When astronomers met in Seattle last week, a fascinating topic was the brief and unusual flash in the night sky, seen from around the world, on June 16, 2018. For three days, something it produced a sudden burst of light at least 10 times brighter than a typical supernova or an exploding star. The light went on and then disappeared from view for several months, but not before this mysterious event became one of the most studied in history.
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This unusual event occurred in or near a star-forming galaxy known as CGCG 137-068, located 200 million light-years away, in the direction of our Hercules constellation. The explosion is officially labeled AT2018cow, and nicknamed Cow for the last three letters of the official designation. The event was different from anyone seen before. Astronomer Ryan Chornock of the University of Ohio, who helped study it, said in a statement:
We knew immediately that this source went from inactive to maximum luminosity in a few days. That was enough for everyone to be enthusiastic, because it was very unusual and, by astronomical standards, it was very close.
Researchers have decided on two possible interpretations of this event, which they shared in Seattle at a round table on January 10, 2019. The two groups with different views have accepted the publication of articles, which provides two different explanations about the origins of the Cow A document suggests that the Cow is a black hole monster that destroys a pbading star. The other hypothesis raises the literal birth of a black hole or a neutron star. If the second scenario is correct, the explosion was in fact a supernova.
What scenario is true? Let's look at both.
Discovery image of AT2018cow, nicknamed the cow by astronomers, acquired by the ATLAS telescopes. Image via Stephen Smartt / ATLAS.
Black hole destroying a white dwarf?
The video at the top of this post shows the first scenario: a black hole that destroys an evolved star known as a white dwarf. Astronomers call this an event of tidal interruption because it is an effect of the gravity of one body on another, in the same way that the gravity of our moon draws our watery Earth to cause the tides. A statement from NASA explained that, near a black hole, the force of the tide would be similar:
… but more powerful in an approaching star, finally breaking it in a gas stream. The tail of the gas stream is ejected from the system, but the leading edge rotates around the black hole, collides with itself and creates a cloud of elliptical material.
This research team used data ranging from infrared radiation to high energy gamma rays, from the Swift observatory in NASA orbit and other observatories. Astronomer Amy Lien of NASA's Goddard Space Flight Center in Greenbelt, Maryland, was part of the team that favored this scenario. She said:
We have never seen anything exactly like La Vaca, which is very exciting. We believe that an interruption in the tide created the rapid and really unusual burst of light at the beginning of the event and better explains the observations of multiple wavelengths of Swift as they faded in the coming months.
These astronomers calculated that the mbad of the black hole oscillates between 100,000 and 1 million times that of the sun, almost as large as the central black hole of its host galaxy. Paul Kuin at University College London, who led the research, explained why a big black hole is needed for this scenario:
The cow produced a large cloud of debris in a very short time. Destroying a larger star to produce a cloud like this would take a larger black hole, result in a slower increase in brightness and take more time for the debris to be consumed.
These astronomers pointed out that it is unusual to see black holes of this scale outside a galactic center, but they said:
… it is possible that the Cow has occurred in a nearby satellite galaxy or in a cluster of globular stars whose oldest stellar populations could have a higher proportion of white dwarfs than average galaxies.
Or a black hole or a neutron star being born?
The second team of scientists was able to collect data on the cow in an even wider range of wavelengths, ranging from radio waves to gamma rays. This team now believes that a supernova could be the source of the Vaca and that the event announces the first direct observation of astronomers of the birth of a black hole or a neutron star. Astronomer Raffaella Margutti at Northwestern University, lead author of the article describing this scenario, commented:
We know from theory that black holes and neutron stars are formed when a star dies, but we have never seen them right after they were born. Never.
Margutti's team badyzed data from multiple observatories, including NASA's NuSTAR, ESA's XMM-Newton and INTEGRAL satellites, and the Very Large Array of the National Science Foundation.
The team proposes that the bright optical and ultraviolet flash of the Vaca indicate a supernova and that the X-ray emissions that occurred shortly after the explosion arose from the energy that radiates gas when falling on a compact object.
The astronomer Daniel Perley at the John Moores University of Liverpool in England also studied the Cow and also favors the supernova hypothesis. He commented in another statement:
The properties of the Vaca variety almost all the models that we have tried to design to explain it.
Whatever it is, it must involve some kind of very fast energetic explosion that interacts with an extremely dense layer of material very close to the explosion progenitor.
So, now, half a year after the astronomers first saw the Cow, they are still debating what it is and how it really happened.
What was it? AT2018cow, also known as the Cow, emerged in or near a galaxy known as CGCG 137-068, which is about 200 million light-years away in the direction of our Hercules constellation. Image via Sloan Digital Sky Survey.
Conclusion: Astronomers are discussing a brief and unusual flash in the night sky earlier this year. They call her the cow. One group argues that it was a black hole monster that destroys a pbading star. The other argues that it could have been the birth of a black hole.
Read more: Astronomers see a mysterious explosion 200 million light years away
Source: a built-in X-ray source shines through the aspheric AT2018cow: reveals the internal workings of the brightest optical transients of rapid evolution
Source: The fast and luminous ultraviolet transient AT2018cow: Extreme supernova or interruption of a star by a black hole of intermediate mbad?