Sun, H.; Wang, C. -W.; Yang, J.; Zhang, B. -B.; Xiong, S. -L.; Yin, Y. -H. I.; Liu, Y.; Li, Y.; Xue, W. -C.; Yan, Z.; Zhang, C.; Tan, W. -J.; Pan, H. -W.; Liu, J. -C.; Cheng, H. -Q.; Zhang, Y. -Q.; Hu, J. -W.; Zheng, C.; An, Z. -H.; Cai, C.; Cai, Z. -M.; Hu, L.; Jin, C.; Li, D. -Y.; Li, X. -Q.; Liu, H. -Y.; Liu, M.; Peng, W. -X.; Song, L. -M.; Sun, S. -L.; Sun, X. -J.; Wang, X. -L.; Wen, X. -Y.; Xiao, S.; Yi, S. -X.; Zhang, F.; Zhang, W. -D.; Zhang, X. -F.; Zhang, Y. -H.; Zhao, D. -H.; Zheng, S. -J.; Ling, Z. -X.; Zhang, S. -N.; Yuan, W.; Zhang, B. March 2025, National Science Review, Vol. 12, Issue 3, article id. nwae401
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 (GRB-less) 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 the associated theoretical challenges have been raised. Here we present a comprehensive analysis of the broad-band prompt emission data of a 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 γ-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 half opening angle of approximately ~3.4deg (R_GRB / 10^15 cm)^(-1/2), where R_GRB is the GRB prompt emission radius.
