A recent study by researchers from the Solar Activity and Coronal Mass Ejection (CME) Research Group and the Stellar Physics Research Group at Yunnan Observatories of the Chinese Academy of Sciences (CAS) has shed new light on the mechanisms behind giant flares from magnetars. Published in Monthly Notices of the Royal Astronomical Society (MNRAS), the research proposes a theoretical model to explain these rare but violent cosmic eruptions, advancing the understanding of magnetar-powered bursts.
Magnetars, a subclass of neutron stars, include anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs). With ultra-strong magnetic fields (1014-1015 Gauss), they are among the most extreme objects in the universe. Only about 30 magnetars have been detected so far. SGRs occasionally produce giant flares—enormous energy releases ranging from 1044- 1047 erg, ranking them as some of the most energetic events known. However, the exact mechanisms triggering these flares remain poorly understood.
To address this gap, the team developed a three-dimensional flux rope catastrophe model, inspired by solar coronal mass ejection (CME) theories. The model simulates how magnetic energy accumulates and releases in magnetars, identifying the conditions required for giant flares.
The study analytically calculated the magnetic field distribution generated by a circular flux rope crossing a neutron star’s surface. The study also examined the equilibrium of forces—including Lorentz force, magnetic tension, and gravity—acting on the system. By analyzing stability under changes in the star’s dipole magnetic field, the team derived a simple criterion for eruption: the parameterη= GMm/(R4B2)<0.48 (where G is the gravitational constant, M and R are the star’s mass and radius, m is ejecta mass, and B is dipole field strength) is a necessary condition for the system becoming unstable and leading to a flare.
This criterion defines the minimum magnetic field needed for giant flares. Combining it with known relationships between magnetar magnetic fields and spin periods, the researchers mapped theoretical flare-prone regions on a period vs. period-derivative diagram, aligning well with observations. The model also successfully estimated a solar ejecta mass limit of 1017 grams, consistent with solar observations.
The work provides a unified framework to explain magnetar flares and solar eruptions, highlighting the role of magnetic instability. Future applications could extend to other astrophysical phenomena involving magnetic energy release.
This study was conducted in collaboration between Assistant Researcher MENG Ying. and Researcher LIN Jun. from the Solar Activity and CME Research Group of Yunnan Observatory, and Associate Researcher ZHANG Quansheng from the Stellar Physics Research Group. This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences.
Figure 1: When a magnetic flux rope is present, the distribution of the magnetic field near the star's surface. The purple sphere represents the star, the blue line represents the magnetic axis, the red line represents the magnetic flux rope, the yellow lines represent the magnetic field lines, and the black lines represent the magnetic field lines near the rope. The left and right figures show the two cases where the magnetic flux rope is perpendicular and parallel to the magnetic axis, respectively. Image by MENG.
Figure 2: The distribution of neutron star periods and period derivatives. Black dots represent ordinary neutron stars, red dots represent magnetars, and green stars denote magnetars that have been observed to produce flares. The three blue lines show theoretical criteria for different ejected masses (solid, dash-dotted, and dashed lines correspond to 1024.5 g, 1025 g, and 1026 g, respectively). Flares can only occur when a neutron star's parameters lie above these lines. Image by MENG.
Contact:
MENG Ying
Yunnan Observatories, CAS
Email:mengy@ynao.ac.cn
