Watching the COW Come Home

Discovery image of AT2018cow – nicknamed The Cow by astronomers – acquired by the ATLAS telescopes. Image via Stephen Smartt/ATLAS.

Discovery image of AT2018cow – nicknamed The Cow by astronomers – acquired by the ATLAS telescopes. Image via Stephen Smartt/ATLAS.

The COW in this case is a star – well, the remnants of a star – in a galaxy called CGCG 137-068. It’s about 200 million light years away.

And COW is a stellar anomaly, something new in the heavens, whose light and other radiation is just now reaching us here on Earth.

COW – technically, the event AT2018cow – isn’t just a supernova. A supernova is a large star collapsing in on itself as its nuclear fuels are exhausted, and then exploding. The Orion Nebula is the debris of a supernova.

COW is something else, because the debris from the stellar explosion is continuing to accelerate as it moves away from the remains of the star. Something is powering the expansion of the debris cloud, and that’s what’s new.

COW was discovered by Stephen Smartt, an astronomer at Queen’s University Belfast, in the United Kingdom, on June 16. Unlike the slow ramp-up of a typical supernova, COW became stupendously bright essentially overnight. So bright that even amateur astronomers could see it, even at a distance of 60 megaparsecs. No other observed star has displayed this pattern of behavior, ever.

The visible light from COW diminished quickly, but the shorter wavelength radiation continued for weeks. It’s as if there were ongoing explosive forces. That can only happen if something is powering the remnant star following the supernova explosion. In the x-ray spectra, it looked as if the remanent was being re-heated from the inside. No other observed supernova has displayed that pattern of behavior, either.

A whole menagerie of stellar mechanisms have been proposed to explain the observed data. Explanations have included a type Ic supernova, a gamma-ray burst, an interaction between a white dwarf and black hole, and as a magnetar. None of them exactly match the data being seen.

But the most likely explanation based on observed data so far is either a black hole or a very rapidly spinning neutron star.

A black hole can develop when an exceptionally large star explodes in a supernova, leaving a dense core. The gravity in such an a dense core is too powerful to permit light (or any other energy) to escape, but in-falling matter, as it accelerates under the influence of that gravity well, becomes very energetic. Another possible outcome for an exploding very large star is a very rapidly spinning neutron star. When a large star collapses to neutronium, the radius shrinks dramatically. The law of conservation of momentum requires the neutron star to spin much, much faster. That, too, can serve as a high energy source, creating the kind of data seen in COW.

WC’s hope is that it does turn out to be the birth of a black hole. And that future data helps sort it out. In the meantime, it’s another interstellar mystery. And mysteries are cool, because sometimes they require new theories.

A kind of Thomas Kuhn moment in astrophysics, perhaps.