“Somewhere,” said Carl Sagan, “something incredible is waiting to be known” –“I’ve never seen anything like this before in the local universe,” said Stephen Smartt, an astrophysicist at Queen’s University Belfast and a lead scientist for the Hawaii-based ATLAS survey, about “something incredible” –a mysterious cataclysm in a neighboring galaxy that rocked the world’s astronomy community with its discovery on June 16, 2018.
“Ten Times as Bright as an Ordinary Supernova”
“It popped up out of nowhere,” says Smartt about the spectacular stellar explosion, weirdly named “The Cow,” that has offered an unprecedented window on the collapse of a star that became unbelievably bright essentially overnight. In the course of just three days, AT2018cow became about 10 times as bright as a normal supernova with a jet of particles moving close to the speed of light. But the Cow was no ordinary supernova, reaching its peak brightness in days, not weeks.
A colleague flagged a bright star at a spot where there was nothing just days before. Smartt initially suspected the flash must have come from something in our Milky Way Galaxy, because it was so bright. But after spectroscopic analyses of the object, separating the light out into its component wavelengths, it turned out that AT2018cow had features associated with CGCG 137-068, a galaxy in the constellation Hercules, stretching its light out over the course of its 200-million-light-year journey to Earth.
“The Cow”
The Cow’s origins are unclear: previous observations of the region around it did not find a type of gas that is a marker of a massive star, reported Nature in “The explosive birth of a celestial ‘Cow’”. “Some astronomers thought the Cow’s unusually quick initial burst of light was caused not by a supernova, but by a white dwarf star being torn apart in a collision with another celestial body.”
At first, reported Nature, Smartt discounted the effect as an unremarkable stellar flare in the Milky Way, but he soon realized that it was probably much farther off, in a galaxy called CGCG 137-068 known to be around 60 megaparsecs (200 million light years) away. “It was 11 o’clock on a Sunday night, and I said to myself, ‘I better tell everybody about this.’” He sent out an alert through the Astronomer’s Telegram, a service for reporting and commenting on transient astronomical observations. Thanks to the site’s randomized three-letter naming system, the object has been dubbed AT2018cow, or “The Cow” for short.
A look at The Cow approximately 80 days after explosion, from the W.M. Keck Observatory in Maunakea, Hawaii.
“Everybody put down what they were doing up to that point” and started following Cow, says Daniel Perley, an astrophysicist at Liverpool John Moores University, UK. Perley and his collaborators commanded a robotic telescope on La Palma, one of Spain’s Canary Islands, to observe Cow nearly every night for a month and a half. They also used a number of other telescopes around the globe that belong to a network designed just for this kind of follow-up study.
“A Central Engine”
Astronomers, reported Nature, arrived at the conclusion that unlike an ordinary supernova, this short-wavelength radiation lasted for weeks revealing the presence“ of a ‘central engine’ that has kept agitating the exploding star from the inside for months and that the energy must have come from either a newly formed black hole in the process of accreting matter, or the frenetic rotation of a neutron star.”
“We were able to show that it’s not consistent with any of the usual mechanisms,” said Anna Ho, an astrophysicist at the University of California, Berkeley, then at Caltech, who moved quickly to observe Cow in the radio spectrum. In a stellar explosion, charged particles emit radio waves as they spiral inside strong magnetic fields, and their wavelengths stretch out as the material spreads out.
Ho made an emergency proposal to the Atacama Large Millimeter/submillimeter Array (ALMA) in the Chilean Andes, where over several weeks, Ho and her collaborators watched the spectrum of the event’s millimeter emissions as it evolved. Their observations revealed that matter was expanding outwards as fast as one-tenth of the speed of light. But unlike an ordinary supernova, this short-wavelength radiation lasted for weeks revealing the presence of the central engine — a black hole or a spinning neutron star. “We were able to show that it’s not consistent with any of the usual mechanisms,” Ho says.
“Pretty much everything about its emission is something we haven’t seen before,” says Iair Arcavi, at the University of California, Santa Barbara, currently an observational astronomer at Tel Aviv University.
A Rare Transient, One-off event?
But was the Cow a rare transient, one-off event that defies explanation?
Now, two-years later the discovery of a supernova-like explosion seen in real-time, dubbed the Camel, appears to suggest, “no” but rather, that The Cow was the result of a newborn black hole eating a star from the inside out.
Beginning on October 12, 2020, telescopes watched as something in a galaxy 3 billion light-years away became incredibly bright, then disappeared from view, reports Jonathan O’Callaghan for Quanta. “ It behaved almost identically to the Cow, astronomers reported in a paper posted to the online preprint site arxiv.org last week, leading them to conclude that it must be the same type of episode. In keeping with tradition, it was given its own animal-inspired name: the Camel.”
The “Camel” Confirms the “Cow” –Core Collapse Straight Into a Black Hole
“It’s really exciting,” said Deanne Coppejans, an astrophysicist at Northwestern University. “The discovery of a new transient like AT2018cow shows that it’s not a complete oddball. This is a new type of transient that we’re looking at.”
Like its predecessor, reports O’Callagan, “the Camel became very bright in a short time, reaching its peak in two or three days. It grew about 100 times brighter than any normal type of supernova. Then it rapidly dimmed in a process that lasted just days, rather than weeks.”
“It fades very fast, and while it’s fading it stays hot,” Perley said.
“The explosion itself and the sort of zombie afterlife behavior, those are quite similar,” said Ho, who was part of the Camel discovery team.
The concluding conjecture according to Quanta, is the “failed-supernova hypothesis” which begins when a star around 20 times the mass of our sun reaches the end of its life and exhausts its fuel causing the core to collapse, beginning what would normally be a regular supernova, where infalling material rebounds back out, leaving behind a dense object called a neutron star.
But with a twist: in cases like the Camel and the Cow, says Perley, “something unusual happens in the process to core collapse. What we claim is that instead of collapsing to a neutron star, it collapsed straight into a black hole, and most of the star fell into the black hole.”
Maxwell Moe, NASA Einstein Fellow, University of Arizona., via Nature and Quanta
Image credit: NASA/JPL
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona. Max can be found two nights a week probing the mysteries of the Universe at the Kitt Peak National Observatory. Max received his Ph.D in astronomy from Harvard University in 2015.