An international team of scientists, including a researcher at the McWilliams Center for Cosmology and Astrophysics at Carnegie Mellon University(opens new window)has discovered the origin of a persistent radio emission that was observed together with a fast radio burst.

The new study led by the Italian National Institute of Astrophysics(opens new window) (INAF) shows that the radiation is generated by a plasma bubble. The data also allows researchers to assess the nature of the “engine” that drives these mysterious sources. The results were published in Nature.(opens new window).

Fast radio bursts (FRBs) are one of the most recent unsolved mysteries in astrophysics. Within a few milliseconds, these powerful events release enormous amounts of energy, among the highest observed in cosmic phenomena. FRBs were discovered just over 10 years ago, but their origin is still unclear.

In very few cases, the fast flash characteristic of FRBs coincides with a sustained emission that is also observed in the radio band. The new study has recorded the weakest sustained radio emission detected to date for an FRB. The subject of the study is FRB 20201124A, a fast radio burst discovered in 2020 whose source is about 1.3 billion light-years away.

The observations were carried out with the most sensitive radio telescope in the world, the Very Large Array (VLA) Radio Telescope(opens new window) in New Mexico. The data allowed scientists to test the theoretical prediction that a plasma bubble is the origin of the persistent radio emission from FRBs.

“We were able to demonstrate through observations that the persistent emission observed together with some fast radio bursts behaves as expected by the nebula emission model, that is, a bubble of ionized gas surrounding the central engine,” explained Gabriele Bruni, INAF researcher in Rome and lead author of the new paper. “In particular, through radio observations of one of the bursts closest to us, we were able to measure the weak persistent emission emanating from the same location as the FRB, thus expanding by two orders of magnitude the range of radio fluxes studied so far for these objects.”

This research also helps narrow down the nature of the engine that drives these mysterious flashes. According to the new data, the phenomenon is due to a magnetar (a highly magnetized neutron star) or a highly accretive X-ray binary, that is, a binary system consisting of a neutron star or a black hole that is accreting material from a companion star at very high speeds. In fact, the winds generated by the magnetar or X-ray binary could “inflate” the plasma bubble, generating the persistent radio emission. There is therefore a direct physical relationship between the engine of the FRBs and the bubble located in its immediate vicinity.

The motivation for this observation campaign stems from another work led by Luigi Piro of INAF and also involving Brendan O’Connor, a McWilliams Postdoctoral Fellow at Carnegie Mellon University. The two are co-authors of the new paper. In their previous work, the researchers had identified the persistent emission in the host galaxy of this FRB, but they had not yet measured the position precisely enough to link the two phenomena. In the new work, the researchers used higher spatial resolution data from the VLA, the Northern Extended Millimeter Array(opens new window) (NOEMA) and the Gran Telescopio CANARIAS(opens new window).

“New data was collected at radio wavelengths that had better angular resolution than previous studies,” O’Connor said. “Basically, it’s like looking at something in 1080p instead of 720p. And in this case, the higher resolution image allows us to better pinpoint what’s going on with this source.”

Using the new data, O’Connor performed a spectral energy distribution (SED) analysis of the host galaxy to infer its properties, such as mass and star formation rate. An SED represents the different amount of energy emitted at each wavelength of light, which can reveal these properties. In this case, O’Connor’s analysis helped determine that the persistent radio emission is not consistent with a star-forming region within the host galaxy.

“The high-resolution data tells us, first, that it’s not spreading out over a large area of ​​the host galaxy, as you would expect for star formation. And second, we can constrain the actual size of the source,” O’Connor said. “And based on the estimated size, it fits the overall picture of what you would expect from a magnetar nebula.”

Most FRBs do not exhibit persistent emission. So far, this type of emission has only been associated with two FRBs, both of which had a low brightness that did not allow the proposed model to be verified. FRB 20201124A, on the other hand, is located at a great but not excessive distance, which made it possible to measure persistent emission despite its low brightness. Understanding the nature of persistent emission allows researchers to add a piece to the puzzle about the nature of these mysterious cosmic sources.

In addition to researchers from INAF, the universities of Bologna, Trieste and Calabria in Italy as well as international research institutes and universities in China, the USA, Spain and Germany are involved in the collaboration.