Possible Multi-band Afterglows of FRB 20171020A and its Implication

TL;DR

This paper analyzes the potential multi-wavelength afterglow of FRB 20171020A within a magnetar external-shock framework, exploring both uniform-density ISM and wind-like environments. It couples relativistic ejecta dynamics with thermal synchrotron emission from a magnetized forward shock, deriving light curves in radio, optical, and X-ray bands and evaluating detectability against current facilities. The optical afterglow emerges as the most promising observable, potentially detectable by LSST within a few hundred seconds after a bright future burst, especially in dense wind environments, while radio detections require favorable surroundings and adiabatic evolution, and X-ray signals are generally too faint for present instruments. The work further estimates a nearby FRB afterglow event rate of about $\sim 0.3$ per CHIME year, underscoring the need for rapid, global multi-wavelength follow-up campaigns to constrain FRB progenitors and their environments.

Abstract

Fast Radio Bursts (FRBs) are millisecond-duration radio transients of mysterious origin, with growing evidence linking at least some of them to magnetars. While FRBs are primarily observed in the radio band, their potential multi-wavelength afterglows remain largely unexplored. We investigate the possible afterglow of FRB 20171020A, a rare nearby and bright FRB localized in a galaxy at only 37 Mpc. Assuming that this source produces a future bright burst, we model the expected afterglow emission in the radio, optical, and X-ray bands under both uniform and wind-like ambient media, within the framework of the magnetar model. Our results show that the optical afterglow is the most promising for detection, but it fades rapidly and requires follow-up within a few hundred seconds post-burst. The radio afterglow may be detectable under favorable conditions in a dense stellar wind, whereas the X-ray counterpart is too faint for current telescopes. These findings suggest that rapid optical follow-up offers the best opportunity to detect the afterglow of the next bright burst from FRB 20171020A, providing unique insights into the progenitor and its environment. To assess observational feasibility, we estimate the event rate of nearby FRBs with sufficient energy to power detectable afterglows, finding a rate of $\sim$0.3 per year for CHIME surveys. Although this rate is low and the optical detection timescale is short, coordinated fast-response strategies using global telescope networks could significantly improve the chance of success. As more nearby FRBs are discovered, multi-wavelength observations will be essential in unveiling the physical nature of these enigmatic events.

Figures (4)

• Figure 1: Multi$-$wavelength afterglow light curves of FRB 20171020A in different uniform media. The radio (1 GHz), optical (R-band), and X-ray (1 keV) light curves are shown for both radiative ($\epsilon = 1$, dashed lines) and adiabatic ($\epsilon = 0$, solid lines) cases. The detection limits of VLA (radio), LSST (optical), and Swift/XRT (X-ray) are indicated by purple dotted lines. Among them, (a), (b), (c) respectively represent the light variation curves under radio, optical and X-ray conditions.
• Figure 2: Same as Figure \ref{['fig1']}, but for a wind-like environment. The afterglow is generally brighter and longer-lasting than the uniform medium case, with the radio and optical signals exceeding detection limits in the adiabatic case. However, the X-ray afterglow remains undetectable with current instruments. Among them, (a), (b), (c) respectively represent the light variation curves under radio, optical and X-ray conditions.
• Figure 3: Contour of optical peak flux in the $E_K$$-$$n$ plane in a uniform medium. Different colors correspond to different optical peak flux levels (magnitude in the R-band). The black dashed lines indicate the LSST observational upper limits. Panel (a) represents the adiabatic case, while Panel (b) corresponds to the radiative case.
• Figure 4: Contour of optical peak flux in the $E_K$$-$$\dot{M}_{\rm w}$ plane for a wind-like environment. Different colors correspond to different optical peak flux levels (magnitude in the R-band). The black dashed lines indicate the LSST observational upper limits. Panel (a) represents the adiabatic case, while Panel (b) corresponds to the radiative case.