Sorting Out "the Cow"

"Holy cow!" was the collective exclamation from astrophysicists when they first glimpsed the cosmic event randomly and amusingly designated AT2018cow. Monikered "the Cow" in short order, this phenomenon blazed into view on June 16, 2018, perplexing researchers with its bizarre properties. Although evidently a supernova (a stellar explosion) given its light spectrum, that spectrum did not mesh well with established theory. Weirder still, the event ramped up to peak brightness within a single day, whereas regular supernovae tend to peak about three weeks after the initial explosion. The peak brightness itself astounded further, measuring about 100 times brighter than a typical supernova.

Researchers—including members of Kavli astrophysics institutes—knew they had a strange one on their hands. Thankfully, subsequent to the Cow's discovery, several more similar events have now been captured and analyzed (and with other amusing nicknames, including Koala and Camel). The accumulating data and models are now starting to pin down just what the Cow and its brethren must be.

"Such behavior has been unknown and is a mystery," says Ken'ichi Nomoto, a Senior Scientist at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo. "It is very interesting and important to explore what is the power source of such a bright light [as the Cow], what is the explosion mechanism, and what kind of stars make such an explosion."

Nomoto is a coauthor of two recent studies (here and here) that have proposed mechanisms behind the Cow-like blasts, which have since come to be dubbed FBOTs, for fast blue optical transients. In addition, a recent study by researchers at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MKI) has reported key data that reinforces the models proposed by Nomoto and company.

The researchers began with the ample observations of the Cow and another explosion, 'the Gep" (technically SN 2018gep), taken by observatories all over and above our planet. Working with the fair assumption that the events were supernovae, the researchers set about constructing plausible supernovae architectures fitting the observations of extreme brightness in visible light, coupled with a fast pulse at higher, or "bluer" energies (that is, ultraviolet light and x-rays that have more energy than the highest-energy visible light).

A good starting point was assuming that FBOTs fundamentally result from a massive star's demise, as many supernova varieties do. As monster stars age, the fusing of chemical elements in their cores can no longer generate enough energy and outward pressure to resist the inward crush of gravity. This loss of equilibrium results in a sudden, catastrophic collapse of the star's core unto itself and culminating in an explosion. Sophisticated models of such supernovae have started granularly accounting for the gradual shedding of stars' outer layers into space before the core collapse occurs. This sloughed-off stellar material likely plays a key role in making the Cow and the Gep appear so bright.

Nomoto and colleagues modeled how the ejected materials could form a dense shell around the dying star. In essence, the shell would act as a means of making the pre-supernova star appear bigger, such that when it blew up, a larger area would glow, thus producing more light than a smaller-sized, normal supernova.

"Our idea is that the progenitor star" — the star that eventually goes kablooie — "has ejected a lot of its outer layer to form a massive, dense circumstellar [shell] just before the explosion," says Nomoto.

Sure enough, this model predicted that the shock wave formed from the exploding star would smash into the previously expelled shell and cause it to light up suddenly. This grand show—duly witnessed by human telescopes hundreds of millions of years later when the light finally arrives at distant Earth—would be short-lived. As the shocked material quickly cooled down over the course of a few days, it would become transparent and allow more conventional light from the supernova in the shell's interior to dominate. Overall, this blow-by-blow matched the weeks-long observations of the light curve of the event, referring to how light intensity and characteristics change with time.

Researchers calculate that a colossal progenitor star with about 100 times the mass of our sun would check many of the progenitor boxes. At such masses, though, stars are expected to die in an unusual way, termed a pair-instability supernova. The pressures and energies get so extreme in the centers of such stars that antimatter forms in ample quantities, only to then undergo immediate annihilation with the ample matter on hand. The tremendous amount of energy these annihilations produce drop the core's total mass, triggering a partial collapse that serves in ramping up the total power of the final supernova explosion.

The expectation is that these exotic supernovae do not leave behind remnants of their collapsed cores in the form of neutron stars or black holes, as is the way of plain ol' supernovae. Yet other observations of the Cow—the focus of the additional MKI-led study—squarely point to the existence of a core remnant. Those results suggest that the Cow-spawning supernovae, while extreme, might not have crossed over into pair-instability territory.

The observations in question consisted of X-ray pulses captured by the NICER instrument on the International Space Station. Like clockwork, the pulses zapped out of the Cow every 4.4 seconds over the course of around two months after its initial appearance. The MKI-led study crunched the numbers on these pulses, finding that a fast-spinning object no more than 1,000 kilometers in diameter and with a mass less than 800 suns could spit out such pulses with such timing.

Nomoto points out that this second study's conclusion is not at odds with his team's findings and actually helps complementarily resolve some of the Cow's features. "The existence of compact remnant can give a 'hint' of the explosion mechanism and can explain the observed X-ray emission from AT2018cow," says Nomoto. "Our light curve model with circumstellar matter can nicely explain the early behavior of the light curve of AT2018cow, but needs an additional energy source at a later phase. The power from the observed compact object is quite consistent with our model."

While the case may therefore be closing for the Cow, much remains unexplained about FBOTs in general. Their explosions are not as symmetrical and "neat" as regular supernovae, and their ultra-brightnesses can vary considerably. "Some variation of FBOTs needs to be explained," says Nomoto. His team's models do offer some flexibility, though. A main source of FBOT variation could stem from the particular properties of the expelled shell, which themselves stem from the progenitor stars coming in a range of conceivable masses.

For now, astrophysicists will keep their eyes peeled and their minds open to whatever FBOTs throw at them next.