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(NASA, ESA, and D. Player/STScI)

Astronomers Are Scrambling to Explain This Unusually Bright Kilonova Explosion

13 NOVEMBER 2020

From across the Universe, 5.5 billion light-years away, a range of telescopes has captured the bright flash of a short gamma-ray burst. It's reminiscent of the kilonova explosion associated with the neutron star collision we detected in a historic first back in 2017, prompting astronomers to wonder if that's what we've witnessed now.

 

That 2017 detection, known as GW 170817, was a great gift: a wealth of data across multiple signals to help us understand these events, and recognise what we're looking at if one shows up again.

But there is something in the kilonova accompanying the new gamma-ray burst, called GRB 200522A, very unlike that neutron star collision. The flash captured in near-infrared wavelengths by the Hubble Space Telescope was incredibly bright - 10 times brighter than predicted by models of neutron star collisions.

"These observations do not fit traditional explanations for short gamma-ray bursts," said astronomer Wen-fai Fong of Northwestern University.

"Given what we know about the radio and X-rays from this blast, it just doesn't match up. The near-infrared emission that we're finding with Hubble is way too bright."

The light was first detected by NASA's Neil Gehrels Swift Observatory, a space telescope designed to detect gamma-ray bursts as early as possible with its Burst Alert Telescope. Once the alert came in, other space and terrestrial telescopes homed in on the burst's location.

The Very Large Array, the W.M. Keck Observatory, and the Las Cumbres Observatory Global Telescope network all worked to obtain an electromagnetic profile of the event from radio wavelengths to X-rays. They showed that the event was a short gamma-ray burst - a type of blast less than two seconds in duration associated with merging neutron stars.

 

But the Hubble Space Telescope, observing in near-infrared, threw a spanner in the works.

"As the data were coming in, we were forming a picture of the mechanism that was producing the light we were seeing," said astronomer Tanmoy Laskar of the University of Bath in the UK.

"We had to completely change our thought process, because the information that Hubble added made us realise that we had to discard our conventional thinking and that there was a new phenomenon going on. Then we had to figure out about what that meant for the physics behind these extremely energetic explosions."

The collision of two neutron stars - the collapsed cores of dead stars - is a momentous event. Neutron stars are tiny and dense, about 1.1 to 2.5 times the mass of the Sun, but packed into a sphere just 20 kilometres (12 miles) across.

When they collide, they release a tremendous amount of energy in a kilonova explosion, 1,000 times brighter than a regular nova. This is accompanied by a burst of high-energy gamma-rays from jets of expelled material travelling at close to the speed of light.

 

The kilonova itself is a glow in optical and infrared wavelengths produced by the radioactive decay of heavy elements. Astronomers believe that the two neutron stars in GW 170817 merged to form a black hole. The near-infrared brightness of the GRB 200522A kilonova, the researchers believe, indicates that these two neutron stars merged to form something else: a magnetar.

Magnetars are a type of neutron star, but they're super weirdos, with insanely powerful magnetic fields - around 1,000 times more powerful than the average neutron star.

"You basically have these magnetic field lines that are anchored to the star that are whipping around at about 1,000 times a second, and this produces a magnetised wind," Laskar said.

"These spinning field lines extract the rotational energy of the neutron star formed in the merger, and deposit that energy into the ejecta from the blast, causing the material to glow even brighter."

Magnetars are also rare; only 24 have been confirmed to date in the Milky Way. That makes it pretty tricky for us to figure out how they got that way. If the two neutron stars associated with GRB 200522A formed a magnetar, that gives us a new mechanism whereby these extreme stars can come into being.

"We know that magnetars exist because we see them in our galaxy," Fong said.

"We think most of them are formed in the explosive deaths of massive stars, leaving these highly magnetised neutron stars behind. However, it is possible that a small fraction form in neutron star mergers. We have never seen evidence of that before, let alone in infrared light, making this discovery special."

It's a little early to know for sure. Only one kilonova to date has been confirmed and well characterised; that, of course, is the kilonova associated with GW 170817.

But the new detection, with its near-infrared weirdness, is a step towards cataloguing the variety possible in kilonovae, and understanding the range of outcomes when two neutron stars collide.

The research has been accepted into The Astrophysical Journal and is available on arXiv.