  We report decaying quasi-periodic intensity oscillations in the X-ray (6-12
keV) and extreme ultraviolet (EUV) channels (131, 94, 1600, 304 \AA) observed
by the Fermi GBM (Gamma-ray Burst Monitor) and SDO/AIA, respectively, during a
C-class flare. The estimated period of oscillation and decay time in the X-ray
channel (6-12 keV) was about 202 s and 154 s, respectively. A similar
oscillation period was detected at the footpoint of the arcade loops in the AIA
1600 and 304 \AA channels. Simultaneously, AIA hot channels (94 and 131 \AA)
reveal propagating EUV disturbances bouncing back and forth between the
footpoints of the arcade loops. The period of the oscillation and decay time
were about 409 s and 1121 s, respectively. The characteristic phase speed of
the wave is about 560 km/s for about 115 Mm loop length, which is roughly
consistent with the sound speed at the temperature about 10-16 MK (480-608
km/s). These EUV oscillations are consistent with the SOHO/SUMER Doppler-shift
oscillations interpreted as the global standing slow magnetoacoustic wave
excited by a flare. The flare occurred at one of the footpoints of the arcade
loops, where the magnetic topology was a 3D fan-spine with a null-point.
Repetitive reconnection at this footpoint could cause the periodic acceleration
of non-thermal electrons that propagated to the opposite footpoint along the
arcade and precipitating there, causing the observed 202-s periodicity. Other
possible interpretations, e.g. the second harmonics of the slow mode are also
discussed.
