The detection of periodicities in light curves of active galacticnuclei (AGN)
could have profound consequences for our understanding of the nature and
radiation physics of these objects. At high energies (HE; E>100 MeV) 5 blazars
(PG 1553+113,PKS 2155-304, 0426-380, 0537-441, 0301-243) have been reported to
show year-like quasi-periodic variations (QPVs) with significance >3 sig. As
these findings are based on few cycles only, care needs to be taken to properly
account for random variations which can produce intervals of seemingly periodic
behaviour. We present results of an updated timing analysis for 6 blazars
(adding PKS 0447-439), utilizing suitable methods to evaluate their long term
variability properties and to search for QPVs in their light curves. We
generate gamma-ray light curves covering almost 10 years, study their timing
properties and search for QPVs using the Lomb-Scargle Periodogram and the
Wavelet Z-transform. Extended Monte Carlo simulations are used to evaluate the
statistical significance. Comparing their probability density functions (PDFs),
all sources (except PG 1553+113) exhibit a clear deviation from a Gaussian
distribution, but are consistent with being log-normal, suggesting that the
underlying variability is of a non-linear, multiplicative nature. Apart from
PKS 0301-243 the power spectral density for all investigated blazars is close
to flicker noise (PL slope -1). Possible QPVs with a local significance ~ 3
sig. are found in all light curves (apart from PKS 0426-380 and 0537-441), with
observed periods between (1.7-2.8) yr. The evidence is strongly reduced,
however, if evaluated in terms of a global significance. Our results advise
caution as to the significance of reported year-like HE QPVs in blazars.
Somewhat surprisingly, the putative, redshift-corrected periods are all
clustering around 1.6 yr. We speculate on possible implications for QPV
generation.Comment: 12 pages, 13 figure