Possible magnetar birth produced brightest ‘kilonova’ ever seen

13 Nov 2020

Illustration of what the kilonova event would have looked like. Image: NASA, ESA, and D Player (STScI)

Researchers have studied a gamma-ray burst that unleashed more energy in half a second than the sun will produce in 10bn years.

A Northwestern University-led astrophysics team looking at observations, including those made by the Hubble Space Telescope, believe it has spotted something never seen before.

Writing in The Astrophysical Journal, the researchers said that an enormous burst of gamma rays – with more energy released in half a second than the sun will release in its entire 10bn-year lifetime – may have been the birth of a magnetar. A magnetar is a type of neutron star believed to have an extremely powerful magnetic field.

After analysing the burst with optical, X-ray, near-infrared and radio wavelengths, the team believes that a magnetar was formed by the merging of two neutron stars. This merger produced the brightest ‘kilonova’ ever seen, the light of which eventually reached Earth on 22 May 2020.

Kilonovae, which are typically 1,000 times brighter than a classic nova, are expected to accompany short gamma-ray bursts. Compared to X-ray and radio observations, the near-infrared emission detected with Hubble was 10 times brighter than predicted.

‘Especially exciting’

“When two neutron stars merge, the most common predicted outcome is that they form a heavy neutron star that collapses into a black hole within milliseconds or less,” said Wen-fai Fong, who led the study.

“Our study shows that it’s possible that, for this particular short gamma-ray burst, the heavy object survived. Instead of collapsing into a black hole, it became a magnetar: a rapidly spinning neutron star that has large magnetic fields, dumping energy into its surrounding environment and creating the very bright glow that we see.”

Jillian Rastinejad, a co-author of the paper, said that so far there has only been one confirmed and well-sampled kilonova.

“It is especially exciting to find a new potential kilonova that looks so different,” she said. “This discovery gave us the opportunity to explore the diversity of kilonovae and their remnant objects. I’m excited for the new surprises that short gamma-ray bursts and neutron star mergers have in store for us in the future.”

The team will now be looking to complete follow-on observations. Researchers said that if this unexpectedness brightness was the result of a magnetar birth, it should produce light that shows up at radio wavelengths in a few years’ time.

Colm Gorey was a senior journalist with Silicon Republic