2025A&A...695A.100L

The radiation physics of repeating fast radio bursts (FRBs) remains enigmatic. Motivated by the observed narrow-banded emission spectrum and ambiguous fringe pattern of the spectral peak frequency (ν_pk) distribution of some repeating FRBs, such as FRB 20121102A, we propose that the bursts from repeating FRBs arise from synchrotron maser radiation in localized blobs within weakly magnetized plasma that relativistically moves toward observers. Assuming the plasma moves toward the observers with a bulk Lorentz factor of Γ = 100 and the electron distribution in an individual blob is monoenergetic (γ_e ∼ 300), our analysis shows that bright and narrow-banded radio bursts with peak flux density ∼1 Jy at peak frequency (ν_pk) ∼3.85 GHz can be produced by the synchrotron maser emission if the plasma blob has a magnetization factor of σ ∼ 10^–5 and a frequency of ν_P ∼ 4.5 MHz. The spectrum of bursts with lower ν_pk tends to be narrower. Applying our model to the bursts of FRB 20121102A, the distributions of both the observed ν_pk and isotropic energy E_iso detected by the Arecibo telescope at the L band and the Green Bank Telescope at the C band are successfully reproduced. We find that the ν_P distribution exhibits several peaks, similar to those observed in the ν_pk distribution of FRB 20121102A. This implies that the synchrotron maser emission in FRB 20121102A is triggered in different plasma blobs with varying ν_P, likely due to the inhomogeneity of relativistic electron number density.