A nearby, long-duration gamma-ray burst that hit Earth has provided new insights into the mysteries of the cosmos, according to a new study. Astrophysicists have believed for nearly two decades that long gamma-ray bursts result solely from the collapse of massive stars. However, the new study overturns this belief.
The study, led by astrophysicists from Northwestern University, was published in the journal Nature on December 7. Earlier, neutron star mergers were believed to produce only short gamma-ray bursts. However, the new study has found that neutron star mergers can produce some long gamma-ray bursts as well.
What happens when neutron stars merge?
A neutron star is formed when a massive star runs out of fuel and collapses. When two neutron stars merge, a strong emission of gravitational waves occurs, along with electromagnetic waves covering the entire spectrum, from gamma-rays to radio.
50-second-long gamma-ray burst detected
The team of astrophysicists detected a 50-second-long gamma-ray burst in December, 2021, following which they started searching for energy burst's afterglow. An afterglow is an incredibly luminous and fast-fading burst of light that often precedes a supernova, the colossal explosion of a star.
What is a kilonova?
Instead of a supernova, the team uncovered the evidence of a kilonova. This is a rare event that occurs only after the merger of a neutron star with another compact object, which could either be another neutron star or a black hole.
New discovery provides insights into heaviest elements of universe
Not only does the new discovery upend long-established beliefs about how long gamma-ray bursts are formed, but also provides new insights into the mysterious formation of the heaviest elements in the universe.
What is surprising about the discovery?
In a statement released by Northwestern University, Jillian Rastinejad, who led the study, said the event looks unlike anything else researchers have seen before from a long gamma-ray burst. She added that the gamma rays resemble those of bursts produced by the collapse of massive stars. She said that since all other confirmed neutron star mergers ever observed have been accompanied by bursts lasting less than two seconds, the team had every reason to expect the 50-second gamma-ray burst was created by the collapse of a massive star.
The discovery provides new insights into gamma-ray burst astronomy.
Wen-fai Fong, a senior author on the paper, said when the researchers followed the long gamma-ray burst, they expected it would lead to evidence of a massive star collapse. Instead, what the researchers found was very different. She added that she believed for a long time that long gamma-ray bursts come from massive star collapses.
Fong said the unexpected finding not only represents a major shift in researchers' understanding, but also excitingly opens up a new window for discovery.
More about gamma-ray bursts
Gamma-ray bursts are the brightest and most energetic explosions since the Big Bang. Gamma-ray bursts are divided into two classes, namely short and long. Short gamma-ray bursts are the ones with durations less than two seconds, while long gamma-ray bursts are the ones which last longer than two seconds. Earlier, researchers believed that short and long gamma-ray bursts must have different origins.
How was the gamma-ray burst observed?
The Neil Gehrels Swift Observatory's Burst Alert Telescope and the Fermi Gamma-ray Space Telescope spotted a bright burst of gamma-ray light in December 2021. They named the gamma-ray burst GRB211211A. The gamma-ray burst was over 50 seconds long, and did not appear to be anything special initially. However, since it was located about 1.1 billion light-years away from Earth, it was relatively close to our planet, and prompted the astrophysicists to study the "nearby" event in detail. They used a multitude of telescopes that could observe across the electromagnetic spectrum.
The team made observations using the Gemini Observatory in Hawaii to image the event with near-infrared wavelengths. Rastinejad said she was worried she would be unable to obtain a clear view because the weather was worsening in Hawaii.
She said that fortunately for the team, Northwestern provided them with remote access to the MMT Observatory in Arizona, and an ideal instrument was being put on that telescope the next day. Though it was cloudy, the telescope operators knew how important the burst was and found a gap between the clouds to take the images.
How the team realised it was not a supernova but a kilnova
According to the statement, the team examined the near-infrared images, and spotted an incredibly faint object that quickly faded. Since supernovae do not fade as quickly and are much brighter, the team realised it found something unexpected that was previously believed impossible.
Fong said there are a lot of objects in the night sky that fade quickly, and that astrophysicists image a source in different filters to obtain colour information. This helps them determine the source's identity.
Fong explained that in their study, the red colour prevailed, and bluer colours faded more quickly. She said the colour evolution is a telltale signature of a kilonova, and kilonovae can only come from neutron star mergers.
Why did scientists believe that neutron star mergers cannot lead to long gamma-ray bursts?
Since neutron stars are clean, compact objects, researchers earlier believed that neutron stars did not contain enough material to power a long-duration gamma-ray burst. On the other hand, massive stars can be tens to hundreds of times the mass of our Sun. Therefore, when a dying star collapses, its material falls inward to feed a newly formed black hole. Due to the black hole's magnetic fields, some of the inward-falling material launches outward at velocities close to the speed of light. This powers a gamma-ray burst.
Fong explained that when one puts two neutron stars together, there is not really much mass there. She added that a little bit of mass accretes and then powers a very short-duration burst. The collapse of a massive star traditionally powers longer gamma-ray bursts.
The gamma-ray burst’s host galaxy is young and star-forming, an unusual discovery
According to the statement, the event was not the only strange part of the study. The gamma-ray burst's host galaxy is quite curious as it is young and star-forming, almost exactly opposite of the only other known local universe host of a neutron star merger event, called GW170817. The team used the WM Keck Observatory to analyse the host galaxy, which Northwestern has special remote access to.
Anya Nugent, a study co-author, said in the statement that after the detection of neutron star merger event GW170817 and its association with a massive, red-and-dead host galaxy, many astronomers assumed that hosts of neutron star mergers in the near universe would look similar to the host galaxy of GW170817, called NGC4993.
She added that the host galaxy of the newly detected gamma-ray burst is fairly young, actively star forming and not actually that massive. This host galaxy looks more similar to short gamma-ray burst hosts seen deeper in the universe.
Nugent said the new discovery changes scientists' view of the types of galaxies they should watch when they are searching for nearby kilonovae.
How the new discovery will help astrophysicists search for heavy elements
According to the study, the new discovery also changes how astrophysicists might approach the search for heavy elements, such as platinum and gold. Helium, silicon and carbon are some of the lighter elements, and researchers have been able to study the astronomical factories that produce these. While astrophysicists argue that supernova explosions and neutron star mergers produce the heaviest elements, clear signatures of their creation are rarely observed.
Rastinejad said radioactive decay of some of the heaviest elements in the universe powers kilonovae. However, kilonovae are very hard to observe and fade very quickly. She explained that now astrophysicists know they can use some long gamma-ray bursts to search for more kilonovae.
With the help of the James Webb Space Telescope (JWST), astrophysicists will be able to look for more clues within kilonovae. Since the JWST is capable of capturing images and spectra of astronomical objects, it can detect specific elements emitted from the object.
Therefore, astrophysicists can obtain observational evidence of heavy elements' formation using Webb.
Rastinejad said with the JWST, one could have obtained a spectrum of the kilonova, and those spectral lines provide direct evidence that one has detected the heaviest elements.