neutron star.

Whatever the signal that ends up being, it will be a first, said Berry: "It's a win-win situation."