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Several billion years ago, a short gamma-ray burst released more energy in half a second than our Sun will produce over its entire 10 billion year lifespan. In May 2020, the light from the event, dubbed GRB 200522A, finally reached Earth and was first detected by NASA’s Neil Gehrels Swift Observatory. NASA / ESA’s Hubble Space Telescope quickly captured the glow in just three days after GRB 200522A and determined its near-infrared emission to be 10 times brighter than expected, defying conventional models.
This image shows the glow of kilonova GRB 200522A caused by the fusion of two neutron stars. Image Credit: NASA / ESA / W. Fong, Northwestern University / T. Laskar, University of Bath.
Lasting less than two seconds, short gamma-ray bursts are among the most energetic and explosive events known in the Universe.
Astronomers believe these events are caused by the fusion of two neutron stars.
Such fusions are very rare and extremely important because scientists believe that they are one of the main sources of heavy elements in the Universe, such as gold and uranium.
In addition to a short gamma-ray burst, astronomers expect to see a kilonova whose maximum brightness is typically 1,000 times that of a conventional nova.
Kilonovae are an optical and infrared glow from the radioactive decay of heavy elements and are unique to the fusion of two neutron stars or the fusion of a neutron star and a black hole.
“It’s amazing to me that after 10 years of studying the same kind of phenomenon, we can discover unprecedented behavior like this,” said Dr. Wen-fai Fong, astronomer at Northwestern University.
“It just reveals the variety of explosions the Universe is capable of producing, which is very exciting.”
“These observations do not correspond to traditional explanations for short gamma-ray bursts,” she added.
“Based on what we know about the radio and x-rays of this explosion, it just doesn’t add up. The near infrared emission that we see with Hubble is far too bright. “
This illustration shows the formation sequence of a kilonova powered by a magnetar, whose maximum luminosity reaches up to 10,000 times that of a classical nova: (i) two orbiting neutron stars are getting closer and closer. ‘one of the other; (ii) they collide and merge, triggering an explosion that releases more energy in half a second than the Sun will produce during its entire 10 billion year lifespan; (iii) the fusion forms an even more massive neutron star called the magnetar, which has an extraordinarily strong magnetic field; (iv) The magnetar deposits energy into the ejected material, causing it to glow unexpectedly at infrared wavelengths. Image Credit: NASA / ESA / D. Player, STScI.
To determine the precise distance to the host galaxy of GRB 200522A, Dr Fong and his colleagues used the instruments of the Low-Resolution Imaging Spectrometer (LRIS) and the DEep and Multi-Object Imaging Spectrograph (DEIMOS) installed on the telescopes of the WM Keck observatory.
They determined that the explosion was from a young star-forming galaxy located 5.5 billion light years away.
They also analyzed the remanence of GRB 200522A in X-rays with Swift Observatory, optical and near infrared with Las Cumbres Observatory Global Telescope, Hubble, and in radio wavelengths with the Very Large Array.
But what the researchers saw was too bright to be explained even by a traditional kilonova.
“As the data came in, we formed a picture of the mechanism that produced the light we were seeing,” said co-author Dr Tanmoy Laskar, astronomer at the University of Bath.
“When we got the observations from Hubble, we had to completely change our thought process, because the information Hubble added made us realize that we had to let go of our conventional thinking and a new phenomenon was happening.
“Then we had to figure out what it meant for the physics behind these extremely energetic explosions.”
Scientists provide a possible explanation for the unusually bright explosion: While most short gamma-ray bursts likely end in a black hole, the fusion of neutron stars in this case may have formed a magnetar, a supermassive neutron star with a very strong magnetic field; the magnetar deposited a large amount of energy into the material ejected from the kilonova, making it glow even brighter.
“What we detected surpasses even that confirmed that the kilonova was discovered in 2017,” said co-author Jillian Rastinejad, a graduate student at Northwestern University.
“As a first-year graduate student working with real-time data for the first time when this explosion occurred, it is remarkable to see our discovery motivate a new and exciting model amplified by magnetar.
With such an event, the team expects the ejecta from the gust to produce light at radio wavelengths within the next few years.
“Follow-up radio observations may ultimately prove that the origin of the burst was indeed a magnetar,” the authors said.
Their article will be published in the Astrophysics Journal.
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W. Fong et al. 2020. The broadband counterpart of short GRB 200522A at z = 0.5536: a luminous kilonova or a collimated output with a reverse shock? A J, in the press; arXiv: 2008.08593
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