In this work, we investigate the 2014–2015 neutrino flare associated with the blazar TXS 0506+056 and a recently discovered muon neutrino event IceCube-200107A in spatial coincidence with the blazar 4FGL J0955.1+3551, under the framework of a two-zone radiation model of blazars where an inner/outer blob close to/far from the supermassive black hole is invoked. An interesting feature that the two sources have in common is that no evidence of GeV gamma-ray activity is found during the neutrino detection period, probably implying a large opacity for GeV gamma rays in the neutrino production region. In our model, continuous particle acceleration/injection takes place in the inner blob at the jet base, where the hot X-ray corona of the supermassive black hole provides target photon fields for efficient neutrino production and strong GeV gamma-ray absorption. We show that this model can self-consistently interpret the neutrino emission from both blazars in a large parameter space. In the meantime, the dissipation processes in outer blob are responsible for the simultaneous multiwavelength emission of both sources. In agreement with previous studies of TXS 0506+056, an intense MeV emission from the induced electromagnetic cascade in the inner blob is robustly expected to accompany the neutrino flare in our model and could be used to test the model using the next-generation MeV gamma-ray detector in the future.

Using the Lomb–Scargle periodogram and the weighted wavelet Z-transform techniques, we have searched for quasi-periodic oscillations (QPOs) of the BL Lacertae object (BL Lac), PKS J2134–0153, in the 15-GHz radio light curve announced by the Owens Valley Radio Observatory 40-m telescope during the period from 2008 January 5 to 2019 May 18. This is the first time that the search for periodic radio signals in the BL Lac PKS J2134–0153 has used these two methods, which consistently reveal a QPO of 4.69 ± 0.14 yr (>5σ confidence level). We discuss possible causes for this QPO, and we expect that the binary black hole scenario, where the QPO is caused by the precession of a binary black hole, is the most likely explanation. Thus, the BL Lac PKS J2134–0153 could be a good candidate for a binary black hole. In the binary black hole scenario, the distance between the primary black hole and the secondary black hole is a ∼ 1.83 × 10¹⁶ cm.

We continuously monitored the blazar S5 0716+714 in the optical g, r, and i bands from 2017 November 10 to 2019 June 6. The total number of observations is 201 nights including 26,973 data points. This is a very large quasi-simultaneous multicolor sample for the blazar. The average time spans and time resolutions are 3.4 hr and 2.9 minutes per night, respectively. During the period of observations, the target source in the r band brightens from 14.ͫ16 to 12.ͫ29 together with five prominent subflares, and then becomes fainter to 14.ͫ76, and again brightens to 12.ͫ94 with seven prominent subflares. For the long-term variations, we find a strong flatter-when-brighter (FWB) trend at a low-flux state and then a weak FWB trend at a higher-flux state. A weak FWB trend at a low-flux state and then a strong FWB trend at a higher-flux state are also reported. Most subflares show strong FWB trends, except for two flares with a weak FWB trend. The particle acceleration and cooling mechanisms together with the superposition of the different FWB slopes from the subflares likely explain the optical color behaviors. A scenario of bent jet is discussed.

The origin of the multiband activities (outbursts/flares) of blazars is still a heavily debated topic. Shock and magnetic reconnection have long been considered as possible triggers of the multiband activities. In this paper, we present an exploration of the origin of multiband activities for a high-redshift (z = 1.8385) flat-spectrum radio quasar PKS 1502+106. Utilizing multiband data from radio to γ-ray and optical polarization observations, we investigate two dramatic activities in detail: a γ-ray-dominated outburst in 2015 and an optical-dominated outburst in 2017. Our main results are as follows. (I) A fast γ-ray flare with a flux-doubling timescale as short as 1 hr in 2015 is discovered. Based on the variability timescale, the physical parameters of the flaring region (e.g., minimum Doppler factor, emission region size, etc.) are constrained. At the peak of the flare, the γ-ray spectrum hardens to Γ _γ = 1.82 ± 0.04 and exhibits an obvious curvature/break characteristic that is caused by the typical “cooling break.” Modeling of multiband spectral energy distributions reveal a very hard electronic energy spectrum with the electronic spectral index of 1.07 ± 0.53. This result suggests that this fast γ-ray flare may be triggered by magnetic reconnection. (II) During the outburst in 2017, the degree of optical polarization and optical fluxes showed a very tight correlation. By analyzing the Stokes parameters of polarization observations, our results show that this outburst could be triggered by a transverse shock with a compression ratio of η > 2.2, and the magnetic field intensity of the shock emission region is about 0.032 G.

In deep X-ray surveys, active galactic nuclei (AGNs) with a broad range of luminosities have been identified. However, cosmologically distant low-luminosity AGN (LLAGN, erg s⁻¹) identification still poses a challenge because of significant contamination from host galaxies. Based on the 7 Ms Chandra Deep Field-South (CDF-S) survey, the longest-timescale (∼17 years) deep X-ray survey to date, we utilize an X-ray variability selection technique to search for LLAGNs that remain unidentified among the CDF-S X-ray sources. We find 13 variable sources from 110 unclassified CDF-S X-ray sources. Except for one source that could be an ultraluminous X-ray source, the variability of the remaining 12 sources is most likely due to accreting supermassive black holes. These 12 AGN candidates have low intrinsic X-ray luminosities, with a median value of 7 × 10⁴⁰ erg s⁻¹. They are generally not heavily obscured, with an average effective power-law photon index of 1.8. The fraction of variable AGNs in the CDF-S is independent of X-ray luminosity and is only restricted by the total number of observed net counts, confirming previous findings that X-ray variability is a near-ubiquitous property of AGNs over a wide range of luminosities. There is an anticorrelation between X-ray luminosity and variability amplitude for high-luminosity AGNs, but as the luminosity drops to ≲10⁴² erg s⁻¹, the variability amplitude no longer appears dependent on the luminosity. The entire observed luminosity–variability trend can be roughly reproduced by an empirical AGN variability model based on a broken power-law power spectral density function.

The correlation between the kinetic jet power P_jet, intrinsic γ-ray luminosity (L^int) and accretion (L_disk) may reveal the underlying jet physics in various black hole systems. We study the relation between kinetic jet power, intrinsic γ-ray luminosity, and accretion by using a large sample of jetted AGN, including flat-spectrum radio quasars (FSRQs), BL Lacertae objects (BL Lacs), gamma-ray Narrow-line Seyfert 1 galaxies (γNLS1s) and radio galaxies. Our main results are as follows: (1) The slope indices of the relation between P_jet and L^int are 0.85 ± 0.01 for the whole sample, 0.70 ± 0.02 for the FSRQs, 0.83 ± 0.03 for the BL Lacs, 0.68 ± 0.11 for the γNLS1s, and 0.93 ± 0.09 for the radio galaxies, respectively. The jets in γNLS1s and radio galaxies almost follow the same P_jet-L^int correlation that was obtained for Fermi blazars. (2) The slope indices of the relation between L^int and L_disk are 1.05 ± 0.02 for the whole sample, 0.94 ± 0.05 for the FSRQs, 1.14 ± 0.05 for the BL Lacs, and 0.92 ± 0.18 for the γNLS1s, respectively. The γNLS1s and radio galaxies almost also follow the L^int-L_disk correlation derived for Fermi blazars. (3) The jet power is larger than the luminosity of accretion disks for almost all jetted AGN. Jet power depends on both the Eddington ratio and black hole mass. We obtain log P_jet ∼ (1.00 ± 0.02)log L_disk for the whole sample, which is consistent with the theoretically predicted coefficient. These results may imply that the jets of jetted AGN are powered by the Blandford-Znajek mechanism.