Left: time derivatives of a section of flare 152 observed by GOES/XRS. The top panel shows the 1–8 Å emission and the bottom panel the 0.5–4 Å emission. Right: the corresponding power spectra, where the red solid lines are broken power law fits to the spectra, the red dotted lines represent the 95% confidence levels, and the red dashed lines the 99% levels. One peak in each is above the 99% level, at a period of 27.2 ± 0.9 s.