What happens when two neutron stars don’t add up to a black hole?

Birth of a Magnetar
This artist's concept of short gamma-ray burst 200522A, which produced what scientists have confirmed to be the brightest kilonova ever recorded, 10 times brighter than any others known.
NASA / ESA / D. Player (STScI)

It turns out, two neutron stars don’t necessarily make a black hole.

Neutron star collisions are likely at the heart of short gamma-ray bursts (GRBs), flashes of gamma radiation that last less than two seconds but carry more power than the Sun will produce in its lifetime.  Simple math suggests that when two neutron stars come together in this way, they ought to have enough mass to make a black hole — provided they don’t lose too much material in the process of merging.

Observing the glow that follows these explosive mergers is difficult, but with the help of the Hubble Space Telescope, astronomers have caught the afterglow of one such burst, GRB 200522A. Its fading radiation carries an important message: Violent as these blasts might be, they’re not necessarily cataclysmic. At least in this case, a highly magnetized neutron star, or magnetar, appears to have survived the event.

Wen-Fai Fong (Northwestern University) and colleagues have posted observations of this GRB on the arXiv preprint server, and the study will appear in the Astrophysical Journal later this year.

An Odd Burst

NASA’s Neil Gehrels Swift Observatory first detected the burst after the radiation had traveled for 5.47 billion years to Earth. Fong’s team observed it again with Hubble Space Telescope and a multitude of other ground-based observatories following the initial GRB. But when it came time to understand the relation between radiation across the electromagnetic spectrum — from radio to infrared to X-rays — the team at first couldn’t make sense of what they were seeing.

After two neutron stars collide, producing the initial burst of gamma-rays, there’s an afterglow of emission that comes from the shock wave that follows. As the shock wave blasts out, electrons from the exploding plasma spiral around the shock’s magnetic fields. Known to astronomers as a kilonova, this emission explains most of the afterglow from other GRBs. But it didn’t work for this one — the infrared emission was 10 times brighter than expected.

“The fact that we see this infrared emission, and that it is so bright, shows that short gamma-ray bursts indeed form from neutron star collisions,” says team member Edo Berger (Center for Astrophysics, Harvard & Smithsonian), “but surprisingly the aftermath of the collision may not be a black hole, but rather likely a magnetar.”

A Magnetar Survives

The team in fact consider two scenarios: One is that the neutron star collision birthed a magnetar. The second is that the collision produced a black hole, accompanied by a jet of plasma traveling at relativistic speed away from the collision with a surprisingly wide angle.

“In my opinion, the magnetar scenario provides a more straightforward explanation for the observations,” says Maria Grazia Bernardini (Astronomical Observatory of Brera, Italy), a GRB expert who was not involved in the study. It’s unlikely, she adds, that a relativistic jet would spray plasma so broadly; such jets are typically quite narrow. A jet also wouldn’t make the right amount of X-rays, Fong’s team notes.

“GRB 200522A is a remarkable example of how short GRB afterglows can still surprise and puzzle us 15 years after their discovery,” Bernardini says.

If a magnetar survived the collision, it will still be around for a long time to come. Within a few years, Fong and his colleagues write, the magnetized remnant should produce observable radio emission.

“If detected, this would not only break the degeneracy between the two possible explanations in this specific case,” Bernardini says, “but it would provide the long-sought smoking gun of the magnetar scenario, and the first direct evidence of a stable magnetar associated with a GRB.”



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