Repeating fast radio bursts from synchrotron maser radiation in localized plasma blobs: Application to FRB 20121102A

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 (νpk) 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 (γe ∼ 300), our analysis shows that bright and narrow-banded radio bursts with peak flux density ∼1 Jy at peak frequency (νpk) ∼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 νP ∼ 4.5 MHz. The spectrum of bursts with lower νpk tends to be narrower. Applying our model to the bursts of FRB 20121102A, the distributions of both the observed νpk and isotropic energy Eiso 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 νP distribution exhibits several peaks, similar to those observed in the νpk distribution of FRB 20121102A. This implies that the synchrotron maser emission in FRB 20121102A is triggered in different plasma blobs with varying νP, likely due to the inhomogeneity of relativistic electron number density.