Magnetar emergence in a peculiar gamma-ray burst from a compact star merger
Hui Sun, Chenwei Wang, Jun Yang, Bin Zhang, Shaolin Xiong, Yi-Han Iris Yin, Yuan Liu, Ye Li, W. C. Xue, Zhen-Yu Yan, Chen Zhang, Wenjun Tan, Haiwu Pan, Jia-Cong Liu, Huaqing Cheng, Yanqiu Zhang, J. Y. Hu, Chao Zheng, Zhenghua An, Ce Cai, Zhiming Cai, Lei Hu, Chichuan Jin, Dongyue Li, Xinqiao Li, Heyang Liu, Mingjun Liu, Wen-Xi Peng, L. M. Song, Shengli Sun, Xiaojin Sun, Xilu Wang, Xiang-Yang Wen, Shuo Xiao, Shu-Xu Yi, Fan Zhang, Wenda Zhang, Xiaofeng Zhang, Yonghe Zhang, Donghua Zhao, S. J. Zheng, Zhixing Ling, Shuang‐Nan Zhang, Weimin Yuan, Bing Zhang
Abstract
The central engine that powers gamma-ray bursts (GRBs), the most powerful explosions in the universe, is still not identified. Besides hyper-accreting black holes, rapidly spinning and highly magnetized neutron stars, known as millisecond magnetars, have been suggested to power both long and short GRBs. The presence of a magnetar engine following compact star mergers is of particular interest as it would provide essential constraints on the poorly understood equation of state for neutron stars. Indirect indications of a magnetar engine in these merger sources have been observed in the form of plateau features present in the X-ray afterglow light curves of some short GRBs. Additionally, some X-ray transients lacking gamma-ray bursts have been identified as potential magnetar candidates originating from compact star mergers. Nevertheless, smoking-gun evidence is still lacking for a magnetar engine in short GRBs, and associated theoretical challenges have been raised. Here we present a comprehensive analysis of the broad-band prompt emission data of the peculiar, very bright GRB 230307A. Despite its apparently long duration, the prompt emission and host galaxy properties are consistent with a compact star merger origin, as suggested by its association with a kilonova. Intriguingly, an extended X-ray emission component shows up as the [Formula: see text]-ray emission dies out, signifying the likely emergence of a magnetar central engine. We also identify an achromatic temporal break in the high-energy band during the prompt emission phase, which was never observed in previous bursts and reveals a narrow jet with a half opening angle of [Formula: see text], where [Formula: see text] is the GRB prompt emission radius.