Explosive neutron star collision may have created a rare, extreme star
When two neutron stars collide, the universe winces. 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. Then, scientists believe, the cosmic smash likely creates a newly merged object that quickly collapses into a black hole. But… what if it survives?
A new study, set to be published in The Astrophysical Journal but available as a preprint on arXiv, describes the brightest kilonova yet and suggests a neutron star collision might sometimes give rise to a magnetar, an 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 distant corner of space, dubbed GRB 200522A. Scientists believe these types of short bursts occur when two neutron stars collide, so when a telescope sees one, there’s a mad scramble to obtain observations at other wavelengths on the electromagnetic spectrum. The collision in question occurred some 5.5 billion years ago but our telescopes only now picked up the signals.
In the new study, the research team pointed a number of different space- and ground-based telescopes at GRB 200522A, including NASA’s Hubble Space Telescope, and observed the fallout after the bright gamma-ray burst.
Using X-ray, radio and near-infrared data, the team were able to measure the brightness of the gamma-ray burst. But there was one particular observation that didn’t fit in. The near-infrared images from Hubble showed an extremely bright burst — about 10 times brighter than any kilonova ever seen (though only a handful have been observed so far).
“We scratched our heads for awhile and pored through all possible models at our disposal,” says Wen-fai Fong, an astrophysicist at Northwestern University and lead author of the new research. “The near-infrared light we saw from GRB 200522A was far too bright to be explained by a standard radioactively powered kilonova.”
Fong and her team eventually settled on 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 — crash into each other. The process of merging ejects a ton of subatomic material into space, including generating the gamma-ray burst. Fong says you can think of it like a smoothie in a blender that you forgot to put the lid on, with “neutron-rich” material streaming out into the cosmos.
The team’s model suggests the creation of a magnetar, a highly magnetized type of neutron star, may have been able to supercharge the kilonova event, making it far brighter than astronomers predicted.
“If confirmed, this would be the first time we were able to witness the birth of a magnetar from a pair of neutron stars,” Fong says.
But there’s some work to be done. Continuing to observe GRB 200522A with radio telescopes will help more clearly determine exactly what happened around the gamma-ray burst. The radio waves from the event should be able to confirm what was seen at infrared wavelengths, but how long those waves take to reach the Earth depends on the environment around GRB 200522A. The model suggests it could be around six years until we pick up such a signal, and Fong says the team will monitor for radio emissions for years to come.
Magnetars have long been mysterious cosmic bodies, but in the last week, astronomers have begun to shed some light on the elusive dead stars. Last week, a team astrophysicists reported the discovery of a fast radio burst (FRB) from a magnetar inside the Milky Way. The momentous discovery suggests magnetars may be able to create these mysterious radio signals sometimes, though the jury is out on whether they can create all FRBs. GRB 200522A may provide an opportunity to test that hypothesis again.
“If we were able to associate an FRB with the location of GRB 200522A, that would be an astounding discovery and would indeed be a smoking gun linking this particular event to a magnetar,” Fong says. However, she cautions it would be surprising if there’s a connection between short gamma-ray bursts themselves and FRBs.
But gamma-ray bursts do keep throwing up new mysteries and cosmic puzzles to solve. “I have studied the same type of explosion for a decade now, and short gamma-ray bursts can still surprise and amaze me,” Fong notes.