Why a global network of telescopes followed the "Cow"

Last year, 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 that showed 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 the more common weeks.

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.

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 the researchers, 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.

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.

About GROWTH

GROWTH is a Caltech-led, NSF-funded project comrpised of 16 international partners including the University of Sydney that conducts frontier research in time domain astronomy.