7, 2022, papers led by Rastinejad, Troja, and Mei were published in the scientific journal Nature, and a paper led by Gompertz was published in Nature Astronomy. The more astronomers can find, the more they can refine their understanding of this new class of phenomena. The scientists noted features that might provide a key for identifying similar incidents - long bursts from mergers - in the future, even ones that are dimmer or more distant. Now they'll need to factor long bursts into their calculations as well.Ī team led by Benjamin Gompertz, an astrophysicist at the University of Birmingham in the United Kingdom, looked at the entire high-energy light curve, or the evolution of the event's brightness over time. They based their estimates on the rate of short bursts thought to occur across the cosmos. Scientists think neutron star mergers are a major source of the universe's heavy elements. The jets could be weakening ones from the original explosion or new ones powered by the resulting black hole or magnetar." "It's possible these high-energy gamma rays come from collisions between visible light from the kilonova and electrons in particle jets. Normally that emission decreases over time," said Alessio Mei, a doctoral candidate at the Gran Sasso Science Institute in L'Aquila, Italy, who led a group that studied the data. "This is the first time we've seen such an excess of high-energy gamma rays in the afterglow of a merger event. (Visible light's energy measures between about 2 and 3 electron volts, for comparison.) These gamma rays reached energies of up to 1 billion electron volts. Fermi detected high-energy gamma rays starting 1.5 hours post-burst and lasting more than 2 hours. The light following the burst, called the afterglow emission, also exhibited unusual features. Perhaps some distant long bursts could also produce kilonovae, but we haven't been able to see them. The event was also relatively nearby, by gamma-ray burst standards, which may have allowed telescopes to catch the kilonova's fainter light. Some have suggested the burst's oddities could be explained by the merger of a neutron star with another massive object, like a black hole. Many research groups have delved into the observations collected by Swift, Fermi, the Hubble Space Telescope, and others. Their observations have provided the earliest look yet at the first stages of a kilonova. "The kilonova we observed is the proof that connects mergers to these long-duration events, forcing us to rethink how black holes are formed."įermi and Swift detected the burst simultaneously, and Swift was able to rapidly identify its location in the constellation Bootes, enabling other facilities to quickly respond with follow-up observations. "Many years ago, Neil Gehrels, an astrophysicist and Swift's namesake, suggested that neutron star mergers could produce some long bursts," said Eleonora Troja, an astrophysicist at the University of Rome who led another team that studied the burst. This decay results in the production of heavy elements like gold and platinum. Heat generated by the radioactive decay of elements in the neutron-rich debris likely creates the kilonova's visible and infrared light. Scientists hypothesize that jets of high-speed particles, launched by the merger, produce the initial gamma-ray flare before they collide with the wreckage. They also generate gravitational waves, or ripples in space-time - although none were detected from this event.Įventually the neutron stars collide and merge, creating a cloud of hot debris emitting light across multiple wavelengths. As the stars circle ever closer, they strip neutron-rich material from each other. A classic short gamma-ray burst begins with two orbiting neutron stars, the crushed remnants of massive stars that exploded as supernovae.
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