Professor Bin-Bin Zhang’s team at Nanjing University, in collaboration with the Institute of High Energy Physics of the Chinese Academy of Sciences and the University of Hong Kong, reported the detection of a short-lived periodic signal at 909 Hz in the gamma-ray burst GRB 230307A, observed by the GECAM satellite. This finding suggests that the central engine of GRB 230307A is likely a newborn “millisecond magnetar”. It marks the first direct observation of a stable millisecond pulsation signal in a gamma-ray burst, providing crucial evidence for understanding the nature of compact star merger remnants. The related results, entitled “Evidence for a brief appearance of gamma-ray periodicity after a compact star merger”, were officially published on September 19, 2025, in the internationally renowned journal Nature Astronomy. The first author of the paper is doctoral student Run-Chao Chen from the School of Astronomy and Space Science at Nanjing University, while Professor Bin-Bin Zhang (Nanjing University), Professor Shao-Lin Xiong (Institute of High Energy Physics, CAS), and Professor Bing Zhang (The University of Hong Kong) are co-corresponding authors.
The “Mysterious Pulse” in Gamma-Ray Bursts
Gamma-ray bursts (GRBs) are among the brightest and most violent explosions in the universe, releasing in just a few seconds more energy than the Sun emits over its entire lifetime. For decades, scientists have believed that some GRBs are powered by the merger of compact stars—such as neutron stars—and may produce remnants like black holes or neutron stars. However, because such events occur at great distances and last only briefly, the nature of the central remnant after a compact star merger has remained difficult to confirm directly.
GRB 230307A is the second brightest GRB ever recorded. It was first detected on March 7, 2023, by China’s GECAM satellite (Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor), which promptly alerted the international astronomy community. Subsequently, near-infrared spectroscopic observations by the James Webb Space Telescope confirmed that the event was triggered by the merger of two compact stars [1]. Because its high-energy gamma-ray emission lasted nearly one minute—far exceeding the typical duration of short GRBs (less than 2 seconds)—it has been classified as a special type of “ultra-long short GRB,” that is, a short GRB with unusually long duration.
Although the prevailing view is that short GRBs are usually powered by black holes formed after mergers, the unusually long duration of GRB 230307A makes it difficult for black hole models to account for its energy output. In response, the collaborative team proposed that this event was instead powered by a newborn millisecond magnetar [2]—a neutron star with an extremely strong magnetic field (up to 1015 gauss) and a spin period on the order of milliseconds. Its rapid rotation could leave a detectable periodic signature in the gamma-ray emission, analogous to a “heartbeat”-like pulsation. To test this possibility, the team conducted targeted searches and analyses of periodic signals in the GRB 230307A data.
First Detection of Millisecond Pulsations with the GECAM Satellite
The research team conducted an in-depth analysis using high time-resolution data from China’s independently developed GECAM satellite series. The gamma-ray detectors onboard GECAM satellites are equipped with sub-microsecond temporal resolution and broad energy coverage [3]. According to Prof. Shao-Lin Xiong, staff scientist at the Institute of High Energy Physics, CAS, and Principal Investigator of the GECAM project: “The unique advantages of the GECAM satellites provide powerful technical support for precisely capturing potential millisecond pulsation signals in GRBs.”
Run-Chao Chen, a doctoral student at Nanjing University, constructed a statistical framework specifically designed for GRB 230307A. Using this framework, the team identified a periodic signal with a central frequency of ~909 Hz, lasting for ~160 ms, appearing around 24 s after the burst was triggered. Its ~1.1 ms pulsation period closely matches the expected spin period of a millisecond magnetar.
To ensure robustness, the team cross-validated the signal using data from GECAM-B, GECAM-C, and the Fermi Gamma-ray Burst Monitor (Fermi/GBM), confirming consistency in time, frequency, and energy, and firmly establishing its association with GRB 230307A. Leveraging the high-quality GECAM-B data, the researchers then constructed a physics-based model to explain the signal’s properties and its strong link to a millisecond magnetar.
Figure 1: a-Light curve of GRB 230307A in the 98–248 keV range, recorded by multiple space observatories (top panel), together with the corresponding dynamical power spectra for GECAM-B data (bottom panel). The red box highlights the interval where the periodic signal was detected. b- High-resolution spectrograms from different instruments show that the signal is observed at the same time and frequency across all of them.
Physical Model of GRB 230307A: Why is the Periodic Signal So Brief?
This significant discovery raises an intriguing question: why does such a prominent periodic signal only last about 160 ms? Prof. Bing Zhang from the Department of Physics at the University of Hong Kong, a co-corresponding author of the paper, explained: “This signal appears in a GRB with luminosity far exceeding the Eddington limit, so it is most likely associated with energy dissipation within the ultra-relativistic jet. If this periodicity is indeed driven by the magnetar’s spin, two key conditions must be met: first, the rotational information of the magnetar must be effectively transmitted into the jet; second, the emission region of the jet must form an asymmetric brightness distribution, allowing the spin modulation to be observable.”
