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 GRBs1–7. 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 stars8, 9. 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 GRBs10, 11. Additionally, some X-ray transients lacking gamma-ray bursts (GRB-less) have been identified as potential magnetar candidates originating from compact star mergers6, 12, 13. Nevertheless, smoking gun evidence is still lacking for a magnetar engine in short GRBs, and the associated theoretical challenges have been addressed14. 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 point toward a compact star merger origin, being consistent with its association with a kilonova15. More intriguingly, an extended X-ray emission component emerges as the γ-ray emission dies out, signifying the 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 3.4◦.

The effect of the divacancy defect on magnetic properties of Fe94V6 alloys was investigated using the first-principles calculations based on density functional theory. The model of Fe28V2 super-lattice for a divacancy in Fe94V6 alloy was established, in comparison with the perfect Fe30V2 super-lattice. For the first time, the effect of the on-site Coulomb repulsion term was considered to correct the underestimation for the bandgap. The magnetism of the Fe, V atoms, and the super-lattice were analyzed by the electron hybridization of the atoms, and the interactions between V and four different neighboring Fe atoms were also investigated. The results show that both Fe28V2 and Fe30V2 super-lattices are ferromagnetic, which is determined mainly by the strong ferromagnetism of Fe 3d electrons, and the hybridizations between Fe 3d electrons and Fe 2p and V 3d electrons account for the weak anti-ferromagnetism. When the Fe divacancy exists, the average magnetic moments of the Fe28V2 super-lattice and Fe atoms inside increase, while that of V atoms decreases due to the change of the Fe-V charges. The variation of the magnetic moments of different neighboring Fe atoms with respect to V atoms depends on the charge transfer and the Fe-V interaction.

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