The mysterious "fast repeating radio burst" comes from an unexpected location
FRBs have intrigued scientists since the first was discovered in 2007. They are made up of extremely powerful signals from deep space, millions of light years away, some of which discharge more energy than 500 million Suns and are only detected in radio wavelengths.
Yet these bursts are incredibly short, shorter than the blink of an eye, only milliseconds in duration, and most of them do not repeat, making them very difficult to predict, track and then understand.
Analyzing fine structure of these radio signals, astronomers focused on the type of object they thought might cause them, with compact objects like neutron stars being the main theory. Then, in 2020, a huge breakthrough came. An FRB has finally been detected from inside the Milky Way, emitted by a magnetar.
Magnetars, of which not many have been confirmed to date, are a rare type of neutron star, the collapsed core of a dead star that began with 8 and 30 times the mass of the Sun. Neutron stars are small and dense, about 20 kilometers in diameter, with a maximum mass of about two Suns.
Magnetars, as the name suggests, add something else to the mix: an absolutely insane magnetic field, about a quadrillion times more powerful than the Earth's magnetic field and a thousand times more powerful than that of a normal star. neutrons. This brings us back to FRB 20200120E. It is a minority among the FRBs, an FRB that repeats its bursts, but other than that it fits the profile perfectly.
As it repeats, however, astronomers were more easily in able to identify the position in the sky from which it originated. By analyzing other properties of the signal, they were able to determine that it had traveled a relatively short distance. This led them to a spiral galaxy called M81 in 2021, albeit with some degree of uncertainty. More specifically, the researchers believed they had tracked FRB 20200120E in a globular cluster. In a study published in Nature this week, a team of astronomers confirmed that position.
Here's why this is a problem. Globular clusters are compact groups of stars that tend to be very old and long-lived, as well as low in mass, none greater than the mass of the Sun. All their stars are thought to have formed from the same gas cloud at the same time; just like a small town, these stars live their mostly peaceful existences together.
Neutron stars, as mentioned above, tend to form from higher-mass stars, which also tend to have a lifespan much shorter life than the main sequence (hydrogen combustion) - those of the OB type. So, as a general rule, you wouldn't expect to find neutron stars or magnetars in a globular cluster.
However, from time to time, a type of rapidly spinning neutron star, known as a pulsar, has been found millisecond, in globular clusters. Because globular clusters are so densely populated, stars can interact and even collide with each other, producing objects such as low-mass X-ray binaries and pulsars.
According to the research team, this introduces other interesting mechanisms for the formation of magnetar. A low-mass white dwarf interacting and accreting material from another star could gain enough mass to collapse into a neutron star; or two white dwarfs could merge.
It is also possible that the source of the FRB is not a magnetar at all, but a low-mass X-ray binary, such as a white dwarf and a neutron star, or a neutron star and an exoplanet. It could also be an accreting black hole. Evidence for these explanations is lacking, there is no X-ray or gamma-ray activity that would typically accompany these systems, but they cannot yet be ruled out.
Whatever the answer, however, it appears that FRB 20200120E is destined to shake things up. Either it will teach us something new about stellar interactions in globular clusters, or it will give us a new training channel for FRBs. Being a repeated FRB so close to us, it represents a rare opportunity to probe these mysterious signals in detail.
