The birth of a black hole or a neutron star observed for the first time

Black holes and neutron stars are some of the most mysterious objects in the cosmos. Although the theoretical models, together with the observations made in recent years, reveal some information on these, many puzzles still remain on them. This is particularly the case of their formation which, for the first time, was observed by an international team of astronomers.

An international team of astronomers, led by Northwestern University, is on the verge of understanding the properties of the mysterious and luminous object that lit up the northern sky this summer, dubbed AT2018cow or "The Cow". With the help of the Maunakea Observatory WM Keck (Hawaii) and the ATLAS Twin Telescopes at the University of Hawaii's Institute of Astronomy, the multi-institutional team now has evidence that they have probably captured the exact moment when a star collapsed to form a compact object, such as a black hole or a neutron star.

Star debris, approaching and swirling around the object's event horizon, caused an extraordinarily brilliant glow. The results of the observations, which will appear in the magazine The Astrophysical Journal, were made public during a press conference during the 233th meeting of theAmerican Astronomical Society in Seattle. This rare event will help astronomers better understand the physics involved in the first moments of the creation of a black hole or a neutron star.

The Cow event was observed in the dwarf CGCG 137-068 galaxy, located 200 million light years away. Credit: R. Margutti / W. Mr Keck Observatory

" According to the X and UV emissions observed, "The Cow" seems to come from the destruction of a white dwarf by a black hole. But other observations on other spectrum wavelengths have led to our interpretation that "The Cow" is actually forming a black hole or a neutron star. Says Raffaella Margutti, an astrophysics at Northwestern University.

" We theoretically know that black holes and neutron stars are formed when a star dies, but we never saw them immediately after their birth. never She adds.

An event with unusual characteristics

The event was first sighted on June 16, after the ATLAS telescopes on Haleakala and Maunaloa captured a spectacular anomaly 200 million light years away in the constellation of Hercules. The object lit up quickly, then disappeared almost as quickly. The event has aroused immediate international interest and left astronomers wary.

" We thought it was a supernova Margutti says. " But what we have observed has questioned our current notions on the processes of stellar death "On the one hand, the phenomenon was exceptionally brilliant: 10 to 100 times brighter than a typical supernova, and it also appeared and disappeared much faster than other known stellar explosions, with particles traveling at 30,000 km / s (or 10% of the speed of light).

Image of AT2018cow and its host galaxy, obtained with Keck's DEIMOS spectrograph. Credit: R. Margutti / W. Mr Keck Observatory

In just 16 days, the object had already emitted most of its power. " We understood that this source had gone from an inactive state to a peak activity peak in a few days He told Ryan Chornock, an astronomer at Ohio University.

On the same topic: The rotation speed of a black hole measured thanks to the gravitational destruction of a star

" Keck has played an important role in determining the chemical composition and geometry of AT2018cow Says Nathan Roth, a doctoral student at the University of Maryland. " Keck's spectroscopic instrumentation allowed us to observe the inside of the explosion and to explore its spectral characteristics, in particular moved towards the red ".

When Margutti and his team examined the chemical composition of The Cow, they found traces of hydrogen and helium, thus excluding the fusion models of compact objects, such as those that produce gravitational waves.

" It took a while to understand what we were looking at, I would say months ago Says Brian Metzger, a physicist at Columbia University. " We tried different possibilities and we had to come back to our hypotheses several times. We finally managed to interpret the results, thanks to the hard work of our incredibly dedicated team ".

A multispectral observation strategy

Researchers looked at the object in hard X-rays using NASA's Nuclear Spectroscopy Telescope Network (NuSTAR) and the gamma-ray gamma-ray laboratory of the European Space Agency (ESA). in mild radiographs (10 times more powerful than normal X-rays). This also includes the X-ray multi-mirror mission of the ESA (XMM-Newton), and for the radio waves, the Very Large Array (VLA) of the National Astronomy Radio Observatory (NRAO).

Left: The Sloan Digital Sky Survey in New Mexico observed the galaxy host Z 137-068 in 2003, The Cow is still undetectable (the green circle indicates where The Cow finally appeared). In the center: the Liverpool telescope located in the Canary Islands, Spain, allowed to observe The Cow near its maximum maximum brightness of the event, on 20 June 2018, while it was much brighter than the host of the galaxy. On the right: the William Herschel telescope, also located in the Canary Islands, has taken a high resolution image of The Cow, almost a month after reaching the maximum brightness as it dissolved and the host galaxy became again visible. Credit: Daniel Perley, Liverpool John Moores University

About 10 times less ejecta revolved around The Cow compared to a typical stellar explosion. This relative absence of debris allowed astronomers to directly examine the "central engine" of the object, which turned out to be a black hole or a neutron star.

Margutti's team also benefited from the relative proximity of the star to the Earth, even though it was in the distant dwarf galaxy CGCG 137-068. " Two hundred million light years represent a short distance in astronomy. It is the closest transient object of this kind that we have ever found Margutti concludes.

Source: NASA
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