Huge Cosmic Explosion Triggered by Birth of Black Hole or Neutron Star

Astronomers have traced the source of a bright blue cosmic explosion back to the birth of a neutron star or a black hole. The supernova, first spotted in 1998 by telescopes around the world, is remarkable because it was faster and 100 times brighter than a typical supernova. However, the source of this powerful explosion has remained a mystery.

Whereas other supernovae take weeks or even months to reach maximum brightness, this one, nicknamed "The Cow" by astronomers, reached its maximum brightness in just a matter of days.

The explosion, officially titled AT2018cow, was logged by astronomers as a fast blue optical transient (FBOT), a bright, short-lived event that is of unknown origin. Now, thanks to a team of scientists led by researchers from the Massachusetts Institute of Technology (MIT), the source of the Cow, located in the spiral arm of a galaxy 200 million light-years away, may have been revealed.

Using regular X-ray pulses traced back to the Cow, the team determined the event was linked to the creation of an object that could be no wider than 630 miles but with a mass of around 800 suns.

This kind of compact massive object marks either a black hole or a neutron star, compact stellar remnants that form when massive stars run out of nuclear fuel and can no longer protect themselves against complete gravitational collapse.

"We have likely discovered the birth of a compact object in a supernova," research scientist at MIT's Kavli Institute for Astrophysics and Space Research, Dheeraj "DJ" Pasham, said. "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."

Following the birth of this black hole or neutron star, it has continued to greedily consume its surrounding material, in the process releasing a massive burst of energy.

Pasham is the lead author of a study detailing the discovery published in the journal Nature Astronomy.

The Cow is only one of a few dozen FBOTs that have been observed in real-time by astronomers, first picked up by observations conducted by the Haleakala Observatory in Hawaii. This initial observation was quickly followed by further sightings of the Cow, which remained bright for three weeks, by telescopes across the globe.

Several attempts had been made prior to this research to explain the source of the Cow, with some suggesting that it could have been created by a black hole stripping material from a companion star. But, what really caught Pasham's attention wasn't optical observations of the region, but X-ray data collected from it by NASA's Neutron Star Interior Composition Explorer (NICER), an X-ray-monitoring telescope aboard the International Space Station.

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

This revealed that the Cow was giving off bursts of X-rays once every 4.4 milliseconds, with Pasham using the frequency information to directly calculate the size of whatever was pulsing.

Reasoning that whatever the object at the heart of the Cow is can't be larger than the distance light can cover in 4.4 milliseconds, Pasham determined that the object can't be wider than around 630 miles. The energy emitted by the object told the MIT researcher and his team that it must have a mass of around 800 times that of the sun.

"The only thing that can be that small is a compact object—either a neutron star or black hole," Pasham said, adding that the mass calculations ruled out a larger, intermediate-mass black hole.

Pasham believes that the technique of X-rays to study FBOTs could be used to find and study more 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."

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An artist's impression of a powerful supernova explosion. Astronomers have discovered that a supernova found in 2018 likely marked the birth of a black hole or neutron star. G.Bacon STSci/ESA/NASA