Quasi-periodic $γ$-ray modulations in the blazars PKS 2155-83 and PKS 2255-282

While there has been an increase in interest in the possibility of quasi-periodic oscillations (QPOs) in blazars, the search has hitherto been restricted to sources with well-sampled light curves. Objects with light curves that include gaps have been, to our knowledge, overlooked. Here, we study two such curves, which have the interesting feature of pertaining to relatively high redshift blazars -- FSRQs, PKS 2155-83 and PKS 2255-282 -- observed by Fermi-LAT. Their redshifts border the 'cosmic noon' era of galaxy formation and merging, and their light curves exhibit a distinctive pattern of repetitive high and low (gap dominant) states for $15.6$ years. To accommodate for the gaps in the curves, data is integrated over extended time intervals of 1 month and 2 months. The resulting curves were also examined using methods suitable for sparsely sampled data. This investigation of PKS 2155-83 and PKS 2255-282 suggests QPOs with periods of $4.69\pm0.79$ yr ($3\sigma$) and $6.82\pm2.25$ yr ($2.8\sigma$), respectively. The flux PDFs of the blazars, along with the correlation between their flux and spectral index, were also analyzed. Given the epochs the objects are observed, the plausibility of a binary black hole scenario as an origin of the apparent periodicity was examined. We estimated the prospective parameters of such a system using a simple geometric model. The total masses were estimated, and found to be consistent, in principle, with independent (dynamical) measurements of the central black hole masses in the two host galaxies.