  Quasi-periodic pulsations (QPPs) are frequently observed in solar and stellar
flare emission, with recent studies suggesting that an increasing instantaneous
period is a common characteristic of QPPs. Determining the prevalence of
non-stationarity in QPPs contributes to a better understanding of which
mechanisms are responsible in QPP generation. We obtain the rate of period
evolution from QPPs in 98 M- and X-class flares from Solar Cycle 24 with
average periods between 8-130s and investigate the prevalence of QPP
non-stationarity. We also investigate whether the presence of a Coronal Mass
Ejection (CME) impacts the period evolution of QPPs. We analyse soft X-ray
lightcurves obtained from GOES' X-Ray Sensor (XRS) and assess the dominant
periods in the impulsive and decay phases of the flares using the Fast Fourier
Transform. We relate the rate of period evolution to flare duration, peak flare
energy, and average QPP period. We find evidence of non-stationarity in 81% of
the flares assessed, with most QPPs exhibiting a period evolution of less than
10s between the impulsive and decay phases, of which 66% exhibited an apparent
period growth and 14% showed an apparent period shrinkage. We find a positive
correlation between the absolute magnitude of period evolution and the duration
of the flare and no correlation between the period evolution of the QPPs and
flare energy or CME presence. Furthermore, we conclude that non-stationarity is
common in solar QPPs and must be accounted for in flare analysis.
