Kilonova Discovery Challenges our Understanding of Gamma-Ray Bursts
Cambridge, Mass. – Gamma-ray bursts (GRBs) — the most energetic explosions in the universe — come in two varieties, long and short. Long GRBs, which last from two to hundreds of seconds, form when a massive star at least 10 times the mass of our Sun explodes as a supernova. Short GRBs, which last less than two seconds, occur when two compact objects, like two neutron stars or a neutron star and a black hole, collide and form a kilonova.
While observing the aftermath of a long GRB detected in 2021, two independent teams of astronomers have found the surprising signs of a neutron-star merger rather than the expected signal of a supernova. This surprising result, described today in the journal Nature, marks the first time that a kilonova has been associated with a long GRB, challenging scientists’ understanding of GRBs.
"Our detailed studies of GRBs over the past two decades have led to the tidy conclusion that a burst's duration directly maps to its progenitor system -- long GRBs from massive stars, and short GRBs from colliding neutron stars," said Edo Berger professor of astronomy at Harvard University and the Center for Astrophysics | Harvard & Smithsonian and co-author of one of the new studies. "It turns out that nature is more complex than that."
The first team to announce this discovery was led by Jillian Rastinejad, a graduate student at Northwestern University. Rastinejad and her colleagues made this startling discovery with the help of Gemini North, which is operated by NSF's NOIRLab. The Gemini North observations revealed a telltale near-infrared afterglow at the precise location of the GRB, providing the first compelling evidence of a kilonova associated with this event. Rastinejad's team promptly reported their Gemini detection in a Gamma-ray Coordinates Network (GCN) Circular.
Astronomers around the world were first alerted to this burst, named GRB 211211A, when a powerful flash of gamma rays was picked up by NASA's Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope. Initial observations revealed that the GRB was unusually nearby, a mere one billion light-years from Earth. Most GRBs originate more than six billion light-years away and the most-distant GRB occurred nearly 13 billion light-years away. The relative proximity of this newly discovered GRB enabled astronomers to make remarkably detailed follow-up studies with a variety of ground- and space-based telescopes.
"Astronomers usually investigate short GRBs when hunting for kilonovae," says Rastinejad. "We were drawn to this longer-duration burst because it was so close that we could study it in detail. Its gamma rays also resembled those of a previous, mysterious supernova-less long GRB."
A unique observational signature of kilonovae is their brightness at near-infrared wavelengths compared to their brightness in visible light. This difference in brightness is due to the heavy elements ejected by the kilonova, which effectively block visible light but allow the longer-wavelength infrared light to pass unimpeded. Observing in the near-infrared, however, is technically challenging and only a handful of telescopes on Earth, like the twin Gemini telescopes, are sensitive enough to detect this kilonova at these wavelengths.
Another team, led by Eleonora Troja, an astronomer at the University of Rome Tor Vergata, independently studied the afterglow using a different approach and a different series of observations, including the Gemini South telescope in Chile, and also concluded that the long GRB came from a kilonova.
"We were able to observe this event only because it was so close to us," says Troja. "It is very rare that we observe such powerful explosions in our cosmic backyard, and every time we do we learn about the most extreme objects in the universe."
The fact that two different teams of scientists working with independent data sets both arrived at the same conclusion about the kilonova nature of this GRB provides confidence in this interpretation.
As well as contributing to the understanding of kilonovae and GRBs, this discovery provides astronomers with a new way to study the formation of gold and other heavy elements in the universe. The extreme physical conditions in kilonovae produce heavy elements such as gold, platinum, and thorium. Astronomers can now identify the sites that are creating heavy elements by searching for the signature of a kilonova following a long-duration gamma-ray burst.
"The hunt for elusive kilonovae -- the cosmic factories of gold and other rare elements -- has so far focused only on short GRBs and gravitational wave events," says Berger. "But now we have a third suspect to chase down, long GRBs."
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About NOIRLab
NSF's NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy's SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du'ag (Kitt Peak) in Arizona, on Maunakea in Hawai'i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O'odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.
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