Based on this idea, the team proposed a physically rich model: the jet of GRB 230307A is powered by a millisecond magnetar, its internal magnetic energy greatly exceeds its kinetic energy, resulting in an energy dissipation process dominated by Poynting flux. Since the magnetar’s magnetic axis is misaligned with its spin axis, the jet develops a highly ordered magnetic field structure that evolves with the rotation, thereby carrying spin information that can be revealed during the energy dissipation process.
In a Poynting-flux dominated jet, magnetic reconnection serves as the main energy dissipation mechanism, producing “mini-jets” at different locations within the emission region [4]. Those directed toward Earth manifest as bright “hot spots.” Under normal circumstances, the large number and roughly uniform distribution of hot spots smooth out any spin modulation, making it difficult to detect a clear periodic signal in the GRB light curve. However, during a particular stage of GRB evolution—when high-latitude emission [5] begins to dominate—geometric light-travel delays cause the observer to first receive radiation from a small number of hot spots. Their limited number breaks the symmetry of the distribution, allowing spin-induced brightness variations to briefly appear as a periodic signal. As emission from more hot spots contributes, the overall symmetry is restored and the periodic signal disappears.
Figure 2: Schematic illustration of the periodic signal in GRB 230307A. Episodes I and III correspond to the phases dominated by beaming and high-latitude emission, respectively. In both phases, the distribution of hot spots is nearly symmetric, so no observable periodic signal arises. Episode II represents the transition from beamed to high-latitude emission; due to the small number and asymmetric distribution of hot spots, spin modulation briefly appears. Panels a and b show the jet structure from different observer angles.
Model Verified by Observation
The physical picture of GRB 230307A constructed by the team makes two key predictions: first, the periodic signal should appear during the transition phase as the GRB emission evolves toward high-latitude dominance; second, its energy dependence should show that the signal amplitude is stronger at higher energies and weaker at lower energies.
Excitingly, the observations of GRB 230307A are in strong agreement with these predictions. First, the appearance of the periodic signal coincides almost exactly with the phase inferred to be dominated by high-latitude emission in a 2024 multi-wavelength fitting study [2]. Second, spectral analyses from GECAM-B, GECAM-C, and Fermi/GBM clearly show that the periodic signal is significantly enhanced at higher energies while being noticeably weaker at lower energies, precisely as predicted.
Furthermore, the brief duration of the periodic signal can be approximately interpreted as the time taken for a single mini-jet beam to sweep across the observer’s line of sight. Using the 160-ms duration, the team estimated the initial radius at which jet energy dissipation begins. The result of about 1015 cm is in excellent agreement with theoretical expectations for a Poynting-flux dominated jet [4].
Scientific Implications: First evidence for the GRB’s “spinning engine”
This study provides the first direct observational evidence that a millisecond magnetar can serve as the remnant of a compact star merger, driving a Poynting-flux dominated jet that produces a gamma-ray burst. Previously, investigations into GRB central engines relied mainly on indirect inferences based on model assumptions—such as burst duration, multi-wavelength components, and afterglow evolution. The millisecond-period signal observed here can instead be regarded as a direct imprint of the magnetar’s spin.
In the vast universe, this 1.1-ms periodic pulse may well represent the “first heartbeat” of a newborn neutron star.
Behind the Paper
Professor Bin-Bin Zhang, corresponding author from the School of Astronomy and Space Science at Nanjing University, remarked: “This result not only advances our understanding of GRB central engine mechanisms, but also provides key clues for uncovering the evolution of compact objects under extreme physical conditions. In the future, our team will continue searching for similar periodic signals in other bright GRBs and, using the parameters obtained from this observation, carry out numerical simulations to explore how millisecond magnetars can effectively transfer their spin information into the jet’s energy dissipation phase.”
This research was supported by the Ministry of Science and Technology of China’s Key R&D Program, the National Natural Science Foundation of China, the China Space Station Program, the Jiangsu Provincial Innovation Programs, and China’s Double First-Class Initiative. The GECAM satellite was developed and deployed under the Strategic Pioneer Program on Space Science (Phase II) of the Chinese Academy of Sciences.
References
1. Levan, A. J. et al. Heavy-element production in a compact object merger observed by JWST. Nature 626, 737–741 (2024).
2. Sun, H. et al. Magnetar emergence in a peculiar gamma-ray burst from a compact star merger. Natl Sci. Rev. 12, nwae401 (2025).
3. Li, X. Q. et al. The technology for detection of gamma-ray burst with GECAM satellite. Radiat. Detect. Technol. Methods 6, 12–25 (2021).
4. Zhang, B. & Yan, H. The Internal-collision-induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-ray Bursts. Astrophys. J. 726, 90 (2011).
5. Kumar, P. & Panaitescu, A. Afterglow Emission from Naked Gamma-Ray Bursts. Astrophys. J. Lett. 541, L51–L54 (2000).
Paper link：https://doi.org/10.1038/s41550-025-02649-w
An artistic illustration of the millisecond magnetar and the gamma-ray burst jet (Illustration: Yuja Tian and Yuting Wu, Nanjing Zhijiao Cloud Intelligent Technology Co., Ltd.; Scientific concept guidance: Runchao Chen and Binbin Zhang, Nanjing University).