Narrowly Banded Spectra with Peak Frequency around 1 GHz of FRB 20201124A: Implications for Energy Function and Radiation Physics

Abstract

The radiation physics of fast radio bursts (FRBs) remains an open question. Current observations have discovered that narrowly banded bursts of FRB 20201124A are active in 0.4–2 GHz, and their spectral peak frequency ( ${\nu }_{p}^{\mathrm{obs}}$ ) is mostly toward ∼1 GHz. Utilizing a sample of 1268 bursts of FRB 20201124A detected with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we show that the 1 σ spectral regime of 71.4% of the events (in-band bursts) is within the FAST bandpass. The intrinsic burst energies ( ${E}_{\mathrm{BWe}}^{\mathrm{obs}}$ ) and spectral widths ( ${\sigma }_{s}^{\mathrm{obs}}$ ) are well measured by fitting the spectral profile with a Gaussian function. The derived ${E}_{\mathrm{BWe}}^{\mathrm{obs}}$ and ${\sigma }_{s}^{\mathrm{obs}}$ distributions are lognormal and centered at $\mathrm{log}{E}_{\mathrm{BWe}}^{\mathrm{obs}}/\mathrm{erg}=37.2\,(\sigma =0.76)$ and $\mathrm{log}{\sigma }_{s}^{\mathrm{obs}}/\mathrm{GHz}=-1.16\,(\sigma =0.17)$ . Our Monte Carlo simulation analysis infers its intrinsic ν _p distribution as a normal function centered at ν _p _, _c = 1.16 GHz ( σ = 0.22) and its intrinsic energy function as ${\rm{\Phi }}(E)\propto {E}^{-0.60}{e}^{-E/{E}_{c}}$ with E _c = 9.49 × 10 ^37 erg. We compare these results with that of typical repeating FRBs 20121102A and 20190520B, which are active over a broad frequency range at several specific frequencies, and discuss possible observational biases on the estimation of the event rate and energy function. Based on these results, we argue that FRB 20201124A likely occurs in a fine-tuned plasma for maser radiation at a narrow frequency range, while FRB 20121102A and FRB 20190520B could involve clumpy plasma conditions that make maser emission around several specific frequencies in a broad range.

Keywords

• Radio bursts
• Radio transient sources