The Self-organized Criticality Behaviors of Pulses in Magnetar Bursts

The overall temporal and spectral characteristics (e.g., duration, fluence, peak flux, peak count rate, and total counts) of X-ray bursts (XRBs) originating from magnetars have been extensively studied to confirm their self-organized criticality (SOC) behaviors. However, for the pulses in XRBs, which are an important part of unraveling radiation processes such as magnetic reconnection and crustal rupture, their variable statistical characteristics are still unclear. In this work, we investigate the distributions of fluctuations in duration, waiting time, peak count rate, and total counts of pulses within bursts from two active and prolific magnetars, SGR J1935+2154 and SGR J1550-5418, observed by Fermi's Gamma-ray Burst Monitor, Insight-HXMT, and GECAM. We report that the waiting time for pulses follows SOC, whereas previous studies indicated for bursts do not, possibly due to incomplete observations. Besides, the duration, peak count rate, and total counts for pulses are also SOC behaviors. The alpha-values for the duration of pulses for both magnetars are similar to 2, and the inferred Euclidean dimension S is 3. Our findings provide evidence for the SOC of each radiation process during a burst, for example, multiple pulses produced by multiple magnetic reconnections triggered by a single starquake. Finally, the similar statistical characteristics for pulses from both magnetars lend support to their shared radiation processes.