Unveil A Peculiar Light Curve Pattern of Magnetar Burst with GECAM observations of SGR J1935+2154

TL;DR

This work addresses the diversity of magnetar X-ray Burst (MXB) light curves by systematically analyzing 159 MXBs from SGR J1935+2154 detected by GECAM-B (2021–2022). It introduces a new pulse pattern, ERCOD, characterized by an exponential rise and a sharp cut-off decay, and contrasts it with the traditional FRED form through joint temporal and spectral fitting using BB and CPL models. The study finds that ~10% of MXBs exhibit ERCOD, these bursts tend to be longer, more luminous, and spectrally harder than FRED bursts, and ERCOD is also present in other magnetars, implying a distinct energy-release mechanism in magnetar bursts. The results motivate theoretical modeling of magnetar emission and call for multi-wavelength observations to pin down the spectral components and physical origin of ERCOD.

Abstract

Magnetar X-ray Burst (MXB) is usually composed of a single pulse or multiple pulses with rapid rise and brief duration mostly observed in hard X-ray (soft gamma-ray) band. Previous work studied the temporal behavior of some magnetar bursts and employed the Fast Rise Exponential Decay (FRED) model to fit pulses of MXB. However, whether there is other kind of pulse shape has not been explored. In this study, we systematically examined light curve of MXBs from SGR J1935+2154 detected by GECAM between 2021 and 2022. We find that there are different light curve morphologies. Especially, we discover a peculiar and new pattern, Exponential Rise and Cut-Off Decay (ERCOD), which is significantly different from FRED and could be well described by a mathematical function we proposed. We find that MXBs with ERCOD shape are generally longer in duration, brighter in the peak flux, and harder in spectrum. We note that the ERCOD shape is not unique to SGR J1935+2154 but also present in other magnetars. This new light curve pattern may imply a special burst and radiation mechanism of magnetar.

Figures (5)

• Figure 1: Light curves of some MXBs from SGR J1935+2154 detected by GECAM. The red lines are the fitting results of the ERCOD or FRED model based on the MCMC method. a: Typical MXBs with ERCOD light curve pattern. b: MXBs with ERCOD light curve pattern and an accompanying initial pulse. The initial pulse is ignored in the fitting. c: MXBs with FERD light curve pattern.
• Figure 2: Fitting of normalized light curves. The FRED (dashed line) and ERCOD (solid line) shape are plot over the normalized light curves. a: Normalized light curves of MXBs with FRED light curve pattern. b: Normalized light curves of MXBs with ERCOD light curve pattern.
• Figure 3: The distribution of $t_{\rm rise}$ and $t_{\rm decay}$ for MXBs with ERCOD (orange triangles) and FRED (blue squares) patterns. The black lines are the fitting results to the data separately. The gray dotted lines indicate the ratio between $t_{\rm rise}$ and $t_{\rm decay}$.
• Figure 4: Spectral properties of ERCOD (orange triangles) and FRED (blue squares) samples.a: Distribution of blackbody temperature ($kT$) and flux (obtained from the BB model). b: Distribution of $E_{\rm p}$ and flux (obtained from the CPL ($\alpha=1$) model).
• Figure 5: Distribution of fluence and duration ($T_{90}$)for MXBs with ERCOD (orange triangles) and FRED (blue squares) patterns.a: Fluence derived from the BB model. b: Fluence derived from the CPL ($\alpha=1$) model. The red and blue lines represent the fitting results for the ERCOD and FRED samples, respectively.