For nearly two decades, astrophysicists have believed that long gamma-ray bursts (GRBs) resulted solely from the collapse of massive stars. Now, a new study overturns this long-held and accepted belief.
Led by Northwestern University, a team of astrophysicists has discovered new evidence that at least some long GRBs may result from neutron star mergers, which were previously thought to produce only short GRBs.
After detecting a 50-second-long GRB in December 2021, the team began searching for the GRB’s long glow, an incredibly bright and fast burst of light that often precedes a supernova. But instead, they discovered evidence of a kilonova, a rare event that only occurs after a neutron star merges with another compact object (either another neutron star or a black hole).
In addition to challenging long-held beliefs about how long GRBs form, the new discovery also leads to new insights into the mysterious formation of the universe’s heaviest elements.
The research was published today (December 7) in the journal Nature.
“This event is unlike anything we’ve seen before from a long gamma-ray burst,” said Northwestern’s Jillian Rastinejad, who led the study. “Its gamma rays resemble bursts produced by collapsing massive stars. Since all other confirmed neutron star mergers we’ve observed have been accompanied by bursts lasting less than two seconds, we had all reasons to expect that this 50-second GRB was created by the collapse of a massive star. This event represents an exciting paradigm shift for gamma-ray burst astronomy.”
“When we followed this long gamma-ray burst, we expected it to lead to evidence of a massive stellar collapse,” said Northwestern’s Wen-fai Fong, lead author of the study. “Instead, what we found was very different. When I entered the field 15 years ago, it was set in stone that long-gamma-ray bursts come from collapsing massive stars. This unexpected finding not only represents a major shift in our understanding, but also excitingly opens a new window for discovery.”
Fong is an assistant professor of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences and a key member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Rastinejad, Ph.D. student in astronomy and a member of Fong’s research group, is the paper’s first author.
long division
The brightest and most energetic explosions since the Big Bang, GRBs fall into two classes. GRBs with durations of less than two seconds are considered short GRBs. If a GRB lasts longer than two seconds, it is considered a long GRB. Researchers previously believed that GRBs on either side of the dividing line must have different origins.
In December 2021, Neil Gehrels’ Swift Observatory’s Burst Alert Telescope and the Fermi Gamma-ray Space Telescope detected a bright burst of gamma-ray light, called GRB211211A. At just over 50 seconds long, GRB211211A didn’t initially appear to be anything special. But located about 1.1 billion light-years away, which, believe it or not, is relatively close to Earth, astrophysicists decided to study this “nearby” event in detail, using a multitude of telescopes that could observe at through the electromagnetic spectrum.
To image the event at near-infrared wavelengths, the team quickly began imaging with the Gemini Observatory in Hawaii. After two days of observing with Gemini, Rastinejad became concerned that he would not be able to get a clear view.
“The weather was getting worse in Hawaii and we were very disappointed because we were starting to see signs that this explosion was unlike anything we had seen before,” he said. “Fortunately, Northwestern gives us remote access to the MMT Observatory in Arizona, and the next day an ideal instrument was being put on that telescope. It was cloudy there, but the telescope operators knew how important this burst was and they found a gap in the clouds to take our pictures. It was stressful but very exciting to get these pictures in real time.”
“Telling sign of a kilonova”
After examining the near-infrared images, the team spotted an incredibly faint object that quickly faded. Supernovae don’t fade as quickly and are much brighter, so the team realized they found something unexpected that was previously thought impossible.
“There are many objects in our night sky that fade quickly,” Fong said. “We image a source with different filters to obtain color information, which helps us determine the identity of the source. In this case, the red color prevailed and the bluer colors faded faster. This color evolution is a telltale signature of a kilonova, and kilonovae can only come from neutron star mergers.”
Because neutron stars are clean and compact objects, researchers previously believed that neutron stars did not contain enough material to power a long-lasting GRB. Massive stars, on the other hand, can be tens to hundreds of times the mass of our sun. When the dying star collapses, its material falls inward to feed a newly formed black hole. But thanks to the black hole’s magnetic fields, some of the inward-falling material is thrown outward at speeds close to the speed of light, fueling a GRB.
“When you put two neutron stars together, there really isn’t too much mass,” Fong explained. “A little bit of mass builds up and then generates a very short-lived explosion. In the case of massive star collapses, which traditionally cause longer gamma-ray bursts, there is a longer feed time.” .
Changing the search
The event wasn’t the only strange part of the study. The GRB’s host galaxy is also quite curious. Named SDSS J140910.47+275320.8, the host galaxy is young and star-forming, almost exactly opposite the only other known host in the local universe for a neutron star merger event: the host galaxy NGC4993 from GW170817. To analyze the host galaxy, the team used data from the WM Keck Observatory, to which Northwestern has special remote access.
“Following the detection of GW170817 and its association with a massive, red, dead host galaxy, many astronomers assumed that hosts of merging neutron stars in the nearby universe would look similar to NGC4993,” he said. Anya Nugent, MD, Northwestern. . astronomy student and co-author of studies. “But this galaxy is quite young, actively forming stars, and actually not that massive. In fact, it looks more like the short GRB hosts that are seen deeper in the universe. I think it changes our view of the types of galaxies we should be looking at when looking for nearby quilonovae.”
It also changes the way astrophysicists might approach the search for heavy elements, such as platinum and gold. Although researchers have been able to study the astronomical factories that produce lighter elements, such as helium, silicon, and carbon, astrophysicists postulate that supernova explosions and neutron star mergers produce the heavier elements. However, clear signatures of their creation are rarely observed.
“Kilonovae are fueled by the radioactive decay of some of the heaviest elements in the universe,” said Rastinejad. “But kilonovae are very hard to observe and fade away very quickly. Now, we know we can also use some long gamma-ray bursts to look for more kilonovae.”
Now that the James Webb Space Telescope (JWST) is up and running, astrophysicists will be able to look for more clues inside quilonovae. Because the JWST is able to capture images and spectra of astronomical objects, it can detect specific elements emitted by the object. Using Webb, astrophysicists could finally obtain direct observational evidence of the formation of heavy elements.
“Unfortunately, even the best ground-based telescopes are not sensitive enough to perform spectroscopy,” said Rastinejad. “With the JWST, we could have obtained a spectrum of the kilonova. These spectral lines provide direct evidence that you have detected the heavier elements.”
The study, “A kilonova after a long-duration gamma-ray burst at 350 Mpc,” was supported by the National Science Foundation (grant numbers AST-1814782, AST-1909358, and AST-2047919), the David and Lucile Packard Foundation, the European Research Council and NASA.
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