  We present multi-wavelength analysis of a quasi-periodic pulsation (QPP)
observed in the hard X-ray, radio, and extreme-ultraviolet (EUV) channels
during an M1.9 flare occurred on 23-24 September 2011. The non-thermal hard
X-ray emission in 25-50 keV observed by RHESSI shows five distinct impulsive
peaks of decaying amplitude with a period of about three minutes. Similar QPP
was observed in the microwave emission recorded by the Nobeyama Radioheliograph
and Polarimeter in the 8.8, 15, 17 GHz channels. Interestingly, the 3-min QPP
was also observed in the metric and decimetric radio frequencies (25-180, 245,
610 MHz) as repetitive type III bursts. Multi-wavelength observations from the
SDO/AIA, Hinode/SOT, and STEREO/SECCHI suggest a fan-spine topology at the
eruption site, associated with the formation of a quasi-circular ribbon during
the flare. A small filament was observed below the fan-loops before the flare
onset. The filament rose slowly and interacted with the ambient field. This
behaviour was followed by an untwisting motion of the filament. Two different
structures of the filament showed $\sim$3-min periodic alternate rotation in
the clockwise and counterclockwise directions. The 3-min QPP was found to
highly correlate with 3-min oscillations in a nearby sunspot. We suggest that
the periodic reconnection (modulated either by sunspot slow-mode wave or by
untwisting filament) at a magnetic null-point most likely causes the repetitive
particle acceleration, generating the QPP observed in hard X-ray, microwave and
type III radio bursts.
