Can the evolutionary patterns of a magnetar’s radio-active phase be reproduced? The Tianma Radio Telescope (TMRT) team at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences has given a conclusive answer.

The team characterized the evolution of the magnetar XTE J1810−197’s radiation and rotation during its latest radio-active phase, and found striking similarities with its previous radio-active phase. To achieve this, they leveraged the telescope’s unique "dual-band simultaneous observation" capability, conducting simultaneous observations of the magnetar at 2.25 GHz and 8.60 GHz over a span of more than 2,100 days.

The relevant findings were published in the international academic journal SCIENCE CHINA Physics, Mechanics & Astronomy (Volume 69, 2026).

Magnetars are a class of special pulsars with extremely strong magnetic fields, typically ranging from 10^13 to 10^15 gauss—more than a billion times stronger than the highest magnetic fields ever produced in terrestrial laboratories. Magnetars often exhibit brief but intense high-energy outbursts, releasing energies that sometimes exceed their rotational energy loss. This suggests that magnetic energy is their primary power source. In addition, magnetars are considered the "engines" behind various short-duration violent burst phenomena, including fast radio bursts.

Currently, about 30 magnetars and candidates are known across the universe, mainly manifesting as soft gamma-ray repeaters and anomalous X-ray pulsars. Among them, only six have been detected emitting radio pulse radiation, an extremely rare phenomenon. Although magnetar research holds great physical significance, our understanding remains limited, partly due to the scarcity of samples and observational data, and partly to their inherently "changeable" nature.

This study focused on the first magnetar ever detected in radio: XTE J1810−197. After it was first detected with radio pulse radiation in 2004, the source remained active until late 2008, when it entered a radio-quiet phase. Following a decade of quiescence, it became radio-active again in late 2018.

The TMRT team promptly coordinated efforts and took advantage of the telescope’s "dual-band simultaneous observation" capability, conducting monitoring of this magnetar at 2.25 GHz and 8.60 GHz. Such "dual-band simultaneous" feature offers distinct advantages over conventional single-frequency monitoring. It not only tracks the evolution of radiation over time but also discerns, with great sensitivity, the frequency-dependent behavior of radiation. For highly variable objects like magnetars, simultaneous dual- or multi-frequency observations are essential for disentangling true frequency-dependent radiation properties; otherwise, temporal evolution can be mistakenly attributed to frequency dependence, leading to a "fixed-time-snapshot" misinterpretation.

Over a period of more than 2,100 days, the team completed 296 observations, covering nearly the entire duration of this active episode, from onset to quiescence. Based on the monitoring results, the team has for the first time conclusively answered whether the evolutionary patterns of a magnetar’s radio-active phase can be reproduced. (Figure 1).

Figure 1: Temporal evolution of the rotation and radiation parameters of XTE J1810−197: (a) duration of dual-frequency observations; (b) rotation frequency; (c) first derivative of rotation frequency; (d) dual-frequency flux density; (e) spectral index; (f) phase modulation index of the intermediate component; (g) width of the intermediate component at 10% of peak intensity. (Image by SHAO)

By comparing the evolution of the first derivative of rotation frequency during the latest radio-active phase with the evolution during the historical active episode in 2003, the team demonstrated that XTE J1810−197 exhibits highly consistent evolutionary characteristics in the two episodes: from intense fluctuations in the early active phase, to a "decline–rebound" in the middle phase, and finally to a stable plateau in the late phase—both the timescales and amplitudes are remarkably similar.

Further analysis indicates that this "reproducibility" is reflected not only in the amplitude and timescale of changes in the first derivative of rotation frequency, but also in the overall evolutionary trend of radio emission behavior. To explain this, the team used a twisted magnetosphere model to fit the measured values, and found that this theoretical model could account for the magnetar’s spin-down evolution and radiation characteristics to a certain extent.

Figure 2: Comparison of the evolution of the first derivative of rotation frequency of XTE J1810−197 during its 2003 (orange) and 2018 (blue) radio-active episodes, with the blue solid line representing the fit to the 2018 data using a twisted magnetosphere model. (Image by SHAO)

This study not only deepens our understanding of the radio emission mechanism of magnetars, but also provides key observational evidence for exploring plasma physics processes under extreme magnetic fields, while offering a new basis for predicting magnetar radio activity.

In addition to the Shanghai Astronomical Observatory, this research was completed in collaboration with Hubei University of Education, Guangzhou University, the Institute of High Energy Physics, and the Xinjiang Astronomical Observatory. The project received substantial support from the Ministry of Science and Technology’s Square Kilometre Array Special Project, the National Key Research and Development Program of China, the National Key Laboratory of Radio Astronomy and Technology, and other related programs. The dedicated support of the Tianma Telescope operations team played a decisive role in achieving the research objectives.