Paired quasi-periodic pulsations of hard X-ray emission in a solar flare

We investigate high time resolution data obtained by the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor(GECAM)during the flare event on 2022 April 21 at 01:52 UT.Several subpeaks with durations of 4-6 s have been detected in the hard X-ray precursor phase,and the key feature is that they appear in pairs and seem like double-peak struc-tures.These subpeaks are rarely observed in hard X-ray band and confirmed by the microwave obtained by Nobeyama Radio Polarimeters(NoRP)and Radio Solar Telescope Network(RSTN).While an exponential function can describe the continuum component of the time profile from the precursor to part of the impulsive phase.The periods of quasi-periodic pulsations(QPPs)are detected to be about 7.3 and 12.8 s for the precursor and impulsive phase,respectively,with at least 95%confidence level.The paired QPPs are assumed to be double-peak QPPs and then the scenario of current loop coalescence model is found to be in good agreement with our observation.The precursor phase can be interpreted as the oscillating coalescence of two islands,while the impulsive phase can be interpreted as more islands to coalesce one by one to form larger islands.