Witnessing a cosmic birth

A brilliant blue flash that telescopes around the world picked up in June 2018 from the spiral arm of a galaxy 200 million light-years away could have been the birth of a compact object such as a black hole or a neutron star, according to a recent study led by an Indian-American scientist.

Researchers led by Dheeraj Pasham, a scientist at the Kavli Institute of Astrophysics and Space Research, Massachusetts Institute of Technology (MIT), presented this theory in a paper published in Nature Astronomy in December 2021.

The powerful burst - a fast blue optical transient, or FBOT - appeared at first to be a supernova, but it was much faster and far brighter than any stellar explosion recorded so far.

Named AT2018cow, or the Cow, the signal was subsequently catalogued as a bright, short-lived event of unknown origin. Now, the team led by the MIT Einstein Fellow has found strong evidence for the signal's source. In addition to a bright optical flash, it detected a strobe-like pulse of high-energy X-rays. They traced hundreds of millions of such X-ray pulses back to the Cow, and found the pulses occurred at regular intervals of 4.4 milliseconds over 60 days.

Based on the frequency of the pulses, the team calculated that the X-rays must have come from an object no more than 1,000-km wide, with a mass smaller than 800 suns. By astrophysical standards, such an object would be considered compact, much like a small black hole or a neutron star. Black holes have strong gravity that does not allow even light to escape; neutron stars are dead stars that are incredibly dense.

"We think we have found the most compelling evidence yet for the birth of a compact object in a supernova," said Pasham.

Pasham, who studied Aerospace Engineering at IIT Bombay (2004-2008), and his colleagues suggested that AT2018cow was likely the product of a dying star that, in collapsing, gave birth to a compact object in the form of a black hole or a neutron star. The newly born object continued to devour surrounding material, eating the star from the inside - a process that released an enormous burst of energy.

"Matter inside neutron stars is compressed to the most extreme densities possible in nature without collapsing into a black hole. If AT2018cow represents the birth of a neutron star, then it may allow us to study the equation of state of hot and young neutron stars and probe fundamental aspects of matter. If it is a black hole, then it is possible that a newly formed black hole formed an accretion disc around it that wobbled and produced this 225-Hz (4.4-millisecond) signal. In either case, as more such events are expected to be discovered in the coming decade, they would provide us a unique window into our universe," he explained.

AT2018cow is among a few dozen known FBOTs, and it is one of only a few such signals that have been observed in real time. Its powerful flash - up to 100 times brighter than a typical supernova - was detected by a survey in Hawaii, which immediately alerted observatories around the world.

"If AT2018cow originated from the death of a massive star, our findings represent the birth of a compact object in a supernova. Similar signals in future FBOTs could allow astronomers to study infant compact objects immediately after birth," said the scientists in the Nature Astronomy paper.