HESS J1857+026 has an energy-dependent morphology in the energy range of 10-500 GeV, and is spatially coincident with the energetic pulsar PSR J1856+0245. We have reanalysed the GeV emission from the HESS J1857+026 region using similar to 16.7 yr of Fermi Large Area Telescope observations. The gamma-ray spectrum is best described by a single power-law model with an index of 1.95 +/- 0.12 in the energy range of 0.03-1 TeV, and could connect smoothly with the TeV gamma-ray spectrum. Given the uncertainty regarding the origin of this emission, we conducted a theoretical analysis to explore the possibility that the multiwavelength emission from HESS J1857+026 originates from a pulsar wind nebula (PWN). Using a time-dependent one-zone model, we found that the observed gamma-ray fluxes can be adequately reproduced under the assumption that particles with broken power-law energy distribution are continuously injected into the nebula. This result indicates that it is reasonable to attribute the multiband non-thermal emission from this source to the PWN powered by PSR J1856+0245. Furthermore, the magnetic field strength in the nebula is constrained to approximately 2.6 mu G.

The prompt-emission spectra of gamma-ray bursts (GRBs) are commonly described by the empirical Band function. The typical low-energy spectral index is ∼ -1, which poses a challenge to standard synchrotron radiation models. We systematically investigate a fast-cooling synchrotron model with a decaying magnetic field and test, within an observation-consistent pipeline, whether it reproduces the Band-fit parameter distributions in the GBM catalog, in a statistical sense. We solve the electron continuity equation with synchrotron, adiabatic, and synchrotron self-Compton cooling to obtain the time-dependent electron distribution and synthetic spectra; we then forward-fold through the GBM response matrices and recover (α, β, E_p) with Band fits. We find that magnetic-field decay can harden the recovered α relative to the fast-cooling limit in part of parameter space, but the effect is not robust and is sensitive to the location of E_p within the finite band and to spectral curvature; varying key physical scales reshapes the recovered α distribution, indicating that catalog α often represents an effective in-band slope rather than the asymptotic index. SSC cooling provides modest additional hardening and, in our setups, does not stabilize α near the observed peak. Using Monte Carlo samples designed to mimic the observations, the model yields α mostly between -1.5 and -0.8, but remains centered around α≈ -1.5. Overall, while decaying-field fast-cooling synchrotron can partially alleviate overly soft spectra expected from standard fast-cooling synchrotron emission, it still falls short of reproducing the GBM α distribution at the population level, implying that additional physical processes are required.

We present a reanalysis of 17 years of Fermi Large Area Telescope (LAT) observations of the Crab pulsar obtained between 2008 August and 2025 August. Using monthly Jodrell Bank radio ephemerides, we assigned pulse phases to the LAT events and aligned the phase zero across the full data set. From this phase-aligned data set, we derived pulse profiles over 100 MeV to 300 GeV. The pulsed emission remains clearly detectable in the 10 to 20 GeV and 20 to 30 GeV bands, with H-test significances of 32.36 sigma and 11.59 sigma, respectively, but is not significantly detected in the 30 to 300 GeV band. Phase-resolved likelihood analysis was performed over 100 MeV to 30 GeV using 14 phase bins with comparable pulsed statistics. The fixed-window fractional fluxes show that the contribution of Peak 1 (P1) decreases steadily with energy, while those of Peak 2 (P2) and the Bridge increase, with P2 exceeding P1 above 10 GeV. Finally, the same phase-assignment framework also enables an off-pulse analysis from 100 MeV to 1 TeV, confirming the synchrotron and inverse-Compton components that dominate the emission in the selected off-pulse interval.

Detecting quasiperiodic oscillations (QPOs) in the light curves of blazars can improve our understanding of the dynamic processes associated with their central engines. We explore the possible QPO of the blazar PKS 0402-362 using over 10 yr of observations from the Fermi Large Area Telescope in the 0.1-500 GeV energy range. Time series analysis techniques, the Lomb-Scargle periodogram, the weighted wavelet z-transform, and phase dispersion minimization were used to search for year-timescale periodicities in the gamma-ray light curve of PKS 0402-362. These methods consistently detect two QPO signals with characteristic periods of similar to 2.7 (similar to 3.5 sigma of significance) and similar to 4.1 yr (>4.0 sigma of significance), which suggest a harmonic relationship in frequency of 2:3, and could be interpreted by the global p-mode oscillation of the accretion disk in a coupled disk-jet system.

