On December 11, 2021, NASA’s Fermi Gamma ray Space Telescope and Neil Gehrels Swift Observatory discovered a burst of high-energy light coming from the edge of a galaxy around 1 billion light-years away. The occurrence has shaken scientists’ perceptions of gamma ray bursts (GRBs), the most potent phenomenon in the cosmos.
Astronomers have typically split GRBs into two types over the past few decades. The development of dense objects like black holes in the centers of huge collapsing stars is the source of long bursts, which last for two seconds or longer and produce gamma ray. Gamma rays are emitted for less than two seconds during short bursts, which are the result of the merger of neutron stars and other massive objects. A kilonova is a brief flash of visible and infrared light that is occasionally seen by scientists.
The first long-duration gamma ray burst linked to a neutron star merger origin, GRB 211211A, was paradigm-shifting, according to Jillian Rastinejad, a PhD student at Northwestern University in Evanston, Illinois, who led one team that analyzed the burst. “We were able to identify a kilonova through subsequent observations after the high-energy burst, which lasted for around a minute. “The origin of the heavy metals in the cosmos is profoundly affected by this discovery.”
Two orbiting neutron stars, the crushed remains of enormous stars that erupted as supernovae, are the precursors of a classic short gamma ray burst. The stars are removing neutron-rich material from one another as they get closer and closer together. They also produce gravitational waves, which are ripples in space-time, although this event didn’t produce any.
The neutron stars eventually collide and fuse, forming a cloud of heated debris that emits light at various wavelengths. The initial gamma ray flare is thought to be caused by jets of fast particles that are launched by the merger before they crash with the debris. The visible and infrared light of the kilonova is most likely produced by heat from the radioactive decay of materials in the neutron-rich debris. Heavy elements like gold and platinum are created as a result of this degradation.
Swift’s namesake and scientist Neil Gehrels had hypothesized that neutron star mergers would result in certain lengthy bursts, according to Eleonora Troja, an astrophysicist at the University of Rome who led another team that investigated the burst. The kilonova that we saw serves as evidence linking mergers to these protracted occurrences, prompting us to reconsider how black holes are created.
Swift was able to immediately pinpoint the burst’s location in the constellation Boötes, allowing other facilities to quickly respond with follow-up observations. Fermi and Swift spotted the burst simultaneously. Their findings gave scientists the earliest glimpse yet at a kilonova’s early phases.
The observations gathered by Swift, Fermi, the Hubble Space Telescope, and other organizations have been studied by numerous study teams. Some have hypothesized that the peculiarities of the burst could be explained by the neutron star merging with another enormous object, such as a black hole. By gamma ray burst standards, the event was also relatively close by, which would have made it possible for telescopes to see the kilonova’s weaker light. Kilonovae may also be produced by some far-off, lengthy bursts, but we haven’t been able to see any of them.
The afterglow emission, the light that appeared after the burst, also exhibited peculiar characteristics. Beginning 1.5 hours after the burst and extending for more than 2 hours, Fermi discovered high-energy gamma rays. The energy of these gamma rays was as high as 1 billion electron volts. For comparison, the energy of visible light is between 2 and 3 electron volts.
“This merger event’s afterglow contains an unusually high number of high-energy gamma rays.” “Usually, the emission gets smaller with time,” said Alessio Mei, the group’s leader and a doctorate student at the Gran Sasso Science Institute in L’Aquila, Italy. “These high-energy gamma rays might be the result of collisions between electrons in particle jets and visible light from the kilonova.” The jets can be dying ones from the original explosion or brand-new ones propelled by the magnetar or black hole that formed as a result.
The heavy elements in the cosmos are believed by scientists to have been produced by neutron star mergers. They based their calculations on the frequency of brief bursts that are believed to occur across the cosmos. Long bursts must now be taken into account in their calculations.
The complete high-energy light curve, or the development of the event’s brightness over time, was examined by a group under the direction of astronomer Benjamin Gompertz from the University of Birmingham in the United Kingdom. Long bursts from mergers were identified as having characteristics that may help identify instances like these in the future, even if they are fainter or farther away. Astronomers can further their understanding of this new class of events by discovering more objects.
The scientific magazine Nature published papers by Rastinejad, Troja, and Mei on December 7, 2022, and Nature Astronomy published a paper by Gompertz.
Regina Caputo, Swift project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, noted that this result “underscores the necessity of our missions working together and with others to provide multiwavelength follow-up on these kinds of events.” “The simple dichotomy we’ve used for years has been put to death by this event; however, similar concerted attempts have revealed that some supernovae may emit brief bursts.” “You never know when you’ll come across anything unexpected.”
NASA’s Goddard Space Flight Center is in charge of the Swift and Fermi missions.
Swift is a partnership between Penn State, the Los Alamos National Laboratory in New Mexico, and Northrop Grumman Space Systems in Dulles, Virginia. Partners from the United Kingdom and Italy have made significant contributions.
The U.S. Department of Energy and partners from France, Germany, Italy, Japan, Sweden, and the United States collaborated on the Fermi project.
The Hubble Space Telescope is a NASA and ESA joint international project (European Space Agency). The telescope is managed by Goddard. Science activities are carried out by the Space Telescope Science Institute (STScI) in Baltimore. The Association of Universities for Research in Astronomy, based in Washington, D.C., runs STScI for NASA.
