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Long ago and throughout the universe, a huge explosion of gamma rays released more energy in half a second than the Sun will produce during its 10 billion years of life. In May 2020, the flash light finally reached Earth and was first detected by NASA’s Neil Gehrels Swift Observatory.
Scientists quickly enlisted other telescopes, including NASA’s Hubble Space Telescope, Very Large Array Radio Observatory, WM Keck Observatory, and the Las Cumbres Observatory Global Telescope Network, to study the aftermath of the explosion and the host galaxy. It was Hubble who provided the surprise.
Based on X-rays and radio observations from other observatories, astronomers were baffled by what they saw with Hubble: the near-infrared emission was 10 times brighter than expected. These findings challenge conventional theories of what happens in the aftermath of a brief gamma-ray burst. One possibility is that the observations may indicate the birth of a massive, highly magnetized neutron star called a magnetar.
“These observations don’t fit traditional explanations for brief gamma-ray bursts,” said study leader Wen-fai Fong of Northwestern University in Evanston, Illinois. “Given what we know about the radio and the X-rays of this explosion, it just doesn’t match. The near-infrared emission we’re seeing with Hubble is too bright. In terms of trying to fit the puzzle pieces of this gamma-ray burst. together, a piece of the puzzle doesn’t fit properly. “
Without Hubble, the gamma-ray burst would have looked like many others, and Fong and his team would not have known about the bizarre behavior of infrared. “It is surprising to me that after 10 years of studying the same type of phenomenon, we can discover unprecedented behavior like this,” Fong said. “It just reveals the diversity of explosions the universe is capable of producing, which is very exciting.”
Fantastic light
The intense gamma-ray bursts of these bursts appear to come from jets of material moving extremely close to the speed of light. The jets don’t contain much mass – perhaps one millionth of the mass of the Sun – but because they move so fast, they release a huge amount of energy across all wavelengths of light. This particular gamma-ray burst was one of the rare instances in which scientists were able to detect light across the entire electromagnetic spectrum.
“As the data was coming in, we were forming an image of the mechanism that was producing the light we were seeing,” said study co-investigator Tanmoy Laskar of the University of Bath in the UK. “When we got the Hubble observations, we had to completely change our thinking process, because the information Hubble added made us realize that we needed to discard our conventional thinking and that there was a new phenomenon going on. Then we have had to understand what it meant for the physics behind these extremely energetic explosions. “
Gamma-ray bursts – the most energetic and explosive events known – live fast and die hard. They are divided into two classes based on the duration of their gamma rays.
If the gamma-ray emission is greater than two seconds, it is called a long gamma-ray burst. It is known that this event results directly from the collapse of the core of a massive star. Scientists expect a supernova to accompany this type of burst longer.
If the gamma-ray emission lasts less than two seconds, it is considered a short burst. This is thought to be caused by the merger of two neutron stars, extremely dense objects about the mass of the Sun compressed into the volume of a city. A neutron star is so dense that on Earth, a teaspoon would weigh a billion tons! It is generally believed that the merger of two neutron stars produces a black hole.
Neutron star mergers are very rare but are extremely important because scientists think they are a major source of heavy elements in the universe, such as gold and uranium.
Accompanying a brief gamma-ray burst, scientists expect to see a “kilonova” whose maximum brightness typically reaches 1,000 times that of a classical nova. Kilonovae are an optical and infrared glow resulting from the radioactive decay of heavy elements and are unique to the merger of two neutron stars or the merger of a neutron star with a small black hole.
Magnetic monster?
Fong and his team discussed several possibilities to explain the unusual brightness Hubble saw. While most of the short gamma-ray bursts likely result in a black hole, the two neutron stars that merged in this case may have combined to form a magnetar, a supermassive neutron star with a very powerful magnetic field.
“Basically you have these magnetic field lines that are anchored to the star that go around about a thousand times per second, and that produces a magnetized wind,” Laskar explained. “These rotating field lines extract the rotational energy of the neutron star formed in the melt and deposit it in the ejecta of the explosion, making the material shine even more.”
If the extra brightness came from a magnetar that deposited energy into the kilonova material, within a few years the team expects the explosion from the burst to produce light that manifests itself at radio wavelengths. Follow-up radio observations could eventually show that it was a magnetar and this could explain the origin of these objects.
“With its incredible sensitivity to near-infrared wavelengths, Hubble really made the deal with this burst,” Fong explained. “Surprisingly, Hubble was only able to take an image three days after the explosion. Through a series of successive images, Hubble showed that a source faded in the aftermath of the explosion. This instead of being a static source. which remains unchanged. With these observations, we knew that we had not only captured the source, but we had also discovered something extremely bright and very unusual. Hubble’s angular resolution was also the key to pinpointing the position of the burst and accurately measuring the light coming from the fusion “.
NASA’s upcoming James Webb Space Telescope is particularly well suited for this type of observation. “Webb will completely revolutionize the study of similar events,” said Edo Berger of Harvard University in Cambridge, Massachusetts, and principal investigator of the Hubble program. “With its incredible infrared sensitivity, it will not only detect such emissions at even greater distances, but will also provide detailed spectroscopic information that will resolve the nature of the infrared emission.”
The team’s findings will appear in an upcoming issue of The Astrophysical Journal.
Research paper
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NASA missions help pinpoint the source of a single X-ray radio burst
Greenbelt MD (SPX) November 5, 2020
On April 28, a supermagnetized stellar remnant known as a magnetar emitted a simultaneous mix of X-rays and radio signals never seen before. The flare-up included the first ever fast radio burst (FRB) seen from within our Milky Way galaxy and shows that magnetars can produce these mysterious and powerful radio bursts previously only seen in other galaxies. “Prior to this event, a wide variety of scenarios could explain the origin of FRBs,” said Chris Bochenek, a PhD student in astrophysi … read more
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