The prompt emission spectra of gamma-ray bursts (GRBs) are commonly described by the empirical Band function. The typical low-energy spectral index is similar to-1, which poses a challenge to standard synchrotron radiation models. We systematically investigate a fast-cooling synchrotron model with a decaying magnetic field and test, within an observation-consistent pipeline, whether it reproduces the Band-fit parameter distributions in the Gamma-Ray Burst Monitor (GBM) catalog, in a statistical sense. We solve the electron continuity equation with synchrotron, adiabatic, and synchrotron self-Compton (SSC) cooling to obtain the time-dependent electron distribution and synthetic spectra; we then forward-fold through the GBM response matrices and recover (alpha, beta, Ep) with Band fits. We find that magnetic-field decay can harden the recovered alpha relative to the fast-cooling limit in part of parameter space, but the effect is not robust and is sensitive to the location of Ep within the finite band and to spectral curvature; varying key physical scales reshapes the recovered alpha distribution, indicating that catalog alpha often represents an effective in-band slope rather than the asymptotic index. SSC cooling provides modest additional hardening and, in our setups, does not stabilize alpha near the observed peak. Using Monte Carlo samples designed to mimic the observations, the model yields alpha mostly between -1.5 and -0.8, but remains centered around alpha approximate to -1.5. Overall, while a decaying-field fast-cooling synchrotron can partially alleviate overly soft spectra expected from standard fast-cooling synchrotron emission, it still falls short of reproducing the GBM alpha distribution at the population level, implying that additional physical processes are required.

The pulsar wind nebula CTB 87 (G74.9+1.2) is one of the sources emitting γ -rays with energies higher than 10 TeV, as measured by the Very Energetic Radiation Imaging Telescope Array System telescope. In this study, we undertake a reanalysis of the GeV emission from the CTB 87 region, utilizing ∼16 yr of high-energy γ -ray data collected with the Fermi Large Area Telescope. In the energy range of 0.03–1 TeV, the spectrum can be adequately described by a power-law model with an index of 1.34 ± 0.18, and the integral energy flux is calculated to be (7.25 ± 1.36) × 10 ^−13 erg cm ^−2 s ^−1 . Based on the multiband data, we have employed a time-dependent model to investigate the nonthermal emission properties of CTB 87. In the model, it is assumed that particles with broken power-law energy distributions are continuously injected into the nebula. This results in multiband nonthermal emission being produced by relativistic leptons via synchrotron radiation and inverse Compton processes. Furthermore, the model suggests an energy of approximately 2.4 PeV for the most energetic particle in the nebula.

1LHAASO J0249+6022 is an extended very-high-energy γ -ray source discovered by the Large High-Altitude Air Shower Observatory. Based on nearly 16.1 yr of data from the Fermi Large Area Telescope, we report the probable γ -ray emission from 1LHAASO J0249+6022 in the 0.03–1 TeV energy range. The results show that its γ -ray spectrum can be well fitted by a single power law with an index of 1.54 ± 0.17, and integral photon flux is (4.28 ± 1.03) × 10 ^−11 photons cm ^−2 s ^−1 . We also considered theoretically whether the nonthermal emission could originate from a pulsar wind nebula (PWN) scenario. Assuming that the particles injected into the nebula have a power-law distribution, the resulting spectrum from the inverse Compton scattering is consistent with the detected GeV and TeV γ -ray fluxes. Our study shows that the PWN scenario is reasonable for 1LHAASO J0249+6022.

We present periodicity search analyses on the long-term gamma-ray light curve of the BL Lacertae object 4FGL J2139.4-4235 observed by the Fermi Large Area Telescope, over a period of more than 15 yr, from 2008 August 4 to 2023 December 10. To determine the quasiperiodic oscillation (QPO) behavior of 4FGL J2139.4-4235 in the 0.3-300 GeV energy range, we used four methods, namely the Lomb-Scargle periodogram, the weighted wavelet z-transform, the phase dispersion minimization, and the autoregressive integrated moving average model. A Monte Carlo simulation technique is used to evaluate the significance level of the QPO signal. Significant levels above 3.5 sigma were detected in the gamma-ray light curve at about 650 days QPO, which is presented throughout the observation period. Interestingly, there was some correlation between the three bands in the discrete correlation function method calculations, which may be an indication that the variability trends between the three bands are similar. We explore the possible physical models and show that a supermassive binary black hole system or a jet helical motion model seem to be reasonable explanations for the potential QPO behavior.

