The mysteriously bright glow of this summer's 'Cow' event gained international interest

The stars are giant gas balls that constantly explode with activity, mainly fusing and incorporating hydrogen into helium. As they age, they make different items. For stars that are five times larger or more massive than our sun, their core is ultimately converted into iron. When the fusion stops and the forces that balance the internal and external pressure cease, the star explodes.





After the combination of several imaging sources, including hard-x-rays and radio waves, the international team led by a Northwestern University is approaching to understand the mysteriously bright object which explodes in the northern sky this summer. The team now estimates that the telescope captured the moment; the star was demolished to make a precise compact object like a black hole or neutron star. Stellar debris rotating around the event horizon of the object caused a remarkable bright glow of the cow.





Birth of a black hole or neutron star captured for the first time





On 17 June 2018,, the twin telescopes of the ATLAS survey in Hawaii found a spectacular bright anomaly and picked up a bright burst in a galaxy 200 million light-years away in the constellation Hercules. AT2018cow or "The Cow" was dubbed, the object quickly flared, then disappeared almost quickly. In the first instance, astronomers believed that this was a supernova, a star that generally explodes at the end of his life. Except that it did not behave as a supernova completely. Now, in a new study report published in the Astrophysical Journal, astronomers believe that they witnessed in this explosion the birth of a black hole or a neutron star - an incredibly dense and powerful stellar object. The explosion seen in June came from a galaxy called CGCG 137-068. Although astronomers were thrilled to capture a supernova, they were somewhat surprised.

Initially, it was extremely bright, almost 10 to 100 times brighter than other supernovae. Also, when a star explodes, it ultimately fades, but in this case, the star - AT2018cow named "The Cow" - faded very fast compared to other supernovae. Also, the particles flew about 30,000 km/second at a speed of about 10 percent of the speed of light, causing it to flare and fade faster than most supernovae. And instead of reaching extreme brightness in weeks, it happened in some days.

Either it was not a supernova or it was something that astronomers had never seen before. And a team of researchers wanted to solve this puzzle.





The team studied the star at different wavelengths, using X-rays, hard x-rays, gamma rays, and radio waves. Given how bright this thing was and how quickly it reached the top. The researchers knew immediately that they needed a different energy source than regular supernovae. The cow started looking very different from anything they had ever seen in x-rays.

They say that stellar debris revolves around the horizon of the object - an area around a black hole where nothing can escape, creating an unmatched bright glow seen in different wavelengths.





This rare phenomenon will help astronomers better understand physics in the first moments of creating a black hole or a neutron star. "We believe that 'The Cow' is the creation of a rising black hole or neutron star. We know from this theory that black holes and neutron stars are created when a star dies, but we have never seen them immediately after birth,” said Raffaella Margutti, an assistant professor of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences and a member of CIERA, who led the research.





"I think this is really a strong possibility that it is driven by an engine that we call it central engine, so there's some kind of energy in the center, which is certainly possible, but that does not mean the issue is closed. I think there are or there will be other suggestions as well. I think it was not what it was last. But he did some serious work of modeling and analyzed the comments. As a whole, I think this is the most comprehensive paper to find out about this object," said Iair Arcavi, an astrophysicist at Tel Aviv University who was not involved in the study.

A second possibility

If a star is sufficiently large or massive enough, then it can fall into a black hole. But sometimes they also make a neutron star. These types of stars are of small city size, but are much larger than the Sun: the weight of a spoon material is 10 million tonnes. They also have an intense gravitational field. The escape velocity of the Earth - the speed at which we can be free from its gravitational pull – is about 11.2 km/second. For the neutron star, this will be 0.4 times the speed of light.

The Curious Cow

After discovering it for the first time, “The cow” took immediate international attention and left astronomers scratching their heads. For example, the anomaly was abnormally bright - 10 to 100 times brighter than the usual supernova. It also collapsed and disappeared much faster than other star explosions, with particles flying at 30,000 kilometers per second. In just 16 days, the object had already issued most of its strength. In a world where some phenomena last for millions and billions of years, two weeks live up to the blink of an eye.

“We thought it should be a supernova and the greatest raider. But what we have observed challenges our current ideas of stellar death. We immediately knew that this source had moved from inactive to peak in just a few days. That was enough to excite everyone's enthusiasm because it was very unusual, and by astronomical standards, it was very close," Margutti said.

Using the access of Northwestern to observational facilities at the MMT Observatory in Arizona and the W.M. Keck Observatory in Hawaii, as well as remote access to the SoAR telescope in Chile, Margotti took a closer look at the makeup of the object. Margotti and his team examined the chemical structure of The Cow, which found clear evidence of hydrogen and helium, which excluded the merger model of compact objects - like those that produce gravitational waves.

Comprehensive strategy

Astronomers have traditionally studied stellar deaths in optical wavelength, which uses telescopes to capture visible light. On the other hand, Margutti's team uses a more comprehensive approach.

The team saw the object with X-rays, hard X-rays (which are 10 times more powerful than normal X-rays), radio waves and gamma rays. This enabled them to continue studying discrepancies after its initial visible glow. After ATLAS spotted the object, the team of Margutti quickly obtained follow-up observations of The Cow with Nuclear Spectroscopic Telescope Array (NuSTAR) of NASA, soft X-rays and INTEGRAL hard X-ray laboratories at radio antennae at the Very Large Array and XMM-Newton toward The Cow.





Margutti showed the relative nudity of the cow's ability to uncover this mystery. Although the stars can collapse in black holes all the time, large quantities of material around the newborn black hole block the sight of astronomers. Fortunately, about a 10-fold less ejecta swirled revolves around a typical stellar blast. The lack of material allowed astronomers to peer directly through the "central engine" of the object, which revealed itself as a potential black hole or a neutron star.





"A 'light bulb' was sitting deep inside the ejecta of the explosion," Margutti said. "It will have been very hard to look at this in a normal stellar explosion. But The Cow had very small ejecta mass, which allowed us to see the radiation of the central engine directly."





Galactic neighbor

The team of Margutti also benefited from the relative proximity of the star of the Earth. Even though it was nested in a dwarf galaxy called CGCG 137-068, astronomers believe that to be right around the corner.”Two hundred million light years are, by the way, closer to us," said Margutti. "This is the closest thing that we have ever seen."



