For the first time ever, astronomers have directly witnessed the birth of a super dense object far outside of our galaxy — the rise of either a black hole or a collapsed star in real time. Up until now, we’ve only ever seen these objects many years after they’ve first formed. But now, we can study this creation in its early days, giving us novel insight into what these mysterious phenomena look like when they first come into existence.
The discovery, nicknamed “The Cow,” came as a delightful surprise during a routine survey of the night sky. Last year, a group of astronomers using the Keck Observatory’s twin telescopes in Hawaii were looking for transients — astronomical explosions that suddenly appear with a flash in the sky and then fade away. On June 17th, an incredibly bright one popped into being, and in just two days it had already peaked in brilliance. The result was a stellar event 10 to 100 times more luminous than your average star explosion or supernova.
“This is an unusual event that’s really exciting.”
At first, astronomers were puzzled. They typically never see supernovae this bright. But upon closer examination, they realized that they had something special on their hands. The radiation from the core of this blast was shining through all of the material that had been ejected during the explosion, revealing something incredibly dense that we’re usually unable to see.
Star explosions normally create giant bubbles of material around them, blocking whatever is inside from our view. But this time, astronomers could actually get a signal from deep inside the blast. “Normally in a supernova, the compact object that’s formed is hidden. This is an unusual event that’s really exciting,” Duncan Brown, a professor of physics at Syracuse University and a gravitational waves researcher, who was not involved with the study, tells The Verge.
It’s too early to say whether or not the explosion resulted in a black hole. It’s possible it formed into a type of stellar corpse, known as a neutron star, which is also incredibly dense. The good news is now that we’ve found it, we can continue observing it and watch it evolve, something we haven’t been able to do before. And how this creation changes could help reshape our theories as to what happens to black holes and neutron stars just after they’re created. “We see them thousands of years afterward, but we know nothing of what they do at the beginning,” Rafaella Margutti, an astrophysicist at Northwestern University who led the research, to be published in the Astrophysical Journal, tells The Verge.
“we know nothing of what they do at the beginning.”
We’ve known for a while that black holes and neutron stars form as leftovers from star explosions. When super massive stars run out of fuel, they burst outward, shedding their outer layers of material. What’s left underneath is a dense core, something much smaller than our Sun in size, but packed with the same amount of material. We have seen evidence of this process, too. When we look at the remnants of supernovae, thousands to millions of years after they occur, we see traces of these dense objects in their place. But the youngest black holes and neutron stars are always hidden from view by the outer layers of the exploding star shooting outward.
The first big clue that Margutti and her team had something truly unique came when they measured the X-rays stemming from The Cow. They found an abundance of “hard” X-rays, which are 10 times more powerful than your average X-ray. This kind of signal is what some astronomers call a “hump,” and it’s usually associated with black holes. This signal strongly suggests that something inside the supernova is gobbling up material, as black holes often do. “So there is something alive in The Cow that is producing these hard X-rays,” says Margutti. “That’s the crucial message from the observation.” She adds: “It’s something we’ve never seen in a transient before. It’s completely unprecedented. Margutti says The Cow got its nickname because it happened to be designated as AT2018cow as part of the team’s naming scheme.
Margutti and her team think they were able to see this object because the star that exploded didn’t shed a lot of material in the blast. That way, there wasn’t as much stuff to shield the inner radiation from view. This may also explain why it got so bright so quickly. Normally supernovae take weeks to reach their peak brightness. The fact that this one got so luminous in just two days is really bizarre, and it may be because there was less material to block the light from our sight. As to why this happened, the team doesn’t know for sure. It may be because most of the material from the blast might have fallen back into the black hole or neutron star. “We speculated about it, but in full honesty, I don’t know,” Margutti says. “We still don’t know.”
It also helped that this explosion happened relatively close by — in the cosmic scheme of things — just 200 million light-years away. It makes everything a bit easier to observe. And to truly understand this event in more detail, astronomers will need to keep watching it in the weeks and months ahead. Right now, it’s too close to the Sun in the sky to see. But after next week, it should be back in a good spot for prime viewing.
And the details we glean from this event could fill us in on what happens to black holes and neutron stars when they are just newborns. How do they change in size? How are they rotating? With The Cow, astronomers hope to get closer to answering those questions. “We’ve seen isolated neutron stars, neutron stars crashing into each other, and we’ve seen material falling into black holes,” says Brown. “This observation could very well be these things being born. That’s pretty cool.”