Colossal explosion challenges our understanding of gamma-ray bursts

The International Gemini Observatory discovers startling evidence of colliding neutron stars after probing the aftermath of a gamma-ray burst.

While investigating the aftermath of a long gamma-ray burst (GRB), two independent teams of astronomers using a multitude of telescopes in space and on Earth discovered the unexpected characteristics of a kilonova. This is the colossal explosion triggered by the collision of neutron stars. This finding challenges the prevailing theory that long GRBs come exclusively from supernovae, the end-of-life explosions of massive stars.

Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe. They come in two varieties, long and short. Long GRBs, which last from a few seconds to a minute, form when a star at least 10 times the mass of our Sun explodes in supernova. Short GRBs, which last less than two seconds, occur when two compact objects, such as two neutron stars or a neutron star and one black holecollide to form a kilonova.

By observing the aftermath of a long GRB detected in 2021, two independent teams of astronomers have found the startling signs of a neutron star merger rather than the expected signal of a supernova. This surprising result marks the first time that a kilonova has been associated with a long GRB and challenges our understanding of these phenomenally powerful explosions.

The first team to announce this discovery was led by Jillian Rastinejad, a doctoral student at[{” attribute=””>Université du nord-ouest. Rastinejad et ses collègues ont fait cette découverte surprenante avec l’aide du télescope Gemini North à Hawai’i, qui fait partie de l’Observatoire international Gemini, qui est exploité par le NOIRLab de la NSF. Les observations de Gemini North ont révélé une rémanence proche infrarouge révélatrice à l’emplacement précis du GRB, fournissant la première preuve convaincante d’une kilonova associée à cet événement.^{” width=”777″ height=”437″ srcset=”” sizes=”” ezimgfmt=”rs rscb2 src ng ngcb2 srcset” loading=”eager” importance=”high”/>}

L’équipe de Rastinejad a rapidement signalé leur détection Gemini dans une circulaire du réseau de coordonnées gamma (GCN).

Les astronomes du monde entier ont été alertés pour la première fois de ce sursaut, nommé GRB 211211A, lorsqu’un puissant flash de rayons gamma a été capté par Nasade l’observatoire Neil Gehrels Swift et du télescope spatial à rayons gamma Fermi. Les premières observations ont révélé que le GRB était exceptionnellement proche, à seulement un milliard d’années-lumière de la Terre.

Interview with Eleonora Troja, an astronomer at the University of Rome Tor Vergata, who has studied GRB remanence using a series of observations, including the Gemini South Telescope in Chile, and has independently concluded that the long GRB came from a kilonova.

Most GRBs come from the distant and early Universe. Typically, these objects are so old and so distant that their light would have had to travel over six billion years to reach Earth. Light from the most distant GRB on record traveled nearly 13 billion years before being detected here on Earth.^[2] The relative proximity of this newly discovered GRB has allowed astronomers to make remarkably detailed tracking studies with a variety of ground-based and space-based telescopes.

“Astronomers typically study short GRBs when hunting for kilonovae,” Rastinejad said. “We were attracted to this longer-lasting burst because it was so close we could study it in detail. Its gamma rays also resembled those of a mysterious, shorter-lived ancient GRB without a supernova.

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 longer wavelength infrared light to pass through unimpeded. Near-infrared observation, however, is technically difficult and only a handful of telescopes on Earth, such as the twin Gemini telescopes, are sensitive enough to detect this kilonova at these wavelengths.

Jillian Rastinejad, a PhD student at Northwestern University, and her colleagues used the Gemini North telescope to reveal a telltale near-infrared afterglow at the precise location of the GRB, providing the first convincing evidence of a kilonova associated with this event.

“Thanks to its sensitivity and our rapid response, Gemini was the first to detect this kilonova in the near infrared, convincing us that we were observing a neutron star merger,” Rastinejad said. “Gemini’s nimble capabilities and variety of instruments allow us to adapt each night’s observing plan based on the previous night’s results, allowing us to make the most of every minute our target was observable. “

Another team, led by Eleonora Troja, an astronomer at the University of Rome Tor Vergata, independently studied afterglow using another series of observations, including the Gemini South telescope in Chile,^[3] and independently concluded that the long GRB originated from a kilonova.

“We were able to observe this event only because it was so close to us,” Troja said. “It is very rare that we observe such powerful explosions in our cosmic backyard, and each time we do, we discover the most extreme objects in the Universe.”

The fact that two different teams of scientists working with independent datasets both came to the same conclusion about the kilonova nature of this GRB gives confidence in this interpretation.

