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From across the Universe, 5.5 billion light-years away, a range of telescopes has captured the bright flash of a brief burst of gamma rays. It’s reminiscent of the kilonova blast associated with the neutron star collision that we detected in a historic first 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 huge amount of data across multiple signals to help us understand these events and recognize what we are seeing if one appears again.
But there is something about the kilonova that accompanies the new gamma-ray burst, called GRB 200522A, very different from the collision of a neutron star. The flare captured at near-infrared wavelengths by the Hubble Space Telescope was incredibly bright, 10 times brighter than predicted by neutron star collision models.
“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 radio and X-rays from this explosion, it just doesn’t match. The near-infrared emission that we are finding with Hubble is too bright.”
The light was first detected by NASA’s Swift Neil Gehrels Observatory, a space telescope designed to detect gamma-ray bursts as quickly as possible with its Burst Alert Telescope. Once the alert came, other space and ground telescopes focused on the location of the blast.
The Very Large Array, the WM Keck Observatory and the Las Cumbres Observatory Global Telescope network worked to obtain an electromagnetic profile of the event from radio wavelengths to X-rays. They demonstrated that the event was a brief gamma-ray burst, a a type of explosion less than two seconds long associated with neutron star fusion.
But the Hubble Space Telescope, looking in the near infrared, threw a wrench into the works.
“As the data came in, we were forming an image of the mechanism that produced the light that 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 Hubble added made us realize that we had to discard our conventional thinking and that a new phenomenon was occurring. Then we had to find out what that meant for the physics behind these explosions. extremely energetic. “
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 clustered together in a sphere only 20 kilometers (12 miles) in diameter.
When they collide, they release an enormous amount of energy in a kilonova blast, 1,000 times brighter than a normal nova. This is accompanied by a high-energy gamma-ray burst of jets of ejected material traveling 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 glow from kilonova GRB 200522A, 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 are super rare, with incredibly powerful magnetic fields, around 1000 times more powerful than the average neutron star.
“You basically have these magnetic field lines that are anchored to the star and that move about 1,000 times a second, and this produces a magnetized wind,” Laskar said.
“These rotating field lines extract the rotational energy of the neutron star formed in the merger and deposit that energy in the ejection of the explosion, making the material glow even more.”
Magnetars are rare too; to date only 24 have been confirmed in the Milky Way. That makes it quite difficult 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 by which these extreme stars can come into existence.
“We know that magnetars exist because we see them in our galaxy,” Fong said.
“We believe that most of them are formed in the explosive death of massive stars, leaving behind these highly magnetized neutron stars. However, it is possible that a small fraction is formed in neutron star mergers. We have never seen evidence of that. before, and much less in infrared light, which makes this discovery special. “
It’s a bit early to be sure. To date, only one kilonova has been confirmed and characterized; that, of course, is the kilonova associated with GW 170817.
But the new detection, with its rarity in the near-infrared, is a step toward cataloging the possible variety in kilonova and understanding the range of results when two neutron stars collide.
The research has been accepted in The Astrophysical Journal and it is available in arXiv.
