  Two kinds of processes could occur during the flare decay phase: processes of
energy release or processes of energy relaxation. Quasi-periodic pulsations
(QPPs) of the broadband emission are a good tool for the verification of
mechanisms. We aim to study the processes during the decay phase of the X-class
solar flare SOL2014-03-29T17:48. The observations in X-ray, microwave, and
extreme ultraviolet (EUV) bands are exploited to study the fine temporal,
spatial, and spectral structures of the flare. The periods, amplitudes, and
phases of both the fluxes and physical parameters (emission measure,
temperature) are studied using standard methods of correlation, Fourier, and
wavelet analyses. It is found that the source of the QPPs is associated with
the uniform post-flare loop. The X-ray source is located at the top of the
arcade. QPPs with the similar characteristic time scales of $P\approx$ 74--80 s
are found in the X-ray (3--25 keV) and microwave (15.7 GHz) emissions. Besides,
QPPs with the same period are found in the time profiles of both the
temperature ($T_e$) and emission measure ($EM$). The QPPs in temperature and
the QPPs in emission measure demonstrate anti-phase behavior. The analysis
reveals the quasi-periodic process of energy relaxation, without any additional
source of energy during the decay phase. The periods of the QPPs are in a good
agreement with second harmonic of standing slow magneto-acoustic wave in the
arcade which could be triggered by a Moreton wave initiated by the flare in the
direct vicinity of the arcade.
