2025MNRAS.538L.100C

We present a timing study of the gamma and X-ray observations and analysis of a sample of bright gamma-ray bursts (GRBs; i.e. GRB 180720B, GRB 181222B, GRB 211211A, and GRB 220910A), including the very bright and long GRB 211211A (a.k.a. kilonova candidate). They have been detected and observed by the Atmosphere-Space Interactions Monitor (ASIM) installed on the International Space Station (ISS) and the Gamma-ray Burst Monitor (GBM) onboard the Fermi mission. The early (${\rm T}-{\rm T}_{0}{\approx }$ s) and high-energy (0.3-20 MeV) ASIM High Energy Detector (HED) and (150 keV-30 MeV) Fermi (BGO) light curves show well-defined peaks with a low quasi-periodic oscillation (QPO) frequency between 2.5 and 3.5 Hz that could be identified with the spin of the neutron star (NS) in the binary mergers originating these GRBs. These QPOs consist on the first detection of low-frequency QPOs (${\le }10$ Hz) detected in magnetars so far. We also detect a strong QPO at $21.8-22$ Hz in GRB 181222B together with its (less significant) harmonics. The low-frequency QPO would correspond to the signal of the orbiting NS previous to the final coalescence giving rise to the gravitational wave (GW) signal.