Astronomers have detected a mysterious cosmic explosion

Artist's conception of a gamma ray burst

Artist’s conception of a gamma-ray burst caused by the violent collision of two massive neutron stars, after their apocalyptic emergence. In addition to high-energy radiation and matter ejected in a narrow jet, the event is thought to be the universe’s main factory for heavy elements, including gold and platinum. Credit: A. Simonnet (Sonoma State University) and Goddard Space Flight Center

Gamma ray bursts are the most powerful explosions in the universe and are usually caused by the collapse of stars or the collision of compact stellar remnants. However, a recent discovery has challenged this understanding, as it does not fit into either of those categories. Astronomers from the Niels Bohr Institute were instrumental in this study, which has the potential to revise existing theories about these powerful events.

Daniel Bjorn Malisani has been doing routine follow-up observations of the gamma-ray burst, named GRB 211211A, using the Scandinavian Optical Telescope on the Canary island of La Palma. Standard procedure after receiving the text message that was automatically triggered by the Neil Gehrels Swift Observatory spacecraft that monitors the skies for gamma ray bursts.

But something wasn’t quite right…

Hubble Space Telescope view of the location and surroundings of GRB 211211A

The Hubble Space Telescope has shown the location and surroundings of GRB 211211A. Zoom in shows the afterglow, as observed with the Gemini North telescope in Hawaii. The binary system that caused the explosion was likely expelled in the past by the large bluish galaxy to its left. Credit: Gemini Observatory International/NOIRLab/NSF/AURA/M. Zamani. NASA/European Space Agency

Malisani is an astronomer at Radboud University in the Netherlands and a visiting researcher at the Cosmic Dawn Center in Copenhagen. It specializes in gamma ray bursts, the most energetic explosions in the universe.

But to understand what isn’t true, let’s first look at what a “gamma ray burst” is:

As bright as the universe itself

Gamma ray bursts are short, super bright flashes of the most energetic form of light, gamma rays. Mostly discovered in the very distant universe, they usually come in two categories that are thought to arise from two different physics scenarios:

The “long” pulses typically last from a few seconds to several minutes, but are often accompanied by prolonged afterglows of less energetic light. They are found in the most star-forming regions of galaxies and are thought to be caused by a massive star collapsing into a compact volume.[{” attribute=””>neutron star or a Nordic Optical Telescope

The Nordic Optical Telescope on the 2400 meters high mountain top Roche de los Muchachos in La Palma. Credit: Peter Laursen (Cosmic Dawn Center)

“Short” bursts are even more fleeting, with typical durations of 1/10 to 1 second. They are often seen offset from the galactic centers, or even outside galaxies. The prevailing theory is that they are the outcome of two massive stars orbiting each other in a “binary” system. At some point, they explode as supernovae, kicking them out of their host galaxy. Eventually, however, the two objects will spiral in and merge, resulting in a gamma-ray burst.

In both cases, the energy released is mind-blowing: At their peak, they can shine as brightly as all the stars in the observable Universe combined (assuming that they emit light equally in all directions; in reality, they are likely somewhat less bright but emit most of their light in narrow jets, where we just happen to lie in this direction).

The enigmatic gamma-ray bursts

Gamma-ray bursts were first discovered in 1967 by the Vela satellite, built to monitor the sky for possible tests of nuclear weapons, which would be a violation of the 1963 Nuclear Test Ban Treaty. First thought to originate from nearby sources within our own galaxy, more sensitive space observatories revealed, in the 1990s, that they must come from far outside the

Gamma-ray bursts come in two versions, “short” and “long”, which have, until now, been thought to arise from two different physical mechanisms, namely the merging of two compact objects, and the collapse of a massive star, respectively. With the new observations, this theory is now being challenged.

Mixed signals

So what was the issue with Malesani’s burst, GRB 211211A? Well, it seemed to fit in neither, or perhaps both, of these categories. “The observations showed that the burst originated outside of a galaxy typical for hosting short bursts. But rather than being a millisecond or a few seconds, this beast lasted for almost a minute,” Malesani says.

The peculiar event prompted an international team of astronomers, led by Jillian Rastinejad of

Luca Izzo, an astronomer at the DARK research section at the Niels Bohr Institute, participated in the study. He comments: “Gamma-ray bursts can show a variety of behaviors, but the distinction between long and short events has been clearly established since the 1990s and is considered one of the pillars in the field. This finding caught us really by surprise.”

A new engine for making gold?

Kilonovae are thought to be the main mechanism for creating heavy elements such as the precious silver, gold, and platinum, the radioactive plutonium and uranium, as well as many others. As always in physics, definite proof that a kilonova is responsible for the long gamma-ray burst does not exist.

When the astronomers nevertheless are confident in their interpretation, it is due to several circumstances. Johan Fynbo, professor at the Cosmic Dawn Center and partaker in the study, explains:

“The afterglow of the burst showed colors and features that are consistent with a kilonova, and which haven’t been seen for any other types of objects. Moreover, we would not expect to see a collapsing star outside of a galaxy, since traveling this far takes hundreds of millions of years, while massive stars collapse on timescales less than 10 million years.”

But in principle, GRB 211211A could be a collapsar inside a faint or dusty, undetected galaxy, although the Hubble images are indeed very deep and ought to have seen this. “Follow-up observations with the more sensitive

Future observations of more long bursts from kilonovae will teach us more about this exciting phenomenon. 

Reference: “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

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