GECAM discovery of a peculiar magnetar X-ray burst (MXB 221120) from SGR J1935+2154 associated with a fast radio burst

¹ State Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
² University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China
³ College of Physics and Hebei Key Laboratory of Photophysics Research and Application, Hebei Normal University, Shijiazhuang Hebei 050024, China
⁴ College of Electronic and Information Engineering, Tongji University, Shanghai 201804, China
⁵ Department of Nuclear Science and Technology, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
⁶ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
⁷ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China
⁸ Department of Astronomy, Peking University, Beijing 100871, China
⁹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
¹⁰ Beijing Laser Acceleration Innovation Center, Huairou Beijing 101400, China
¹¹ School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 611756, China
¹² Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
¹³ School of Physics and Electronic Science, Guizhou Normal University, Guiyang 550001, China
¹⁴ Guizhou Provincial Key Laboratory of Radio Astronomy and Data Processing, Guizhou Normal University, Guiyang 550001, China
¹⁵ Institute of Astrophysics, Central China Normal University, Wuhan 430079, China
¹⁶ Key Laboratory of Lithium Battery New Energy Materials and Devices of Jiangxi Education Department, College of Intelligent Manufacturing and Materials & Chemical Engineering, Yichun University, Yichun, Jiangxi Province 336000, China
¹⁷ School of Computer and Information, Dezhou University, Dezhou 253023, China

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Received: 1 January 2026
Accepted: 8 February 2026

Abstract

Fast radio bursts (FRBs) are enigmatic cosmic transients of millisecond duration observed in the radio band. The identification of FRB-associated magnetar X-ray bursts (MXBs) from the galactic magnetar SGR J1935+2154 suggests that at least a fraction of FRBs can be produced from magnetar activity. However, the sample size of FRB-associated MXBs is still very small. Here we report a bright and peculiar FRB-associated MXB from SGR J1935+2154 detected by GECAM on November 20, 2022, dubbed MXB 221120. We find that the temporal and spectral properties of MXB 221120 exhibit distinctive features. Its light curve could generally be described by a single FRED function with the superposition of several narrow pulses. Interestingly, we identified a possible quasiperiodic oscillation feature with a center frequency of ∼18 Hz in this MXB. The time-integrated spectrum is best fit by a blackbody model with a temperature (kT) of 18.6 keV, rendering it the first thermal spectrum FRB-associated MXB from SGR J1935+2154. Compared to other MXBs with a single emission episode, MXB 221120 has a longer duration and a higher blackbody temperature, making it an outlier in the burst sample. These results indicate that MXB 221120 may have been produced by a special mechanism with extreme physical conditions.