The Cow in the sky: Observing a neutron star or black hole birth

The Cow in the sky: Observing a neutron star or black hole birth
An extraordinary outburst produced by a black hole in a nearby galaxy. © NASA CXC Curtin University R. Soria

A team of astronomers has discovered that The Cow, a mysterious bright object that burst in the northern sky this summer, may be the result of a neutron star or black hole birth.

The team was led by Northwestern University and found that the bright object, which has been called both AT2018cow or The Cow, could be caused by a neutron star or black hole birth.

It was first observed on 16 June after the ATLAS telescopes on Haleakala and Maunaloa captured an incredibly bright anomaly 200 million light years away in the Hercules constellation. The Cow flared up quickly, and then vanished.

Astronomers were unsure of the cause of the mysterious bright object. The lead author, Raffaella Margutti, assistant professor of physics and astronomy at Northwestern University and faculty member of Northwestern’s CIERA (Center for Interdisciplinary Exploration and Research for Astrophysics), said: “We thought it must be a supernova. But what we observed challenged our current notions of stellar death.”

A neutron star or black hole birth

Margutti’s team quickly conducted follow-up observations using multiple observatories. They analysed The Cow in different wavelengths.

Margutti explained: “A ‘lightbulb’ was sitting deep inside the ejecta of the explosion,” Margutti said. “It would have been hard to see this in a normal stellar explosion. But The Cow had very little ejecta mass, which allowed us to view the central engine’s radiation directly.”

She added: “Based on its X-ray and UV emission, ‘The Cow’ may appear to have been caused by a black hole devouring a white dwarf. But further observations of other wavelengths across the spectrum led to our interpretation that ‘The Cow’ is actually the formation of an accreting black hole or neutron star. We know from theory that black holes and neutron stars form when a star dies, but we’ve never seen them right after they are born. Never.”

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