FRB 20200428 and potentially associated hard X-ray bursts: Maser emission and synchrotron radiation of electrons in a weakly magnetized plasma?

Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China

^⋆ Corresponding author: lew@gxu.edu.cn

Received: 22 April 2025
Accepted: 4 August 2025

Abstract

The temporal and spatial coincidence between FRB 20200428 and hard peaks of the X-ray burst from SGR 1935+2154 suggests their potential association. We attributed them to the plasma synchrotron maser emission and synchrotron radiation of electrons in weakly magnetized, relativistically moving plasma blobs, and Monte Carlo simulation analysis shows that our model can predict observable fast radio burst outbursts and associated hard X-ray bursts with current telescopes. We constrained the properties of the blobs, including the Lorentz factor Γ = 5 − 30, the magnetization factor σ = 6 × 10⁻⁵ ∼ 2 × 10⁻⁴, the electron Lorentz factor γ_e, s = (1.8 − 3.3)×10⁴, and the plasma frequency ν_P = 2.48 − 42.61 MHz. The inferred size of the blobs is ∼10^9 − 10 cm, and it is located ∼10^{12 − 14} cm from the central engine. By adopting fine-tuned parameter sets, the observed spectra of both the FRB 20200428 outbursts and X-ray bursts can be well represented. The peak flux density (F_νpk) of plasma maser emission is sensitive to σ and ν_P. Variation in F_νpk can be more than 10 orders of magnitude, while the flux density of the synchrotron emission only varies by 1–2 orders of magnitude. This can account for the observed sub-energetic radio bursts or giant radio pulses from SGR 1935+2154.