Recurrent double-peaked gamma-ray sub-flares in PKS 1424-418

We report a hint of recurrent double-peaked γ-ray sub-flares in PKS 1424--418 during two similar major active epochs, MJD 56117--56498 and MJD 59669--59978. The candidate double-peaked sub-flares show a characteristic intra-subflare peak separation of about 11.8 d and a neighbouring-subflare spacing of about 67 d. Red-noise Monte Carlo tests indicate that the candidate recurrent double-peaked morphology is not easily reproduced by stochastic variability alone, with false-alarm probabilities of p a=0.0216 and p b=0.0006 for the two active epochs, respectively. In the simulations of the entire Fermi Large Area Telescope (LAT) light curve, no red-noise realisation reproduces both epoch-like structures. The spectral energy distribution (SED) modelling shows similar radiative properties for the two peaks of one double-peaked candidate, while changes in the Doppler factor appear to play an important role in the flaring activity. Together with the previously reported near-zero lag between the millimetre and γ-ray emission, these results suggest that the recurrent activity may be associated with the compact mm/sub-mm core region. We discuss a possible structured-jet scenario in which a single disturbance propagates through a chain of quasi-stationary recollimation shocks. In this picture, repeated interactions between the disturbance and the shock structure may qualitatively account for both the double-peaked sub-flare profiles and the observed about 11.8 d and about 67 d time-scales. Further active epochs are required to determine whether this similarity represents a recurrent long-term behaviour.