Explosive neutron star fusion captured in millimeter light for the first time

Scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) – an international observatory co-operated by the US National Science Foundation’s National Radio Astronomy Observatory (NRAO) – have for the first time recorded millimeter-wave light from a fiery explosion caused by the fusion of a neutron star with another star. The team also confirmed that this burst of light was one of the most energetic short-lived gamma-ray bursts ever observed, leaving one of the brightest afterglows ever recorded. The research results will be published in an upcoming issue of Letters from the Astrophysical Journal.

Gamma-ray bursts (GRBs) are the brightest and most energetic explosions in the universe, capable of emitting more energy in a few seconds than our sun will emit in its entire lifetime. GRB 211106A belongs to a GRB subclass known as short-lived gamma-ray bursts. These explosions – which scientists believe are responsible for creating the heaviest elements in the universe, such as platinum and gold – are the result of the catastrophic merger of binary star systems containing a neutron star. “These mergers occur due to gravitational wave radiation that removes energy from the orbits of binary stars, causing the stars to spiral toward each other,” said Tanmoy Laskar, soon to be an assistant professor of physics and astronomy at the university. University of Utah. “The resulting explosion is accompanied by jets that move close to the speed of light. When one of these jets points towards Earth, we observe a short pulse of gamma radiation, or a short-lived GRB. »

A short-lived GRB typically lasts only a few tenths of a second. The researchers then look for an afterglow, an emission of light caused by the jets’ interaction with the surrounding gas. Even still, they are hard to detect; only half a dozen short-lived GRBs have been detected at radio wavelengths, and so far none have been detected at millimeter wavelengths. Laskar, who led the research while an excellence fellow at Radboud University in the Netherlands, said the difficulty is the enormous distance to the GRBs and the telescopes’ technological capabilities. “The short-lived GRB afterglows are very bright and energetic. But these explosions take place in distant galaxies, which means that the light they emit can be quite weak for our telescopes on Earth. Before ALMA, millimeter telescopes were not sensitive enough to detect these afterglows.”

About 20 billion light years from Earth, GRB 211106A is no exception. The light from this short-lived gamma-ray burst was so faint that while early X-ray observations with NASA’s Neil Gehrels Swift Observatory saw the explosion, the host galaxy was undetectable at that wavelength, and scientists have been unable to determine precisely. where the explosion came from. “The afterglow is crucial to determining which galaxy an outburst originated from and to learning more about the outburst itself. Initially, when only the X-ray counterpart had been discovered, astronomers thought this outburst might have come from a nearby galaxy. Laskar said and added that a significant amount of dust in the area also obscured the object from detection in optical observations with the Hubble Space Telescope.

Each wavelength added a new dimension to GRB scientists’ understanding, and the millimeter in particular was the key to revealing the truth about the burst. “Hubble’s observations have revealed an unchanging field of galaxies. ALMA’s unparalleled sensitivity allowed us to more precisely locate the GRB’s location in this field, and it turned out to be in another faint galaxy, which is further away. This means on the other hand, that this short-lived gamma-ray burst is even more powerful than we first thought, making it one of the brightest and most energetic on record, says Laskar.

Wen-fai Fong, assistant professor of physics and astronomy at Northwestern University, added: “This short gamma-ray burst was the first time we have tried to observe such an event with ALMA. Afterglows for short bursts are very difficult to find, so after many years observing these bursts, this surprising discovery opens up a new field of study as it motivates us to observe many more with ALMA and other telescope arrays in the future. »

Joe Pesce, National Science Foundation program manager for NRAO/ALMA, said: “These observations are amazing on many levels. They provide more information to help us understand enigmatic gamma-ray bursts (and neutron star astrophysics in general), and they demonstrate how important multiwavelength observations with space and ground-based telescopes are, and complementary to understand astrophysical phenomena. »

And there is still much work to be done across multiple wavelengths, both with the new GRBs and with GRB 211106A, which may reveal additional surprises about these bursts. “The study of short-lived GRBs requires rapid coordination of telescopes around the world and in space operating at all wavelengths,” said Edo Berger, professor of astronomy at Harvard University. “In the case of GRB 211106A, we used some of the most powerful telescopes available—ALMA, the National Science Foundation’s Karl G. Jansky Very Large Array (VLA), NASA’s Chandra X-ray Observatory, and the Hubble Space Telescope. With the James Webb Space Telescope (JWST) now operational and future 20-40 meter optical and radio telescopes such as the Next Generation VLA (ngVLA), we will be able to produce a complete picture of these cataclysmic events and study at unprecedented distances. »

Laskar added: “With JWST we can now take a spectrum of the host galaxy and know the distance easily, and in the future we can also use JWST to capture infrared afterglows and study their chemical composition. With ngVLA we will be able to study the geometric structure of afterglows and the star-forming fuel found in their host environments in unprecedented detail.I am excited about these upcoming discoveries in our field.

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