We systematically search for quasiperiodic oscillatory (QPO) signals on the month timescale among the 1525 sources given in the Fermi Large Area Telescope Light Curve Repository. We find a transient QPO of 31.3 ± 1.8 days in the gamma-ray band light curve of the TeV blazar S5 0716+714, which has seven cycles (MJD 55918–56137) for the first time by weighted wavelet Z-transform and Lomb–Scargle periodogram methods. Monte Carlo simulations based on the power spectral density and probability distribution function were used to evaluate the confidence level of the QPO, and the result is ∼4.1σ. Seasonal autoregressive integrated moving average modeling of the light curve revealed it is a significant physical QPO. The physical models to explain the sporadic month-timescale QPOs in the blazar were discussed. Our studies indicate that the helical jet model and blob move helically in a curved jet model to properly explain this kind of transient QPO.

We analyze the quasiperiodic oscillation (QPO) of the historical light curve of flat-spectrum radio quasars PKS 0405-385 detected by the Fermi Large Area Telescope from 2008 August to 2021 November. To identify and determine the QPO signal of PKS 0405-385 in the γ-ray light curve, we use four time series analysis techniques based on frequency and time domains, i.e., the Lomb–Scargle periodogram (LSP), the weighted wavelet z-transform (WWZ), the REDFIT, and the epoch folding. The results show that PKS 0405-385 has a quasiperiodic behavior of ∼2.8 yr with the significance of ∼4.3σ in Fermi long-term monitoring. Remarkably, we also performed QPO analysis in the G-band light curve observed from 2014 October to 2021 October using LSP and WWZ technology, and the results (∼4σ of significance) are consistent with the periodic detection in γ-ray. This may imply that the optical emission is radiated by an electron population in the same way as the γ-ray emission. In discussing the possible mechanism of quasiperiodic behavior, either the helical motion within a jet or the supermassive black hole binary system provides a viable explanation for the QPO of 2.8 yr, and the relevant parameters have been estimated.

LHAASO J1908+0621 has recently been detected as a source emitting γ-rays with energies above 100 TeV, and multiband observations show that a break around 1 TeV appears in the γ-ray spectrum. We have reanalysed the GeV γ-ray properties of the 100-TeV source using 14 years of data recorded by the Fermi Large Area Telescope (Fermi-LAT). The spectrum in the energy range range 30–500 GeV has an index of 1.50 ± 0.26, which is much smaller than that detected in TeV γ-rays. Additionally, the radiation properties of this source are investigated based on a one-zone time-dependent model. In the model, LHAASO J1908+0621 is associated with a pulsar wind nebula (PWN) powered by the pulsar PSR J1907 + 0602. High-energy particles composed of electrons and positrons are injected into the nebula. Multiband non-thermal emission is produced via synchrotron radiation and inverse Compton scattering (ICS). Taking the effect of radiative energy losses and adiabatic cooling into account, the spectral energy distribution from a model with a broken power law for the distribution of the injected particles can explain the fluxes detected in the γ-ray bands. The results support the idea that LHAASO J1908 + 0621 originates from the PWN powered by PSR J1907 + 0602, and γ-rays with energy above 100 TeV are produced by electrons/positrons in the nebula via ICS.

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In this work, we report periodicity search analyses in the gamma-ray light curve of the blazar S4 0954+658 in monitoring undertaken by the Fermi Large Area Telescope. Four analytical methods and a tool are adopted to detect any periodic flux modulation and corresponding significance level, revealing: (i) a quasi-periodic oscillation (QPO) of 66 days with a significance level of >5σ spanning over 600 days from 2015 to 2016 (MJD 57,145–57,745), resulting in continuous observation of nine cycles, which is one of the longest cycles discerned in blazar gamma-ray light curves; (ii) a possible QPO of 210 days at a moderate significance of ∼3.5σ, which lasted for over 880 days from 2020 to 2022 (MJD 59,035–59,915) and for four cycles. In addition, we discuss several physical models to explain the origin of the two transient QPOs and conclude that a geometrical scenario involving a plasma blob moving helically inside the jet can explain the timescale of the QPO.