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Holy Cow! A super-bright stellar blast is likely a dying star giving birth to baby black hole or neutron star

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The find, which is based on an unusual cosmic flash known as “the Cow”, could provide astronomers with a new approach to detect young tiny objects.

A dazzling blue flare from the spiral arm of a galaxy 200 million light years away was detected by telescopes all over the world in June of 2018. The strong blast appeared to be a supernova at first glance, but it was considerably faster and brighter than any star explosion scientists had previously witnessed. The signal, codenamed AT2018cow, has subsequently been called “the Cow” by astronomers, who have classified it as a fast blue optical transient, or FBOT, a brilliant, short phenomenon of unknown origin.

Now, a team led by MIT has discovered compelling evidence for the signal’s origin. A strobe-like pulse of high-energy X-rays was discovered in addition to a dazzling visual flash. They tracked hundreds of millions of such X-ray pulses back to the Cow and discovered that they occurred at the same time every 4.4 milliseconds for 60 days.

Based on the frequency of the pulses, the scientists concluded that the X-rays must have come from an object no larger than 1,000 kilometers across and weighing no more than 800 suns. Such an item, by astronomical standards, would be considered compact, similar to a miniature black hole or neutron star.

Their observations, published in the journal Nature Astronomy, clearly suggest that AT2018cow was most likely the result of a dying star collapsing and giving birth to a compact object in the form of a black hole or neutron star. The infant body continued to consume surrounding matter, devouring the star from within – a process that generated a massive explosion of energy.

“We have likely discovered the birth of a compact object in a supernova,” said Dheeraj “DJ” Pasham, lead author.

“This happens in normal supernovae, but we haven’t seen it before because it’s such a messy process. We think this new evidence  opens possibilities for finding baby black holes or baby neutron stars.”

“The core of the Cow”

AT2018cow was detected as one of various “astronomical transients” in 2018. The “cow” in its name is a natural outcome of the astronomical naming process (for example, “aaa” refers to the first astronomical transient found in 2018). The signal is one of a few dozen known FBOTs, and it is one of just a few that have been observed in real-time. Its spectacular light, which was up to 100 times brighter than a usual supernova, was identified by a survey in Hawaii, which promptly alerted observatories all over the world.

“It was exciting because loads of data started piling up,” Pasham added.

“The amount of energy was orders of magnitude more than the typical core collapse supernova. And the question was, what could produce this additional source of energy?”

Astronomers have proposed a number of explanations for the super-bright signal. It could, for example, be the result of a black hole born in a supernova. It might also be the result of a middle-weight black hole stealing material from a passing star. However, the data obtained by optical telescopes has not definitively established the source of the signal. Pasham wondered if X-ray data may provide a solution.

“This signal was close and also bright in X-rays, which is what got my attention,” said Pasham.

“To me, the first thing that comes to mind is, some really energetic phenomenon is going on to generate X-rays. So, I wanted to test out the idea that there is a black hole or compact object at the core of the Cow.”

Finding a pulse

NASA’s Neutron Star Interior Composition Explorer (NICER), an X-ray-monitoring telescope atop the International Space Station, allowed the researcher to obtain X-ray data. NICER began watching the Cow about five days after it was first seen by optical telescopes, and continued for the next 60 days. This information was saved in a publicly accessible archive, which Pasham and his colleagues accessed and evaluated.

The team examined the data for X-ray signals coming from near AT2018cow and confirmed that the emissions were not caused by instrument noise or cosmic background phenomena. They concentrated on the X-rays and discovered that the Cow was emitting bursts at a frequency of 225 hertz, or once every 4.4 milliseconds.

Pasham was drawn to this pulse because he realized that its frequency could be used to immediately calculate the magnitude of whatever was pulsing. The size of the pulsing item in this example must not exceed the distance that light can reach in 4.4 milliseconds. Using this logic, he concluded that the object’s dimensions must be no more than 1.3×108 cm, or around 1,000 kilometers wide.

“The only thing that can be that small is a compact object — either a neutron star or black hole,” added Pasham.

Based on the energy emitted by AT2018cow, the team concluded that it must be no larger than 800 solar masses.

“This rules out the idea that the signal is from an intermediate black hole,” Pasham said.      

Apart from identifying the source of this signal, Pasham claims that the research shows that X-ray analysis of FBOTs and other ultrabright occurrences could be a new tool for investigating infant black holes.

“Whenever there’s a new phenomenon, there’s excitement that it could tell something new about the universe,” Pasham said.

“For FBOTs, we have shown we can study their pulsations in detail, in a way that’s not possible in the optical. So, this is a new way to understand these newborn compact objects.”

Image Credit: NASA

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