On the Fast-radio-burst-associated X-ray Bursts: Inverse Compton Scattering of Radio Photons by an Extreme Pair Flow During Magnetosphere Activities

TL;DR

This paper models the X-ray bursts associated with fast radio bursts from magnetars as inverse Compton scattering of radio photons by an extreme pair plasma flow, explaining observed spectral and timing features.

Contribution

It introduces a novel inverse Compton scattering model involving an extreme pair flow to explain FRB-associated X-ray burst properties, linking magnetar activity to observed emissions.

Findings

The model reproduces the spectral features of the XRBs.

It explains the timing offset between FRB and XRB.

The scenario accounts for multiple observed events.

Abstract

The Galactic fast radio burst (FRB) FRB 200428 was associated with a short X-ray burst (XRB) from the magnetar SGR J1935+2154 during one of its active phases. This FRB-associated XRB exhibits distinct properties compared to other typical XRBs, including a significantly higher cutoff energy and a steeper power-law index. Its recovered X-ray light curve shows a multiple-peak structure, with the time of arrival offset from that of the FRB. These unique features imply a special physical link between the FRB and X-ray emissions. In 2022 October, a similar FRB-XRB association was detected from the same source. In this paper, we propose a model in which the observed spectral and temporal features of the associated XRBs can be attributed to the inverse Compton scattering (ICS) of FRB photons by an extreme pair flow around the light cylinder, with a bulk Lorentz factor of $\Gamma\sim10$ and a power-law distribution in the comoving frame, characterized by a typical Lorentz factor $\gamma^\prime_\mathrm{m}\sim5\times 10^4$. This extreme pair flow could originate from the compression of a transient pulse of $\sim10^{40}-10^{41}\mathrm{erg\,s^{-1}}$ and the acceleration through magnetic reconnection in the current sheet during magnetar activity. The Doppler-boosted ICS spectra and the arrival time shifts in such a scenario can well explain the observed features of the FRB 200428-associated XRB and can also account for another associated event in 2022.