Scientists have used the Atacama Large Millimeter/submillimeter Array (ALMA) – an international observatory in partnership with the US National Science Foundation’s National Radio Astronomy Observatory (NRAO) – to record millimeter-wave light for the first time from a fiery explosion caused by the merger of a neutron star with another star. The team also confirmed that this flash is one of the most energetic short-duration gamma-ray bursts ever observed, leaving behind one of the most luminous afterglows ever recorded. The results of the research will be published in an upcoming issue of the Astrophysical Journal Letters.
Gamma-ray bursts (GRBs) are the brightest and most energetic explosions in the Universe, capable of emitting more energy in a matter of seconds than our Sun emits in its lifetime. GRB 211106A belongs to a GRB subclass known as short-duration gamma-ray bursts. These explosions – which scientists believe are responsible for the formation of the heaviest elements in the universe, such as platinum and gold – result from the catastrophic merger of binary star systems containing a neutron star. “These mergers occur because of gravitational-wave radiation, which siphons energy from the binary stars’ orbit and causes the stars to spiral toward each other,” said Tanmoy Laskar, Excellence Fellow at Radboud University. “The resulting explosion is accompanied by jets traveling at nearly the speed of light. When one of these jets is aimed at Earth, we observe a short burst of gamma rays, or a short-lived GRB.”
A short-duration GRB usually lasts only a few tenths of a second. Scientists then look for an afterglow, an emission of light caused by the jets’ interaction with surrounding gas. Despite this, they are difficult to spot; only half a dozen short-duration GRBs have been detected at radio wavelengths, and so far none have been detected at millimeter wavelengths. Laskar said the difficulty lies in the immense distance to GRBs and the technological capabilities of telescopes. “Short duration GRB afterglows are very bright and energetic. But these explosions take place in distant galaxies, meaning the light from them can be quite faint for our telescopes on Earth. Before ALMA, millimeter telescopes were not sensitive enough to detect this afterglow.”
Located about 20 billion light-years from Earth, GRB 211106A is no exception. The light from this brief gamma-ray burst was so faint that early X-ray observations with NASA’s Neil Gehrel’s Swift Observatory saw the explosion, but the host galaxy was undetectable at that wavelength and scientists couldn’t pinpoint exactly where the explosion came from. “Afterglow is essential for finding out which galaxy a burst came from and for learning more about the burst itself. Initially, when only the X-ray counterpart had been spotted, astronomers thought this burst might be coming from a nearby galaxy,” Laskar said, adding that a significant amount of dust in the area also obscured detection of the object in optical observations with the Hubble Space Telescope.
Luminous and energetic gamma-ray burst
Each wavelength added a new dimension to scientists’ understanding of the GRB, and millimeters in particular were crucial in uncovering the truth about the eruption. “The Hubble observations showed an unchanging galaxy field. ALMA’s unprecedented sensitivity allowed us to more accurately pinpoint the position of the GRB in this field, and it turned out to be in another faint galaxy, more distant. That in turn means this brief gamma-ray burst is even stronger than we first thought, making it one of the brightest and most energetic on record,” Laskar said.
Wen-fai Fong, assistant professor of physics and astronomy at Northwestern University, added: “This brief gamma-ray burst was the first time we attempted to observe such an event with ALMA. Short burst afterglows are very hard to come by, so seeing this event shine so brightly was spectacular. After many years of observing these outbursts, this surprising discovery opens up a new area of research as it motivates us to observe many more of them with ALMA and other telescope arrays in the future.”
Joe Pesce, National Science Foundation program officer for NRAO/ALMA, said, “These observations are fantastic on many levels. They provide further information to help us understand the enigmatic gamma-ray bursts (and neutron star astrophysics in general), and they show how important and complementary multi-wavelength observations with space- and ground-based telescopes are for understanding astrophysical phenomena.”
More surprises to discover
And there’s still plenty of work to be done across multiple wavelengths, both with new GRBs and GRB 211106A, that could uncover additional surprises in these outbursts. “Studying short-duration GRBs requires the rapid coordination of telescopes around the world and in space operating at all wavelengths,” said Edo Berger, a 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 in operation and future 20-40 meter optical and radio telescopes such as the Next Generation VLA (ngVLA), we will be able to create a complete picture of these catastrophic events and to examine them in unprecedented ways distances.”
Laskar added: “With JWST we can now take a spectrum of the host galaxy and easily determine the distance, and in the future we could also use JWST to detect infrared afterglows and study their chemical composition. With ngVLA we will be able to study in unprecedented detail the geometric structure of the afterglow and star-forming fuel found in their host environment. I am excited about these upcoming discoveries in our field.”