Detecting fast-variation pulsations in solar hard X-ray and radio emissions

Abstract: Quasi-periodic pulsations (QPPs) at sub-second periods are frequently detected in the time series of X-rays during stellar flares. However, such rapid pulsations are rarely reported in the hard X-ray (HXR) emission of the small solar flare. We explored the QPP patterns with fast-time variations in HXR and radio emissions produced in a small solar flare on 2025 January 19. By applying the Fast Fourier Transform, the fast-variation pulsations at a quasi-period of about 1 s are identified in the HXR channel of 20-80 keV, which were simultaneously measured by the Hard X-ray Imager and the Konus-Wind. The rapid pulsations with a same quasi-period were also detected in the radio emission at a lower frequency range of about 40-100 MHz. The restructured HXR images show that the QPP patterns mainly locate in footpoint areas that connect by hot plasma loops, and they appear in the flare impulsive phase. Our observations suggest that the fast-variation pulsations could be associated with nonthermal electrons that are periodically accelerated by the intermittent magnetic reconnection, and the 1-s period may be modulated by the coalescence instability between current-carrying loops and magnetic islands.