Astronomy Now - January 2021

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Monster blast could have been magnetar birth

he birth of one of the most magnetic objects in the Universe may have been witnessed during the

collision of two neutron stars, according to observations of an exceptionally bright kilonova.

On 22 May 2020, a short blast of gamma rays, lasting just half a second, was detected by NASA’s
orbiting Neil Gehrels Swift Observatory. Such a short gamma-ray burst is the signal of a neutron-
star collision in a distant galaxy. ree days later, the Hubble Space Telescope was able to capture the
near-infrared light of the expanding cloud of radioactive debris. Astronomers call this a kilonova, so-
named because kilonovae are a thousand times brighter than typical novae. Only, there was a catch:
its light was ten times more luminous than what theory predicts for a kilonova.

Based on its brightness, the event liberated more energy than the Sun will do over its entire ten-
billion-year lifetime.

Ordinarily, it is thought that two colliding neutron stars will form a black hole. However, “ese
observations do not t traditional explanations for short gamma-ray bursts,” says Wen-fai Fong, of
Northwestern University in the United States, and lead author of a paper describing the ndings in a
forthcoming issue of e Astrophysical Journal. “e near-infrared emission that [we found] with
Hubble is way too bright.”

eir solution is that the neutron-star merger produced an excessively massive and highly magnetic
neutron star called a magnetar. Over two dozen magnetars are known to exist in our Galaxy alone,
and despite being only ten kilometres or so in diameter, they are so magnetically powerful that they
could wipe your credit card from the distance of the Moon. Magnetars are born spinning a thousand
times a second and whipping their magnetic eld lines around with them. e eld lines extract
energy from a magnetar’s rotation, slowing it, and via a ‘magnetic wind’, they dump that energy into
the cloud of debris, causing the extra brightness.

If Fong’s team is correct, then in a few years the magnetar should start producing detectable radio
waves, which could conrm their theory.

An artist’s impression of a magnetar. Image: ESO/L. Calçada.

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Monster blast could have been m...

January 2021

Astronomy Now