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

TL;DR

This study addresses the origin of intermediate X-ray bursts from magnetars by searching for eclipse-like features in the plateau phases of bursts from SGR J1935+2154. The authors develop an eclipse fireball model treating the emission region as a co-rotating spherical fireball whose visibility is modulated by the magnetar’s shadow, and they fit four eclipsed bursts (B–E) using Burst A as a template. They derive a consistent viewing angle $\chi \approx 17^\circ \pm 10^\circ$ and place fireballs at distances $d/R_{ns} \gtrsim 5$ with radii $l \approx 17-20$ km, placing the fireballs in the magnetosphere rather than on the surface; spectral CRSFs around $E_{cyc} \approx 35$ keV corroborate this geometry. The results support a magnetospheric origin for at least some intermediate bursts, likely driven by magnetic reconnection in magnetospheric structures, and demonstrate a novel method to infer magnetar viewing geometry from eclipse-like light curves, with implications for burst triggering mechanisms and the connection to magnetospheric dynamics.

Abstract

X-ray bursts from the magnetar can lead to the formation of fireballs trapped by the magnetic field and co-rotating with the star. The fireball emission could occasionally be eclipsed by the magnetar, especially when the burst duration is comparable to the magnetar's spin period. In this work, we discover a peculiar type of burst whose light curve has a plateau-like feature among the long bursts of the magnetar SGR J1935+2154. 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 $17^\circ \pm 10^\circ$. The distances from the fireballs to the magnetar are found to be more than five times the magnetar's radius, indicating that the fireballs are suspended in the magnetosphere rather than adhering to the magnetar surface. We also find this configuration is well consistent with the implication of the cyclotron resonance scattering feature in their spectra. Our results suggest that some intermediate X-ray bursts of SGR 1935+2154 may originate from magnetic reconnection within the magnetosphere rather than the starquake.

Figures (9)

• Figure 1: Illustration of the eclipse geometry of a fireball co-rotating with the magnetar.
• Figure 2: The histogram (blue line) and kernel density (orange line) of the duration distribution of the 696 X-ray bursts from SGR J1935+2154 detected by Fermi/GBM Lin2020apjLin2020apjlKaneko2021ApJRehan2023apjRehan2024ApJ and GECAM Xie2022mnrasXie2025ApJS up to the end of 2022. The red and green vertical dot lines represent the mean value ($104.12$ ms) of burst duration and the spin period ($3.24$ s) of SGR J1935+2154, respectively. The duration range where we search for the candidates of eclipsed X-ray bursts is highlighted by the shade.
• Figure 3: Left: The light curve of a plateau burst observed by Fermi/GBM at 2016-06-26 13:54:30.720 (labeled Burst A), where the dashed line represents the average of the plateau flux, and the fluctuation, as shown by the shaded band, is no more than $\sim6$% of the average. Right: The time-resolved spectra of Burst A, which can be fitted by a non-evolving black body with a CRSF absorption (the red solid histogram). The residuals of the spectral fit are plotted in the lower panel. The dashed line represents the unabsorbed black body. The center of the CRSF line is labeled by the vertical dotted line, while the optical depth $\tau_{\rm CRSF}$ and width $\sigma$ of the absorption are given in the legend.
• Figure 4: The temporal evolution behaviors of the eclipsed bursts during the plateau phase. The top panels present the evolution of the emission flux, where the dashed lines provide the fittings of these light curves with the eclipse model. In the middle panels, we plot the model-predicted evolutions of the black body emission areas (dotted line), in comparison with the data derived from the fittings of the time-resolved spectra. The residuals of the light curve fittings are shown in the bottom panels, there the shadows represent $1\sigma$ and $3\sigma$ confidence levels.
• Figure 5: Posteriors of the light curve fittings for Bursts B-E with the eclipse model. Medians and $1\ \sigma$ ranges of the parameters are labeled.