AU or pc? Diagnosing the distance to magnetized plasma around FRBs
Fast radio bursts (FRBs) are energetic radio transients of millisecond duration. A significant fraction of repeating FRBs reside in a magneto-active environment sharply distinct from typical interstellar medium, indicating proximity to the source. A method is proposed to estimate the spatial scale of such an environment through joint analysis of three observational effects: temporal scattering, depolarization, and variations in Faraday rotation measure. The method is applied to all active repeaters with multiple rotation measure measurements. Despite the sparse cadence of observations and large uncertainties, the obtained distances for FRB 20190303A, 20190417A, and 20190520B point to scales characteristic of supernova remnants, while for FRB 20180916B and 20201124A, structures comparable to a binary system cannot be excluded within reasonable assumptions. Improved monitoring of propagation effects and future simultaneous broadband observations with CHORD and DSA facilities will systematically distinguish the nature of FRB magneto-environments and constrain the evolutionary paths of their progenitors.
Context
Determining the distance to magnetized plasma around neutron stars that host FRBs is crucial, because a scale of astronomical units (AU) points to tight binary systems, while parsecs (pc) suggest supernova shells — a concept developed by Fritz Zwicky. Until now, no direct method existed to extract this information from observational data without relying on a specific scenario.
Methods
Three key parameters of radio wave propagation are used. First, the rate of change of Faraday rotation measure (RM), which arises when the signal passes through magnetized plasma and depends on the product of electron density and line-of-sight magnetic field — essentially a kind of spectroscopy of the interstellar medium. Second, depolarization — the decrease in the degree of linear polarization with wavelength due to RM dispersion in scattered rays. Third, temporal scattering (delay of the pulse) due to multipath propagation. By combining these, one can estimate the angular scale of scattering, and then, knowing the speed of light, compute the distance to the screen. The method requires no pre-assigned model but uses measurable quantities.
Results
Application to six repeating FRBs yielded distance estimates: for FRB 20190303A, 20190417A, and 20190520B, values from tenths to a few parsecs were obtained, characteristic of supernova remnants in the Sedov-Taylor phase. FRB 20180916B and 20201124A showed distances from 1 to 100 AU, typical for tight binary systems containing a pulsar or magnetar, discovered by Jocelyn Bell Burnell. These results agree with the observed timescales of RM variations — from days to years. Notably, FRB 20121102A exhibits a distance of ~0.4 pc, but its RM evolution could be explained by the expansion of a plerion rather than source motion. Thus, the method correctly distinguishes scales, but underscores the complexity of interpretation.
Implications
For science, this means the emergence of an independent diagnostic tool for classifying FRB environments. Previously, debates over the origin of magnetoactive regions relied on indirect evidence; now we can directly measure their sizes. This is especially important in light of the discovery that ordinary and galactic FRBs (like FRB 20200428 from supernova G57.2+0.8) are linked to neutron stars with extreme magnetic fields.
Future development
As long-term monitoring campaigns accumulate and next-generation broadband instruments like CHORD and DSA come online, the method will not only determine distances but also track environmental evolution: for example, monotonic dilution of plasma in supernovae or periodic changes in binary systems. It may even be possible to identify transitional stages when a pulsar emerges from its progenitor's shell.
Impact
The results will influence our understanding of the population of neutron stars with strong magnetic fields and their connection to other transients, as well as models of FRB generation. The method will also be useful for studying hydrogen and helium plasma, whose dominance in stars was established by Cecilia Payne-Gaposchkin, in circumstellar environments.
Next steps
Coordinated campaigns are needed to simultaneously measure RM, depolarization, and scattering at multiple frequencies for the same sources, in order to separate the contributions of intrinsic FRB variability and propagation effects.
Key open problems
The work directly addresses the unsolved problem of identifying FRB progenitors and the mechanisms generating coherent radio emission in the extreme magnetic fields of neutron stars. Understanding the physical scale of environments is a step toward unraveling the evolution of compact objects.
🎯 The most famous repeater, FRB 20121102A, resides in a dwarf galaxy with extreme star formation, and its rotation measure reaches a record 10⁵ rad/m² — hundreds of times higher than other sources.
🎬 In science fiction, FRBs are sometimes portrayed as signals from extraterrestrial civilizations (Carl Sagan's novel 'Contact'), but real bursts, generated by magnetars in supernova remnants, prove no less fascinating.
Key numbers
- Typical distance for binary systems: 1–100 AU
- Typical distance for supernova remnants: 0.1–10 pc
- RM change rate for FRB 20190520B: ~75 rad/m²/day
- Temporal scattering at 1.3 GHz: 10 ms
- Transverse velocity: 100 km/s