Celestial Event Dubbed “The Cow” Puzzles Astronomers

Astronomers can’t agree on what’s behind the brilliant source dubbed “the Cow.” Did a medium-mass black hole tear a white dwarf apart, or are we seeing a strange new type of supernova?

Multiple multinational, multi-telescope teams of astronomers recently tracked a celestial blast with unprecedented precision. The results reveal an unresolved mystery: the teams are torn as to whether the burst of light marked the death of a white dwarf torn apart by a black hole or a never-before-seen supernova. Astronomers are still studying the fading remnants to settle the debate.

Astronomers using ground-based observatories caught the progression of a cosmic event nicknamed “the Cow,” as seen in these three images. Left: The Sloan Digital Sky Survey in New Mexico observed the host galaxy Z 137-068 in 2003, and the Cow nowhere in sight. (The green circle indicates the location where the Cow eventually appeared). Center: The Liverpool Telescope in Spain’s Canary Islands saw the Cow very close to the event’s peak brightness on June 20, 2018, when it was much brighter than its host galaxy. Right: The William Herschel Telescope, also in the Canary Islands, took a high-resolution image of the Cow nearly a month after it reached peak brightness, as it faded and the host galaxy came back into view.
Daniel Perley (Liverpool John Moores University, UK)

The event — sudden, luminous, and weird — was picked up by the robotic Asteroid Terrestrial-impact Last Alert System (ATLAS) survey and received the automatically generated name AT2018cow. Naturally, astronomers dubbed the event “the Cow,” and it tore through existing theories of stellar death like a bull in a china shop. At a press conference yesterday at the American Astronomical Society meeting in Seattle, astronomers revved up the puns while also presenting very different views about how the Cow came to be.

Here’s what everyone agrees on: The explosion took place on June 16, 2018, appearing to originate from the spiral arm of a star-forming galaxy 200 million light years away called CGCG 137-068. The explosion took place in the constellation Hercules (which is fitting because one of the mythological Hercules’ labors was to capture a Cretan bull – a male cow).

An image of AT2018cow and its host galaxy was obtained on August 17, 2018, using W. M. Keck Observatory’s instrument, the Deep Imaging and Multi-Object Spectrograph (DEIMOS).
R. Margutti / W. M. Keck Observatory

In just two days, the burst went from nonexistent to a peak luminosity of 100 billion Suns — about 10 times brighter than a typical supernova. Early spectroscopy showed very broad features, indicating that the explosion had tossed out material at incredibly high speeds: 10% of the speed of light.

But what really cowed astronomers was the explosion’s glowing aftermath: rather than showing the heavy and radioactive elements characteristic of “ordinary” supernovae, the spectrum showed only the chemical fingerprints of hydrogen and helium. And weirdly, followup observations showed that the source remained hot for weeks after its discovery, rather than cooling down as supernova ejecta typically do.

Telescopes monitoring the source’s brightness also discovered oddities: up-and-down “bumps” in its brightness, coupled with fluctuating X-rays. Together, those observations suggested that some sort of “engine” was keeping the explosion going for weeks and weeks. The engine’s source is unclear, however; understanding the nature of the engine is central to the ongoing debate.

An Unlucky White Dwarf?

If AT2018cow were a tidal disruption event, it would have had to be a helium white dwarf encountering a medium-mass black hole. This is unlike other tidal disruption events seen to date.
Sophia Dagnello / NRAO / AUI / NSF

One option is that a star came too near a black hole and was torn to shreds, said Amy Lien (University of Maryland and NASA Goddard). Her team used NASA’s Neil Gehrels Swift Observatory to observe the flood of X-rays, ultraviolet, and visible light coming from the explosion.

The Cow appeared so suddenly that the shredded star would have had to be small: a white dwarf, typically about the size of Earth, would fit the bill. Specifically, the star must have been a so-called “helium white dwarf,” as these are rich in hydrogen and helium and lack heavier elements. Moreover, the black hole doing the shredding would also have to be relatively small — 100,000 to 1 million times the mass of the Sun — so it would have been a member of the elusive class of intermediate-mass black holes.

But this theory does have a rub: The explosion took place in a spiral arm of CGCG 137-068. It wouldn’t be a surprise to find a supernova in a spiral arm, but an intermediate-mass black hole is unexpected. Lien argued that the Cow might live in satellite galaxy or globular star cluster, where intermediate-mass black holes are more likely to be found, and the alignment with the spiral arm might just be chance.

A Strange Supernova?

Likewise, if AT2018cow is in fact a supernova, it’s a strange sort, unlike any we’ve seen before.
NASA, ESA, G. Bacon (STScI)

Another explanation could also work to explain the observations so far: a strange supernova. Granted, the observations are pretty atypical for ordinary exploding stars, but some astronomers think this one might have had special circumstances. In this case, astronomers may have peered through the exploding debris to actually witness the birth of the black hole or neutron star at the center of the explosion. Such a find would fill in a huge “knowledge gap” in the astronomy community, said Raffaella Margutti (Northwestern University).

One line of evidence pointing to this picture is that the X-ray spectrum shows the signature of iron and a “Compton Hump,” a wide bump associated with X-rays reflecting off colder, denser material. These signatures could come from a newborn black hole feeding on material that’s falling back onto it.

Longer-wavelength observations provide additional evidence for this line of thinking. Anna Ho (Caltech) had the rare chance to observe millimeter-wave emission from the Cow using the Submillimeter Array in Mauna Kea, Hawai‘i. In typical supernovae, this emission fades too fast, before telescope time becomes available. But observations of the Cow showed it was actually getting brighter, Ho said, and the Cow remained bright for several weeks before fading away.

Ho suggested that this emission could come from a shockwave propelled by a black hole or neutron star engine, as it plowed into a dense cloud of dust and gas. Once the shockwave passed through, the submillimeter signal would have tailed off. This dense cloud, suggested by both the X-ray and millimeter-wavelength observations, is a strike against the intermediate-mass black hole idea, as medium black holes aren’t thought to exist in such environments.

Ongoing and future observations may eventually be able to see what, if anything, remains from this mysterious explosion.

References:

N. Paul M. Kuin, A. Lien, et al. “Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?” arXiv.org, 2018 August 26.

D. Perley et al. “The fast, luminous ultraviolet transient AT2018cow: Extreme Supernova, or disruption of a star by an intermediate-mass black hole?” To appear in Monthly Notices of Royal Astronomical Society. (Preprint available here.)

R. Margutti et al. “An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients.” arXiv.org, 2018 October 25.