Observational studies of the X-ray quasi-periodic oscillations of a solar flare

We study an M2.6 flare observed with RHESSI on 22 August 2005. The light curves of the hard X-rays (counts and photon fluxes), the derived number fluxes, as well as the energy fluxes of energetic electrons all presented a damped quasi-periodic oscillation. The modulation depth of the hard X-rays increased with the energies. For the energy fluxes of energetic electrons, the modulation depth can be as high as 90%. During the oscillations, however, the plasma temperature had no apparent change. No correspondence was found between the motions of the flare loops and the quasi-periodic oscillations. We conclude that an oscillation with a high modulation depth for a period of about four minutes cannot be easily explained with the existing mechanisms.