Plasma bubbles and the ‘engine’ behind fast radio bursts

Artistic illustration of a magnetar, surrounded by the plasma bubble responsible for the persistent emission observed in some fast radio bursts (image: NSF/AUI/NRAO/S. Dagnello)

The Very Large Array (VLA) – the world’s most sensitive radio telescope, located in the United States – provided observations that confirmed the theory suggesting a plasma bubble as the origin of persistent radio emissions of fast radio bursts’. The research, published in Nature, was led by researchers of the Italian National Institute for Astrophysics (INAF), in collaboration with the Universities of Bologna, Trieste and the Calabria region. International research institutes and universities in China, the United States, Spain and Germany participated as well.

Fast Radio Bursts (FRBs) are among the most recent unsolved mysteries in modern astrophysics. In just a few milliseconds, they release one of the highest amounts of energy possibly observable in cosmic phenomena. Discovered just over ten years ago, these powerful bursts emitted in the radio spectrum mostly derive from extragalactic sources, but their origin is still uncertain. The world’s astrophysics community is insistently trying to understand the physical processes at their origin.

In very few cases, the fast burst characterising fast radio bursts corresponds to a persistent emission in the radio spectrum. Thanks to the Very Large Array (VLA), researchers were able to record the weakest persistent radio emission ever recorded for an FRB: FRB20201124A, a fast radio burst discovered in 2020, whose source is located about 1.3 billion light-years away from us. The recording confirmed that the burst originated from a plasma bubble.

In addition, this new research helps to circumscribe the nature of the engine of these mysterious bursts. According to new data, the phenomenon would be caused by either a magnetar (a highly magnetised neutron star) or an X-ray binary with a very high accretion rate, i.e. a binary system formed by either a neutron star or a black hole that accumulates material from a companion star at very intense rates. In this scenario, it would be the winds produced by the magnetar or the X binary system that ‘inflate’ the plasma bubble, originating the persistent radio emission. Therefore, a direct physical relationship is instantiated between the FRB’s ‘engine’ and the bubble, which lies in its closest surroundings.

The majority of fast radio bursts do not exhibit persistent emission. Until now, this kind of radiation had been associated with only two FRBs, and at a luminosity level that did not allow for verification of the proposed model. However, given FRB20201124A’s distance is great but not excessive, it was possible to measure the persistent emission despite the low luminosity. Understanding the nature of the persistent radiation helps adding a piece to the puzzle regarding the nature of these mysterious cosmic sources.