The variability data for the BL Lacertae object TXS 1902+556 in the optical and gamma-ray wavebands were obtained from the 0.76-m Katzman Automatic Imaging Telescope and the Fermi Large Area Telescope (Fermi-LAT), covering periods of 14.4 and 14.7 yr, respectively. The variability properties were systematically analysed, with particular emphasis on the first comprehensive investigation of radiation variation in the optical waveband. Four well-established techniques were employed for this purpose: the Lomb-Scargle periodogram, REDFIT program, Jurkevich method, and discrete correlation function (DCF) approach. The optical waveband exhibits quasi-periodic oscillations (QPO) with a time-scale of PO=276.8 +/- 6.1 d at a significance level 3.87 sigma, while the gamma-ray waveband does not exhibit any significant periodicity. However, it should be noted that the QPO time-scale is consistent with the Sun-gaps in the optical light curve within 2 sigma uncertainties. The optical QPO behaviour is most likely attributed to the helical motion of the jet driven by the orbital motion in a supermassive black hole binary system. Moreover, we have provided an explanation for the absence of QPO in the gamma-ray light curves. Furthermore, utilizing the DCF method, a weak correlation between the variability in the optical and gamma-ray wavebands was observed, suggesting that the emission of TXS 1902+556 may be generated through a combination of synchrotron self-Compton (SSC) and external Compton (EC) processes, or a leptonic-hadronic hybrid process.

By means of astronomical observation data from the 60 cm Telescope at Yunnan Observatory, optical data in the g, r , and i bands were collected for BL Lac S5 0716+714, spanning from 2017 November 10 to 2018 May 15. The original data set contains 21,396 quasi-simultaneous multiband points, with 7132 data points for each band. The Lomb–Scargle periodogram method and the weighted wavelet Z-transform method were used to search for a quasi-periodic oscillation (QPO) signal in the data. For the first time, we report a QPO signal at 44 ± 6 days with a final significance of 3.98 σ . Further analysis of the spectrum index reveals that the 44 day QPO signal is most likely explained by a helical motion of a blob with velocity β in the jet, where the viewing angle of the emission region in the jet undergoes periodic variations. In addition, we employed the hypothesis testing method (the null hypothesis) to analyze the flux distribution and determined that a double log-normal distribution provides a better fit; thus, there may be two radiative mini-regions within a jet in this source, so this 44 day QPO signal may be superimposed on a longer-term outburst.

ABSTRACT We report a detection of GeV γ-ray emission potentially originating from the pulsar wind nebula in CTA 1 by analysing about 15 yr of Fermi Large Area Telescope data. By selecting an energy range from 50 GeV to 1 TeV to remove contamination from the γ-ray pulsar PSR J0007+7303, we have discovered an extended γ-ray source with a test statistic value of ∼44.94 in the region of CTA 1. The obtained flux is measured to be 6.71 ± 2.60 × 10−12 erg cm−2 s−1 with a spectral index of 1.61 ± 0.36, which allows for a smooth connection with the flux in the TeV band. CTA 1 is also considered to be associated with 1LHAASO J0007+7303u, which is an ultra-high-energy source listed in the recently published catalogue of the Large High Altitude Air Shower Observatory. We assume that the radiation originates from the pulsar wind nebula and that its multiwavelength spectral energy distribution can be explained well with a time-dependent one-zone model.

ABSTRACT The observation data of blazar 1ES 1426+42.8 were obtained using the 1.02 m optical telescope of Yunnan Observatories during 2021 to 2023. Intraday variability (IDV) is detected on seven nights. We use the turbulent model to investigate the mechanism of IDV in 1ES 1426+42.8. The fitting light curves match the actual IDV curves well. Using this model, we obtain the parameters such as the size of turbulent cells and the width of pulses in the jet. A possible short-lived quasi-periodic oscillation (QPO) of $58.55 \pm 8.09$ min was detected on 2022 April 26 whose light curve exhibits eight cycles at $\gt 3\sigma$ global significance and confirmed by several different techniques. Through a more detailed analysis of the light curve of this night, we find that the period is shortened from 54.23 min ($4\sigma$) to 29.71 min ($3\sigma$). The possible QPO and period shortening phenomenon are best explained by the processes of magnetic reconnections.

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 >5 σ 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 ∼3.5 σ , 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.

ABSTRACT N 157B located in the Large Magellanic Cloud is the first pulsar wind nebula detected outside of the Galaxy in γ-rays. In this paper, we analyse the emission above 30 GeV from N 157B using ∼14.3 yr of Fermi-LAT data. The γ-ray spectrum between 30 and 500 GeV is well described by a single power-law function with a photon index of 1.83 ± 0.26, and its integral photon flux is (4.10 ± 0.83) × 10−11 photons cm−2 s−1. We adopt a one-zone leptonic model to investigate whether the multiband non-thermal emission of the target source can be generated by synchrotron radiation and inverse-Compton scattering of the electrons/positrons. Assuming the electrons/positrons in the nebula have a broken power-law spectrum with two breaks, the model can reproduce the observed fluxes in the radio, X-ray, and γ-ray bands. This result indicates that the γ-ray emission from N 157B can be explained by the leptonic process of electrons/positrons via inverse-Compton scattering.

