Plasma Bubble Confirmed as Source of Persistent Emission in Fast Radio Burst FRB20201124A
In rare instances, the rapid bursts characteristic of FRBs coincide with persistent radio emissions. A recent study led by the Italian National Institute for Astrophysics (INAF) has detected the faintest persistent radio emission ever associated with an FRB. The focus of this research is FRB20201124A, first identified in 2020, situated approximately 1.3 billion light-years from Earth. The study is a collaborative effort involving INAF, the Universities of Bologna, Trieste, and Calabria in Italy, as well as international partners from research institutions in China, the United States, Spain, and Germany.
Using the world's most sensitive radio telescope, the Very Large Array (VLA) in the United States, researchers confirmed that a plasma bubble is responsible for the persistent radio emission linked to fast radio bursts. These findings are published in the journal Nature.
"We were able to demonstrate through observations that the persistent emission observed along with some fast radio bursts behaves as expected from the nebular emission model, i.e., a 'bubble' of ionised gas that surrounds 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 that is nearest to us, we were able to measure the weak persistent emission coming from the same location as the FRB, extending the radio flux range explored so far for these objects by two orders of magnitude."
The study also advances understanding of the engine powering these mysterious radio flashes. According to the new data, the phenomenon is likely driven by a magnetar (a strongly magnetised neutron star) or a high-accretion X-ray binary system, which includes a neutron star or black hole accreting material from a companion star at high rates. The winds generated by the magnetar or the X-ray binary would produce the plasma bubble responsible for the persistent radio emission. Thus, there is a direct physical relationship between the FRB's engine and the plasma bubble in its vicinity.
The research initiative stemmed from earlier work led by Luigi Piro of INAF, a co-author of the current study. Previous research had identified persistent emission in the host galaxy of this FRB but lacked the precision to confirm a direct association with the FRB source. "In this new work, we conducted a campaign at higher spatial resolution with the VLA, along with observations in different bands with the NOEMA interferometer and the Gran Telescopio Canarias (GranTeCan), which allowed us to reconstruct the general picture of the galaxy and discover the presence of a compact radio source - the FRB plasma bubble - immersed in the star-forming region," added Piro. "In the meantime, the theoretical model on the nebula had also been published, allowing us to test its validity and, finally, to confirm the model itself."
A significant part of the study involved excluding the possibility that the persistent radio emission originated from a star-forming region unrelated to the FRB source. NOEMA observations in the millimetre band assessed the amount of dust, an indicator of star-forming regions, while GranTeCan optical observations measured ionised hydrogen emission, another marker of star formation rates.
"Optical observations were an important element to study the FRB region at a spatial resolution similar to that of radio observations," noted co-author Eliana Palazzi from INAF in Bologna. "Mapping hydrogen emission at such a great level of detail allowed us to derive the local star formation rate, which we found to be too low to justify continuous radio emission."
Most FRBs do not exhibit persistent emission. Until now, such emissions had been linked to only two FRBs, both of which were too faint to verify the proposed model. FRB20201124A, while distant, was sufficiently bright to allow for the measurement of its persistent emission. Understanding this persistent emission adds a crucial piece to the puzzle of these cosmic sources.
Research Report:A nebular origin for the persistent radio emission of fast radio burst