They probably witnessed the birth of a small black hole or neutron star.

In June 2018, several telescopes observed a brilliant blue flash from a galaxy 200 million light-years away. The powerful eruption — dubbed “the Cow” — initially appeared to be a supernova, though it was much faster and brighter than any other stellar explosion scientists have spotted so far. Since then, the flash has been known as “a bright, ephemeral event of mysterious origin.” But now, three years later, astronomers believe they can finally explain the Cow.

More about the Cow
What made the Cow so special was the sheer brightness of the object. For example, the object was 10 to 100 times brighter than typical supernovae. The Cow flared up and then disappeared as quickly as it came. Within sixteen days, the object had already radiated most of its power. And that is very unusual. In the universe, where some phenomena last for millions and billions of years, two weeks is just the blink of an eye.

Several scientists have already made frantic efforts to solve the mysterious problem. And although several explanations have already been suggested, each time it turned out that there were loose ends. In a new study, the researchers wondered if there might be important clues to be found in X-ray data. “The signal was close and clear in X-rays, which caught my attention,” said study researcher Dheeraj Pasham.

X-rays

The team analyzed X-ray data collected by NASA’s Neutron Star Interior Composition Explorer (NICER), an X-ray telescope aboard the International Space Station. NICER observed the Cow about five days after its first detection through optical telescopes. This data is contained in a publicly available archive, which Pasham and his colleagues downloaded and analyzed.

‘The Cow’ photographed by the Sloan Digital Sky Survey in a galaxy 200 million light-years away, in the constellation Hercules. Image: Sloan Digital Sky Survey via Wikimedia Commons

pulses

Indeed, in addition to a bright optical flash, the researchers discovered pulses of high-energy X-rays. The research team traced hundreds of millions of such X-ray pulses to the Cow. And it seems these pulses were repeating like clockwork: one appeared every 4.4 milliseconds over a span of 60 days. Based on the frequency of the pulses, the researchers then found that the X-rays must come from an object no more than 1,000 kilometers in diameter, with a mass less than 800 suns. In astrophysical terms it is in that case a ‘compact’ object.

Small black hole or neutron star

According to the researchers, the mystery has now been unraveled. Because it means that the Cow is probably the product of a dying star that, when it collapsed, gave birth to a compact object: probably a small black hole or neutron star. The newborn object continued to devour surrounding material, eating the star from within — a process that produced a massive burst of energy.

supernova

“We probably discovered the birth of a compact object in a supernova,” Pasham says. “This happens in normal supernovae, but we haven’t seen it before because it’s such a messy process. We suspect this new evidence will make it easier to find baby black holes or baby neutron stars.”

It means astronomers have finally pinpointed the mysterious, bizarrely bright flash spotted in space in 2018. But beyond determining the source of this particular signal, Pasham underlines that the study also shows that X-ray analysis could be a new tool for studying compact “baby” objects. “New phenomena are exciting because they can also tell something new about the universe,” Pasham says. “We have now shown that we can study pulsations in detail, in a way that is not optically possible. So this is a new method to learn more about newborn compact objects.”