  Quasi-periodic pulsations (QPP) are often observed in X-ray emission from
solar flares. To date, it is unclear what their physical origins are. Here, we
present a multi-instrument investigation of the nature of QPP during the
impulsive and decay phases of the X1.0 flare of 28 October 2013. We focus on
the character of the fine structure pulsations evident in the soft X-ray time
derivatives and compare this variability with structure across multiple
wavelengths including hard X-ray and microwave emission. We find that during
the impulsive phase of the flare, high correlations between pulsations in the
thermal and non-thermal emissions are seen. A characteristic timescale of ~20s
is observed in all channels and a second timescale of ~55s is observed in the
non-thermal emissions. Soft X-ray pulsations are seen to persist into the decay
phase of this flare, up to 20 minutes after the non-thermal emission has
ceased. We find that these decay phase thermal pulsations have very small
amplitude and show an increase in characteristic timescale from ~40s up to
~70s. We interpret the bursty nature of the co-existing multi-wavelength QPP
during the impulsive phase in terms of episodic particle acceleration and
plasma heating. The persistent thermal decay phase QPP are most likely
connected with compressive MHD processes in the post-flare loops such as the
fast sausage mode or the vertical kink mode.
