Far from across the Universe, a set of telescopes has captured the bright flash of a short gamma-ray burst. The brightest kilonova ever seen.
About 5.5 billion light-years away, an enormous burst of gamma rays unleashed more energy in a half-second than the sun will produce over its entire 10-billion-year lifetime. That’s a kilonova.
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After gauging the incredibly bright burst with optical, X-ray, near-infrared, and radio wavelengths, a Northwestern University-led astrophysics team believes it potentially spotted the birth of a strange magnetic star.
Two dense neutron stars collided to form a magnetar and scientists have never before seen such an event. The merger resulted in a brilliant kilonova—the brightest ever seen—whose light finally reached Earth on May 22, 2020.
“When two neutron stars merge, the most common predicted outcome is that they form a heavy neutron star that collapses into a black hole within milliseconds or less,” said Northwestern’s Wen-fai Fong, who led the study. “Our study shows that it’s possible that, for this particular short gamma-ray burst, the heavy object survived. Instead of collapsing into a black hole, it became a magnetar: A rapidly spinning neutron star that has large magnetic fields, dumping energy into its surrounding environment and creating the very bright glow that we see.”
Neutron stars are tiny and dense, about 1.1 to 2.5 times the mass of the Sun. But they are just 20 kilometers (12 miles) across.
The short gamma-ray burst, detected here on Earth, lasted less than two seconds. These types of bursts are among the most energetic, explosive events know in the Universe.
Such mergers are very rare and extremely important. That’s because they are one of the main sources of heavy elements in the Universe, such as gold and uranium.
Scientists first detected the light using NASA’s Neil Gehrels Swift Observatory, a space telescope that detects gamma-ray bursts as early as possible with its Burst Alert Telescope. Once the alert came in, other space and terrestrial telescopes located the burst and started to study the explosion’s aftermath.
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.
Compared to X-ray and radio observations, the near-infrared emission detected with Hubble was too bright to be explained even by a traditional kilonova. In fact, it was 10 times brighter than predicted.
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“As the data were coming in, we were forming a picture of the mechanism that was producing the light we were seeing,” said the study’s co-investigator, Tanmoy Laskar of the University of Bath in the United Kingdom.
“As we got the Hubble observations, we had to completely change our thought process, because the information that Hubble added made us realize 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 scientists provide one possible explanation for the unusually bright blast. Most short gamma-ray bursts probably result in a black hole. But the neutron star merger may have instead formed a magnetar, a dense highly magnetized neutron star. The magnetar deposited a large amount of energy into the ejected material of the kilonova. Thus, causing it to glow even brighter.
Magnetars are young neutron stars that are the most magnetic objects in the universe. Their powerful magnetic field is about 1,000 times more powerful than the average neutron star.
These strange magnetic stars are also rare; only 24 have been confirmed to date in the Milky Way.
Observing a possible magnetar formation will help scientists understand how magnetars become so magnetic.
Scientists described the research in a paper announced for publication in The Astrophysical Journal today (Nov. 12) and available to read on the preprint server arXiv.org.