  Temporal analysis of blazar flux is a powerful tool to draw inferences about
the emission processes and physics of these sources. In the most general case,
the available light curves are irregularly sampled and influenced by gaps, and
in addition are also affected by correlated noise, making their analysis
complicated. Gaussian processes may offer a viable tool to assess the
statistical significance of proposed periods in light curves characterized by
any sampling and noise pattern. We infer the significance of the periods
proposed in the literature for two well known blazars with multiple claims of
possible year-long periodicity: PG1553+113 and PKS2155-304, in the high-energy
and optical bands. Adding a periodic component to the modeling gives a better
statistical description of the analyzed light curves. The improvement is rather
solid for PG1553+113, both at high energies and in the optical, while for
PKS2155-304 at high energies the improvement is not yet strong enough to allow
cogent claims, and no evidence for periodicity emerged by the analysis in the
optical. Modeling a light curve by means of Gaussian processes, in spite of
being relatively computationally demanding, allows us to derive a wealth of
information about the data under study and suggests an original analysis
framework for light curves of astrophysical interest.
