Explosive neutron star collision may have created a rare and extreme star

When two neutron stars collide, the universe is shaking. The extreme crash is explosive and creates a “kilonova”, which sends out a bright, rapid burst of gamma rays. It also sends ripples through the fabric of space-time. Next, scientists believe the cosmic crash likely creates a newly merged object that quickly collapses into a black hole. But … what if he survives?

A new study, which is expected to be published in The Astrophysical Journal but available for preprint on arXiv, describes the brightest kilonova to date and suggests that a neutron star collision could sometimes give rise to a magnetar, a extreme neutron star with dense magnetic fields.

On May 22, NASA’s Neil Gehrels Swift Observatory, a space telescope, spotted a gamma-ray burst in an extremely remote corner of space, dubbed GRB 200522A. Scientists believe that these types of short bursts occur when two neutron stars collide, so when a telescope sees one, there is a mad rush to get observations at other wavelengths of the electromagnetic spectrum. The collision in question happened about 5.5 billion years ago, but our telescopes are only now picking up the signals.

In the new study, the research team pointed to a number of different space and ground telescopes at GRB 200522A, including NASA’s Hubble Space Telescope, and observed the fallout after the bright gamma-ray burst.

Using radiographic, radio and near infrared data, the team was able to measure the brightness of the gamma-ray burst. But there was one particular observation that didn’t fit. Hubble’s near-infrared images showed an extremely bright burst – about 10 times brighter than any kilonova ever seen (although only a handful have been seen so far).

“We scratched our heads for a while and looked at every possible model available to us,” says Wen-fai Fong, an astrophysicist at Northwestern University and lead author of the new research. “The near infrared light we saw from the GRB 200522A was far too bright to be explained by a standard radioactive kilonova.”

Fong and his team finally opted for a model they dubbed a “magnetar-boosted kilonova” to explain the extreme brightness.

Kilonova are created when two dense cosmic objects – like neutron stars and black holes – collide. The fusion process ejects a ton of subatomic material into space, including the generation of the gamma-ray burst. Fong says you can think of it as a smoothie in a blender that you forgot to put on the lid, with “neutron-rich” material flowing into the cosmos.

The team’s model suggests that creating a magnetar, a type of highly magnetized neutron star, could have supercharged the kilonova event, making it much brighter than astronomers had predicted.

“If this is confirmed, this would be the first time we could witness the birth of a magnetar from a pair of neutron stars,” Fong explains.

But there is work to be done. Continuing to observe GRB 200522A with radio telescopes will help determine more clearly what happened around the gamma-ray burst. Radio waves from the event should be able to confirm what was seen at infrared wavelengths, but the time it takes for these waves to reach Earth depends on the environment around GRB 200522A. The model suggests it could take about six years before we pick up such a signal, and Fong says the team will be monitoring radio broadcasts for years to come.

Magnetars have long been mysterious cosmic bodies, but last week astronomers began to shed light on the elusive dead stars. Last week, a team of astrophysicists reported the discovery of a rapid radio burst (FRB) from a magnetar inside the Milky Way. The momentous discovery suggests that magnetars can sometimes be able to create these mysterious radio signals, although the jury is out on whether they can create all FRBs. GRB 200522A may be the opportunity to test this hypothesis again.

“If we could match an FRB with the location of GRB 200522A, that would be an amazing find and indeed it would be a smoking gun connecting this particular event to a magnetar,” Fong says. However, she cautions that it would be surprising if there was a link between short gamma-ray bursts themselves and FRBs.

But gamma-ray bursts continue to create new cosmic mysteries and puzzles to solve. “I’ve been studying the same type of explosion for about ten years now, and short gamma-ray bursts can still surprise and amaze me,” notes Fong.