Scientists use ALMA observatory to capture neutron star merger for the first time ever

This artist’s conception shows the merger between a neutron star and another star (seen as a disk, lower left) which caused an explosion resulting in the short-duration gamma-ray burst, GRB 211106A (white jet, middle), and left behind what scientists now know to be one of the most luminous afterglows on record (semi-spherical shock wave mid-right). While dust in the host galaxy obscured most of the visible light (shown as colors), millimeter light from the event (depicted in green) was able to escape and reach the Atacama Large Millimeter/submillimeter Array (ALMA), giving scientists an unprecedented view of this cosmic explosion. From the study, the team confirmed that GRB 211106A is one of the most energetic short-duration GRBs ever observed. Credit: ALMA (ESO/NAOJ/NRAO), M. Weiss (NRAO/AUI/NSF)

Scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) have, for the first time, recorded millimeter-wavelength light from a ‘fiery explosion caused by the merger of a neutron star with another star.’ In addition to the incredible observation, the team also confirmed that the flash of light caused by the star merger is among the most energetic short-duration gamma-ray bursts ever observed, ‘leaving behind one of the most luminous afterglows on record.’

Gamma-ray bursts (GRBs) are the most energetic explosions in the universe and can emit more energy in a few seconds than the Sun will emit during its entire lifespan. GRB 211106A is part of a GRB sub-class known as short-duration gamma-ray bursts. Scientists believe that GRBs create some of the heaviest elements in the universe.

The GRBs, which result from the ‘catastrophic’ merger of binary star systems including a neutron star, ‘occur because of gravitational wave radiation that removes energy from the orbit of the binary stars, causing the stars to spiral in toward each other,’ said Tanmoy Laskar. Laskar will soon begin working as an Assistant Professor of Physics and Astronomy at the University of Utah. ‘The resulting explosion is accompanied by jets moving at close to the speed of light. When one of these jets is pointed at Earth, we observe a short pulse of gamma-ray radiation or a short-duration GRB,’ Laskar continued.

Atacama Large Millimeter/submillimeter Array (ALMA) Observatory in northern Chile, located in the heart of the Atacama Desert.

Short-duration GRBs last only a few tenths of a second. Following the short, explosive event, scientists search for an afterglow, which is emitted light caused by the interaction of the energetic jets with surrounding gas. They’re difficult to detect and observe. Only about half a dozen short-duration GRBs have been detected using radio wavelengths and, until the new, momentous observation, had only been detected using millimeter wavelengths. Laskar, who led the research as an Excellence Fellow at Radboud University in The Netherlands, explained, ‘Short-duration GRB afterglows are very luminous and energetic. But these explosions take place in distant galaxies which means the light from them can be quite faint for our telescopes on Earth. Before ALMA, millimeter telescopes were not sensitive enough to detect these afterglows.’

The observation also allowed researchers to measure the opening angle of the jet. Rouco Escorial, research coauthor and postdoctoral fellow at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University, said, ‘The millimeter and radio bands provided us with information we needed to measure the jet opening angle. This is essential to infer the real rates of short GRBs in our universe and to compare them with the rates of binary neutron star or neutron star and black hole mergers.’

GRB 211106A is quite far and faint, having occurred when the universe was about 40% of its current age. The light from the short-duration gamma-ray burst was so faint that even though NASA’s Neil Gehrels Swift Observatory did see the explosion using X-ray observation, it was unable to observe the host galaxy at that wavelength, meaning that scientists couldn’t pinpoint the origin of the explosion. Afterglow is important to help locate the source of this type of explosion. The Hubble Space Telescope also couldn’t help much as there was too much dust in the area, obscuring the object from detection.

‘In the first-ever time-lapse movie of a short-duration gamma-ray burst in millimeter-wavelength light, we see GRB 21106A as captured with the Atacama Large Millimeter/submillimeter Array (ALMA). The millimeter light seen here pinpoints the location of the event to a distant host galaxy in images captured using the Hubble Space Telescope. The evolution of the millimeter light’s brightness provides information on the energy and geometry of the jets produced in the explosion.’ Credit: ALMA (ESO/NAOJ/NRAO), T. Laskar (Utah), S. Dagnello (NRAO/AUI/NSF) If the GIF doesn’t display properly, click it to open it in its own window.

‘This short gamma-ray burst was the first time we tried to observe such an event with ALMA. Afterglows for short bursts are very difficult to come by, so it was spectacular to catch this event shining so bright,’ said Wen-fai Fong, an Assistant Professor of Physics and Astronomy at Northwestern University. ‘After many years of observing these bursts, this surprising discovery opens up a new area of study, as it motivates us to observe many more of these with ALMA, and other telescope arrays, in the future.’

Joe Pesce, the National Science Foundation Program Officer for NRAO/ALMA, said, ‘These observations are fantastic on many levels. They provide more information to help us understand the enigmatic gamma-ray bursts (and neutron-star astrophysics in general), and they demonstrate how important and complementary multi-wavelength observations with space- and ground-based telescopes are in understanding astrophysical phenomena.’ ALMA is an international astronomy facility co-operated by the US National Science Foundation’s National Radio Observatory (NRAO) and other research groups worldwide.

Researchers will now use other telescopes and observatories to investigate GRB 21106A. The James Webb Space Telescope can take a spectrum of the host galaxy, determining its distance and possibly even its chemical composition. The next generation VLA (ngVLA) will allow better observation of very distant events, like GRB 211106A. Laskar added, ‘With JWST, we can now take a spectrum of the host galaxy and easily know the distance, and in the future, we could also use JWST to capture infrared afterglows and study their chemical composition. With ngVLA, we can study the geometric structure of the afterglows and the star-forming fuel found in their host environments in unprecedented detail. I am excited about these upcoming discoveries in our field.’

The research paper is available in a draft version. ‘The First Short GRB Millimeter Afterglow: The Wide-Angled Jet of the Extremely Energetic SGRB 211106A’ is led by Tanmoy Laskar. Additional researchers include Alicia Rouco Escorial, Genevieve Schroeder, Wen-fai Fong, Edo Berger, Péter Veres, Shivani Bhandari, Jillian Rastinejad, Charles D. Kilpatrick, Aaron Tohuvavohu, Raffaella Margutti, Kate D. Alexander, James DeLaunay, Jamie A. Kennea, Anya Nugent, K. Paterson, and Peter K. G. Williams.