2025ApJ...993...24W

A magnetar X-ray burst (MXB) is usually composed of a single pulse or multiple pulses with rapid rise and brief duration mostly observed in the 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 another kind of pulse shape has not been explored. In this study, we systematically examined the light curve of MXBs from SGR J1935+2154 detected by the Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor between 2021 and 2022. We find that there are different light-curve morphologies. Especially, we discover a peculiar and new pattern, exponential rise and cutoff 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 than those with FRED. 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.