A recent study by Ph.D. student SONG Feifan and Researcher MAO Jirong from the  Yunnan Observatories of the Chinese Academy of Sciences (CAS) has reported the detection of quasi-periodic oscillation (QPO) signals in an unusual gamma-ray burst event. The findings appear in their paper, “Possible Quasi-Periodic Oscillation Signals in the Unique Event of GRB 250702DBE/EP250702a?” published in The Astrophysical Journal.

Gamma-ray bursts (GRBs) are short-timescale, highly energetic explosive phenomena, typically associated with the collapse of massive stars or mergers of compact objects. On July 2, 2025, the Gamma-ray Burst Monitor (GBM) onboard the Fermi satellite detected an unusual high-energy burst event, designated GRB 250702DBE. This event triggered the Fermi/GBM three times. Although named according to the standard GRB conventions, the burst exhibited unusual traits: a duration of several hours, far longer than typical GRBs. The same source, also detected in X-ray band by the Einstein Probe (EP) as EP250702a, has attracted significant interest due to its  extraordinarily long duration, and unclear physical origin and radiation mechanism.

Using Fast Fourier Transform (FFT) and Weighted Wavelet Z-transform (WWZ) methods, the teams at Yunnan Observatories conducted a detailed time-series analysis  of the Fermi GBM gamma-ray data. While no QPO signals were found in the first trigger (GRB 250702D), the second trigger (GRB 250702B) revealed a QPO signal with a frequency of about 0.024 Hz (period ≈ 41.7 s). The third rigger (GRB 250702E) showed another QPO signal with a frequency of about 0.046 Hz (period ≈ 21.7 s). The researchers completed this analysis promptly after the event, yielding key clues about the burst’s astrophysical origin and physical mechanisms.

The detected QPO signals may be related to helical structures or precession within the relativistic jet produced by the source. If the event indeed originated from a GRB, it would represent an extremely unusual case compared to typical GRBs. Alternatively, a tidal disruption event involving an intermediate-mass black hole (a micro-TDE) could also produce such an ultra-long gamma-ray emission accompanied by quasi-periodic oscillations.

This work is supported by the National Key R&D Program of China, the Natural Science Foundation of China, and the Yunnan Revitalization Talent Support Program.

Figure 1: the light curve of the third trigger (GRB 250702E) along with its FFT and WWZ power spectra. Image by SONG.

Contact:
MAO Jirong
Yunnan Observatories, CAS
e-mail:jirongmao@ynao.ac.cn