The cosmic ‘Cow’ may be a strange supernova

The cosmic oddity known as the Cow may have been a dying star that shed its skin like a snake before it exploded.

Newly released observations support the idea that the burst occurred in a dense environment with strong magnetic fields, astronomer Kuiyun Huang and colleagues report in The Astrophysical Journal Letters June 12.

These new measurements “for the mysterious transient … provide one of the strong hints of its nature,” says Huang, of the Chung Yuan Christian University in Taoyuan City, Taiwan.

Since the Cow appeared in June 2018 as a brief burst of light in a galaxy about 200 million light-years away, astronomers haven’t been sure what to think of it. The initial glow flared more quickly and seemed 10 times brighter than an ordinary supernova, the violent explosion that marks the death of a massive star (SN: 2/18/17, p. 20).

Follow-up observations of the Cow — which got its nickname from the randomly assigned name “AT2018cow” — left two main theories for what it could be: a strange sort of supernova, or an exotic star being shredded by a black hole (SN: 2/2/19, p. 13). But neither theory alone could explain all the Cow’s weird features.

Astronomer Anna Ho of Caltech and colleagues published work in April at arXiv.org that analyzed light from the Cow in a range of wavelengths, from short gamma rays to long radio waves. That work suggested that the light was getting distorted on its journey. So if the Cow is a supernova, it must have exploded in a very dense environment that squashed some of the light emerging from the dying star. But to come to that conclusion, the team had to simplify assumptions about how the explosion’s energy was released.

Now, Huang and colleagues have released new radio wave observations that back up the findings by Ho’s team, without relying on those assumptions. In June and July 2018, Huang’s group used the Atacama Large Millimeter-submillimeter Array in Chile to look at the way the Cow’s light was polarized, a measurement of the light’s preferred direction. Imagine holding a jump rope: If you swing your arm up and down, the jump rope will take on an up-down wave pattern. Swinging left to right gives the rope a side-to-side wave.

The radio waves emitted in the wreckage of a supernova should do the same thing, Ho explains. But if the waves travel through an environment filled with gas, charged particles and magnetic fields, the waves’ preferred direction can get rotated or smeared out. “By the time it all gets out at the end, it can look like a blurred mess,” Ho says.

That’s what Huang and colleagues saw from the Cow: The radio waves essentially had no polarization by the time they reached Earth, suggesting the waves had been tossed about in a dense and turbulent environment.

That environment probably came from the Cow itself, Ho says. Toward the end of the star’s life, it started shedding outer layers of gas, similar to a snake shedding its skin. Those discarded layers were still nearby when the star finally ran out of fuel and exploded, so the light and material from the explosion plowed through the debris from the star’s death throes.

“That might actually be a common thing that stars do,” Ho says. She and her colleagues observed another stellar explosion in September, SN2018gep, that first appeared to be a Cow-like event. It ended up looking more like a straightforward supernova, with ordinary speed and brightness — but one that was also surrounded by the dense layers the star tossed off before it died.

The new polarization observations by Huang’s team aren’t the final word on the Cow’s identity, though, says astronomer Daniel Perley of Liverpool John Moores University in England. “It supports one argument,” he says, “but doesn’t overall change the balance of the somewhat contradictory evidence pointing in different directions for this event.” More work on the shredded star theory could help break the tie, he says.