CSIRO, Alex Cherney

Why a global network of telescopes followed the “Cow”

Embargoed until: Publicly released:
In 2018 an initially unremarkable cosmic flare soon became one of astronomy's most observed and puzzling objects. The comprehensive observations of the GROWTH team of astronomers shed light on what may have happened.

Journal/conference: 233rd meeting of the American Astronomical Society

Organisation/s: The University of Sydney

Media Release

From: The University of Sydney

In 2018 a cosmic event captivated astronomers around the world when a distant supernova quickly appeared and shone more intensely than any witnessed before it.

Nicknamed the “Cow” after the computer-generated catalogue name “AT2018cow”, the intense flare was first registered in The Asteroid Terrestrial-impact Last Alert System (ATLAS) survey and has been monitored by a global team including University of Sydney researchers.

The survey had been designed to search for transients — flashes in the sky that carry signals from violent cosmic explosions such as supernovae, merging neutron stars and stars being consumed by black holes.

The flare quickly captured the attention of the Global Relay of Observatories Watching Transients Happen (GROWTH) co-investigator and Liverpool John Moores University Assistant Professor of Astronomy Daniel Perley, who today presented the latest Cow results at the 233rd meeting of the American Astronomical Society in Seattle.

“This was an incredibly luminous event, brighter than almost any supernova we've ever seen before. The Cow also appeared and faded away very quickly, so quickly that existing supernova models can't properly explain it. It must be a new type of extremely energetic, explosive event," he said.

Perley employed various optical telescopes within the global network of GROWTH observatories, which closely monitored the object for over a month after detection.

The international network provided a rich dataset which showed that material ejected after the explosion was expanding at very high speeds. The team believes this may have contributed to the very rapid brightening of the object, which reached peak brightness in just two days rather than weeks, which is more common for known types of supernovae.

As GROWTH's optical astronomers were keeping their telescopes on the Cow, two students were observing the explosion with radio telescopes. University of Sydney PhD student Dougal Dobie was observing the flare with the CSIRO’s Australia Telescope Compact Array in Narrabri, while Caltech PhD student Anna Ho was observing the event with the Submillimeter Array in Hawaii and the Atacama Large Millimeter Array in Chile.

“Radio observations let us track the shock from the explosion as it travels out into the interstellar medium. Our radio data shows that the shock wave is travelling at about one-tenth the speed of light,” Dougal Dobie said.

As the Cow's luminosity began to fade, the GROWTH team observed subtle bumps and wiggles in the optical data rather than the smooth decline in brightness that is more typical for declining supernovae. According to Perley, this suggests that there is an additional source of power — a “central engine” — pumping energy into the expanding material.

Dobie’s radio observations combined with submillimeter and X-ray data also point to the conclusion that Cow was an engine-driven explosion continuously energised by either a black hole or a fast-spinning neutron star formed in a supernova, known as a magnetar.

“This is the first time we’ve seen a transient source with these extreme properties — radio observations add an important part of the picture as we try to understand what’s going on," said Dobie.

Over six months since the Cow was first spotted, astronomers continue to debate what it is or how it appeared. While the supernova-turned-magnetar central engine is the most favoured interpretation, other teams have suggested the Cow has been caused by a black hole ripping apart a white dwarf star.

“The properties of the Cow strain nearly all models we have tried to devise to explain it. Whatever it is, it must involve some form of energetic and very fast explosion interacting with an extremely dense shell of material very close to the explosion progenitor," concluded Perley.

Supervising Dougal Dobie’s work was University of Sydney radio astronomy expert Professor Tara Murphy who believes a sustained, collaborative effort by global researchers is fundamental to mapping future astronomical events.

“Fast transient surveys with well-coordinated follow-ups from a network of observatories are critical. In the past it has been almost impossible to detect these rare events, and now we are watching them evolve in real time.”

“This event was unlike anything we’ve seen before, and we’re excited about what we will find as next generation radio telescopes come online”, concluded Professor Murphy.

The study “The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole” led by Assistant Professor Daniel Perley has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

The study "AT2018cow: a luminous millimeter transient" led by Anna Ho is accepted for publication in The Astrophysical Journal.

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