  In the last four decades it has been observed that solar flares show
quasi-periodic pulsations (QPPs) from the lowest, i.e. radio, to the highest,
i.e. gamma-ray, part of the electromagnetic spectrum. To this day, it is still
unclear which mechanism creates such QPPs. In this paper, we analyze four
bright solar flares which show compelling signatures of quasi-periodic behavior
and were observed with the Gamma-Ray Burst Monitor (\gbm) onboard the Fermi
satellite. Because GBM covers over 3 decades in energy (8 keV to 40 MeV) it can
be a key instrument to understand the physical processes which drive solar
flares. We tested for periodicity in the time series of the solar flares
observed by GBM by applying a classical periodogram analysis. However, contrary
to previous authors, we did not detrend the raw light curve before creating the
power spectral density spectrum (PSD). To assess the significance of the
frequencies we made use of a method which is commonly applied for X-ray
binaries and Seyfert galaxies. This technique takes into account the underlying
continuum of the PSD which for all of these sources has a P(f) ~ f^{-\alpha}
dependence and is typically labeled red-noise. We checked the reliability of
this technique by applying it to a solar flare which was observed by the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) which contains, besides
any potential periodicity from the Sun, a 4 s rotational period due to the
rotation of the spacecraft around its axis. While we do not find an intrinsic
solar quasi-periodic pulsation we do reproduce the instrumental periodicity.
Moreover, with the method adopted here, we do not detect significant QPPs in
the four bright solar flares observed by GBM. We stress that for the purpose of
such kind of analyses it is of uttermost importance to appropriately account
for the red-noise component in the PSD of these astrophysical sources.