“Kilonova’s interpretation was so far removed from anything we knew about long GRBs that we couldn’t believe our own eyes and spent months testing every other possibility,” Troja said. “It was only after excluding everything else that we realized that our decade-long paradigm had to be overhauled.”

In addition to contributing to our understanding of kilonovae and GRBs, this discovery offers astronomers a new way to study the formation of gold and other heavy elements in the Universe. The extreme physical conditions of kilonovae produce heavy elements such as gold, platinum, and thorium. Astronomers can now identify sites that create heavy elements by looking for the signature of a kilonova following a long-lasting gamma-ray burst.

“This discovery is a clear reminder that the Universe is never fully understood,” Rastinejad said. “Astronomers often take it for granted that the origins of GRBs can be identified by their length, but this discovery shows us that there is still much to understand about these amazing events.”

“NSF congratulates the science teams on this new and exciting discovery, opening a new window into cosmic evolution,” said National Science Foundation Director Sethuraman Panchanathan. “The International Gemini Observatory continues to provide powerful and agile resources open to the entire scientific community through innovation and partnership.”

For more on this research, see Undetected neutron-star hybrid fusion event revealed by unusual burst of gamma rays.

The Gemini International Observatory is operated by a partnership of six countries, including the United States through the National Science Foundation, Canada through the National Research Council of Canada, Chile through the Agencia Nacional de Investigación y Desarrollo, Brazil through the Ministério da Ciência, Tecnologia e Inovações, Argentina through the Ministerio de Ciencia, Tecnología e Innovación, and Korea through the Korea Institute of Astronomy and Space Science. These participants and the University of Hawaii, which has regular access to Gemini, each maintain a national Gemini office to support their local users.

Remarks

1. Rastinejad and his colleagues made initial follow-up observations of the burst using the Nordic Optical Telescope. Following critical observations from Gemini North, they continued their observations of kilonova fading with the Karl G. Jansky Very Large Array, Calar Alto Observatory, and MMT Observatory, and obtained subsequent observations with the large Binocular Telescope, WM Keck Observatory, Gran Telescopio Canarias and NASA/ESA[{” attribute=””>Hubble Space Telescope.
2. Light that has traveled nearly 13 billion years to reach Earth would have a redshift (z) of about 7. Due to the accelerating expansion of the Universe, that would roughly equate to a distance of 24.5 billion light-years today. When talking about large redshifts, those greater than 1, and cosmically distant objects, it is more accurate to state how many billions of years the light has traveled rather than a distance in light-years.
3. Troja and her colleagues initially observed the afterglow of this event with the Devasthal Optical Telescope, the Multicolor Imaging Telescopes for Survey and Monstrous Explosions, and the Calar Alto Observatory. They obtained observations of the host galaxy with the NASA/ESA Hubble Space Telescope.

References:

“A kilonova following a long-duration gamma-ray burst at 350 Mpc” by Jillian C. Rastinejad, Benjamin P. Gompertz, Andrew J. Levan, Wen-fai Fong, Matt Nicholl, Gavin P. Lamb, Daniele B. Malesani, Anya E. Nugent, Samantha R. Oates, Nial R. Tanvir, Antonio de Ugarte Postigo, Charles D. Kilpatrick, Christopher J. Moore, Brian D. Metzger, Maria Edvige Ravasio, Andrea Rossi, Genevieve Schroeder, Jacob Jencson, David J. Sand, Nathan Smith, José Feliciano Agüí Fernández, Edo Berger, Peter K. Blanchard, Ryan Chornock, Bethany E. Cobb, Massimiliano De Pasquale, Johan P. U. Fynbo, Luca Izzo, D. Alexander Kann, Tanmoy Laskar, Ester Marini, Kerry Paterson, Alicia Rouco Escorial, Huei M. Sears and Christina C. Thöne, 7 December 2022, Nature.
DOI: 10.1038/s41586-022-05390-w

“A nearby long gamma-ray burst from a merger of compact objects” by E. Troja, C. L. Fryer, B. O’Connor, G. Ryan, S. Dichiara, A. Kumar, N. Ito, R. Gupta, R. Wollaeger, J. P. Norris, N. Kawai, N. Butler, A. Aryan, K. Misra, R. Hosokawa, K. L. Murata, M. Niwano, S. B. Pandey, A. Kutyrev, H. J. van Eerten, E. A. Chase, Y.-D. Hu, M. D. Caballero-Garcia, A. J. Castro-Tira, 7 December 2022, Nature.
DOI: 10.1038/s41586-022-05327-3