Searching for quasi-periodicities in short transients: The curious case of GRB 230307A

Context. Gamma-ray bursts (GRBs) are the most powerful explosions in the Universe; their energy release reaches us from the end of the re-ionisation era, making them invaluable cosmological probes. GRB 230307A is the second brightest GRB ever observed in the 56 years of observations since the discovery of the phenomenon in 1967. Follow-up observations of the event at longer wavelengths have revealed a lanthanide-rich kilonova with long-lasting X-ray emission immediately following the prompt gamma-rays. Moreover, the gamma-ray light curve of GRB 230307A exhibits high amplitude variability, especially within the first 15 s. Aims. We performed a timing analysis of the prompt emission of GRB 230307A collected with INTEGRAL’s SPectrometer of INTEGRAL AntiCoincidence Shield (SPI-ACS) and Fermi’s Gamma-Ray Burst Monitor (GBM). Methods. We used Fourier analysis, wavelets, and Gaussian processes. We critically assessed all three methods in terms of their robustness for detections of quasi-periodic oscillations (QPOs) in fast transients such as GRBs. Results. Our analyses reveal QPOs at a frequency of ∼1.2 Hz (0.82 s period) near the burst’s peak emission phase, consistent across instruments and detection methods. We also identify a second, less significant QPO at ∼2.9 Hz (0.34 s) nearly simultaneously. We hypothesise that the two QPOs originate from the transition epoch at the end of the jet acceleration phase. These QPOs represent plasma circulation periods in vorticity about the jet axis carried outwards to the prompt radiation zone at much larger radii. They are sampled by colliding structures (e.g. shocks) in the spinning jet, possibly marking the evolution of plasma rotation during the final stages of the progenitor neutron star coalescence event.