Magnetar emergence in a peculiar gamma-ray burst from a compact star merger
astro-ph.HE
/ Authors
H. Sun, C. -W. Wang, J. Yang, B. -B. Zhang, S. -L. Xiong, Y. -H. I. Yin, Y. Liu, Y. Li, W. -C. Xue, Z. Yan
and 35 more authors
C. Zhang, W. -J. Tan, H. -W. Pan, J. -C. Liu, H. -Q. Cheng, Y. -Q. Zhang, J. -W. Hu, C. Zheng, Z. -H. An, C. Cai, Z. -M. Cai, L. Hu, C. Jin, D. -Y. Li, X. -Q. Li, H. -Y. Liu, M. Liu, W. -X. Peng, L. -M. Song, S. -L. Sun, X. -J. Sun, X. -L. Wang, X. -Y. Wen, S. Xiao
/ 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 (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 $\sim 3.4^\circ (R_{GRB}/10^{15}~{cm})^{-1/2}$, where $R_{GRB}$ is the GRB prompt emission radius.