Eclipsed X-ray Bursts from Magnetar SGR J1935+2154 and the Fireball Measurements

TL;DR

This study presents observational evidence of eclipsed X-ray bursts from magnetar SGR J1935+2154, supporting the existence of magnetospheric fireballs and offering insights into burst origins and magnetic reconnection processes.

Contribution

It introduces a new eclipse fireball model to interpret burst light curves and estimates the fireball locations, providing novel evidence for magnetospheric fireballs in magnetars.

Findings

Identified four burst candidates with eclipse-like features.

Estimated the fireball distance to be over five times the magnetar radius.

Supported the magnetospheric origin of some X-ray bursts.

Abstract

In theory, burst activity of the magnetar can lead to the formation of fireballs trapped by the magnetic field and corotating with the star. However, the smoking-gun observational evidence of the fireball is elusive. We envisage that the fireball emission should occasionally be eclipsed by the magnetar, especially when the burst duration is comparable to the magnetar's spin period. In this work, we first discover a peculiar type of burst whose light curve has a plateau-like feature among the long bursts of the magnetar SGR J1935+2154 detected by GECAM and Fermi/GBM. Then, based on these bursts, we identified four burst candidates with eclipse-like characteristics. By fitting their light curves with the eclipse fireball model, the viewing angle of the magnetar relative to its spin axis is estimated to be $17^\circ \pm 10^\circ$, and the distances from the fireballs to the magnetar are found to be more than 5 times the magnetar's radius, indicating that the fireballs are suspended in the magnetosphere rather than adhering to the magnetar surface. Furthermore, we find that this configuration is well consistent with the implication of the cyclotron resonance scattering feature we found in their spectra. Our results suggest that some intermediate X-ray bursts may originate from magnetic reconnection within the magnetosphere rather than the starquake.