HESS J1303-631 is an extended TeV pulsar wind nebula powered by the pulsar PSR J1301-6305 detected with the High Energy Stereoscopic System. We present an analysis of the GeV γ -ray region of HESS J1303-631 with about 14 yr of Fermi Large Area Telescope data. The GeV γ -ray emission, coincident with the very-high-energy source, has a photon index of 1.69 ± 0.09 in 10–500 GeV band, and the GeV morphology has an extension to the same direction as indicated in the TeV band. Moreover, the observed multi-wavelength spectral energy distribution of the nebula is studied with a one-zone time-dependent leptonic model, in which the electrons/positrons injected into the nebula are assumed to have a broken power-law spectrum. The result indicates that the multi-wavelength non-thermal emission can be well reproduced via synchrotron radiation and inverse Compton scattering of the particles.

HESS J1420−607 is a γ -ray emitting source associated with the pulsar wind nebula (PWN) powered by the energetic pulsar PSR J1420−6048. Based on 14 yr of data obtained with the Fermi Large Area Telescope, we re-analyzed its GeV γ -ray radiative properties, resulting in detailed spectra obtained within the band 10–200 GeV. Moreover, we use a one-zone time-dependent model for the multiband nonthermal emission from pulsar wind nebulae to investigate the radiative properties of the nebula associated with HESS J1420−607. Assuming that the electrons/positrons are injected into the PWN with a broken power law spectrum with indexes of ∼1.6 and ∼2.7, as well as a break Lorentz factor of ∼5 × 10 6 , the results indicate that the multi-wavelength spectral energy distribution is consistent with the detected fluxes in both X-rays and γ -rays. The results support that the γ -rays detected can be produced via inverse Compton scattering of the energetic electrons/positrons within the PWN.

We propose a jet helical motion model to explain the γ -ray quasiperiodic oscillation of PG 1553+113. In this model, we hypothesize that a relativistic jet with constant angular velocity rotates around the axis, causing the Doppler factor of the jet to vary over time, leading to periodic changes in flux. In addition, we consider that the parameters of the emission field vary slightly during the observation period and can be obtained by directly fitting their multiwavelength spectra. Then, we use the resulting emission field information and the model to reproduce the light curve. Our model successfully explains the 2.18 ± 0.08 yr γ quasiperiodic oscillation of PG 1553+113, which is consistent with previous studies. Most importantly, we notice an upward trend in the light curve, and we suggest that this upward trend is due to long-term variability with a timescale of ∼42 yr.

耀变体是一种非常活跃的活动星系核，研究它的有效谱指数是认识其内部结构和辐射机制的有效方法。文中数据采用目前已公布出的SMARTS数据库数据,共682组具有B,V,R,J,K波段的准同时性的观测数据，用LombScargle Periodogram（LSP）方法研究了其有效谱指数的特性，结果得出3C 454.3光学和红外波段光变之间呈正相关；光学和红外波段光变存在2个主导周期，分别约为1.2yr，4.5yr；双黑洞结构模型中双黑洞质量比为2/1 。

LHAASO J1908+0621 has recently been detected as a source emitting gamma-rays with energies above 100 TeV, and multiband observations show that a break around 1 TeV appears in the gamma-ray spectrum. We have reanalysed the GeV gamma-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 gamma-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 gamma-ray bands. The results support the idea that LHAASO J1908 + 0621 originates from the PWN powered by PSR J1907 + 0602, and gamma-rays with energy above 100 TeV are produced by electrons/positrons in the nebula via ICS.

We carried out a high time-resolution, multicolour optical observing campaign for eight γ-ray detected blazars during 2010-2020. We analyze flux variations, correlations between magnitudes and colours on different timescales. Intraday variability (IDV) is detected in all eight sources of our sample. A bluer-when-brighter (BWB) chromatic trend is dominant on intraday timescales. On the short timescales, the BWB trend only shows up in ON 231, 3C 279, BL Lacertae and 1E 1458.8+2249. There is a BWB trend in 3C 279 on the long timescale. We estimate the upper limits of black hole mass for three blazars (i.e. ON 321, 1ES 1426+42.8, PKS 1510-089) using variability timescales. On April 13, 2010 a potential quasi-periodic oscillation (QPO) with the period of P=48.67±13.90 minutes is found in 1ES 1426+42.8. The light curve on March 16, 2021 further shows the existence of the QPO phenomenon. The QPO in this target deserves further observation and confirmation.