Discovery of the linear energy-dependence of the spectral lag of X-ray bursts from SGR J1935+2154

Shuo Xiao, You-Li Tuo, Shuang-Nan Zhang, Shao-Lin Xiong, Lin Lin, Yan-Qiu Zhang, Yue Wang, Wang-Chen Xue, Ce Cai, He Gao, Cheng-Kui Li, Xiao-Bo Li, Chao Zheng, Jia-Cong Liu, Ping Wang, Jin Wang, Wen-Xi Peng, Cong Zhan Liu, Xin-Qiao Li, Xiang-Yang Wen, Zheng-Hua An, Li-Ming Song, Shi-Jie Zheng, Fan Zhang, Ai-Jun Dong, Wei Xie, Jian-Chao Feng, Qing-Bo Ma, De-Hua Wang, Xi-Hong Luo, Shi-Jun Dang, Lun-Hua Shang, Qi-Jun Zhi, Ti-Pei Li

Published 2023-03-22Version 1

Spectral lag of the low-energy photons with respect to the high-energy ones is a common astrophysical phenomenon (such as Gamma-ray bursts and the Crab pulsar) and may serve as a key probe to the underlying radiation mechanism. However, spectral lag in keV range of the magnetar bursts has not been systematically studied yet. In this work, we perform a detailed spectral lag analysis with the Li-CCF method for SGR J1935+2154 bursts observed by {\it Insight}-HXMT, GECAM and Fermi/GBM from July 2014 to Jan 2022. We discover that the spectral lags of about 61\% (non-zero significance >1$\sigma$) bursts from SGR J1935+2154 are linearly dependent on the photon energy ($E$) with $t_{\rm lag}(E)=\alpha (E/{\rm keV})+C$, which may be explained by a linear change of the temperature of the blackbody-emitting plasma with time. The distribution of the slope ($\alpha$) approximately follows a Gaussian function with mean and standard deviation of 0.02 ms/keV (i.e. high-energy photons arrive earlier) and 0.02 ms/keV, respectively. We also find that the distribution can be well fitted with three Gaussians with mean values of $\sim$ -0.009, 0.013 and 0.039 ms/keV, which may correspond to different origins of the bursts. These spectral lag features may have important implications on the magnetar bursts.