Yingkang Zhang

We investigate X-shaped radio galaxies (XRGs) with optical double-peaked narrow emission lines (DPNELs) as potential hosts of dual or binary supermassive black holes (SMBHs). Using available optical spectra from the Sloan Digital Sky Survey and the Dark Energy Spectroscopy Instrument survey, we identify 55 DPNEL sources among a sample of 187 XRGs. We find that the occurrence of [O iii ] DPNELs in XRGs is ∼30%, which is substantially higher than the ∼1% observed in the general galaxy population. Using optical (involving [O iii ] λ 5007, H α , H β , [N ii ] λ 6584, and [S ii ] λλ 6716, 6731) and mid-infrared diagnostic diagrams, we found that both components are predominantly powered by active galactic nuclei (AGNs), implying a ∼95% probability of hosting dual-AGN systems. In comparison, a stellar-mass-, color-, and redshift-matched control sample of non-XRG DPNEL galaxies shows a lower dual-AGN fraction, which depends strongly on radio luminosity, rising from 25% in radio-undetected galaxies to 54% in radio-detected galaxies, and reaching 95% in radio-bright XRGs and FR II radio galaxies. Interestingly, more than 30% of DPNEL XRGs have companion galaxies, supporting a merger-driven origin for these systems. Finally, we analyze parsec-scale radio structure in seven XRGs using Very Long Baseline Array (VLBA) observations at 1.4, 4.3, and 7.6 GHz, resolving the core in only one source. Nevertheless, flat radio spectral indices, AGN-like emission-line properties, and radio–optical VLBA–Gaia positional offsets found in five cases together support the interpretation that XRGs are strong dual/binary-AGN candidates.

Unveiling the launching and driving mechanisms of powerful jets in active galactic nuclei is crucial for understanding the coevolution of supermassive black holes and their host galaxies. 1156+295 is a blazar at a redshift of z = 0.729 and exhibits significant variability in long-term radio monitoring. Using multifrequency Effelsberg single-dish flux density data from 2007 to 2012, we performed synchrotron self-absorption (SSA) spectral modeling and extracted the turnover frequency and turnover flux density. By combining SSA spectral modeling with the core size and brightness temperature from quasi-simultaneous very long baseline interferometry images, we estimated the jet magnetic field strength and magnetic flux and investigated their temporal evolution in 1156+295. The evolution of radio flux density, spectral shape, and jet structure is consistent with the shock-in-jet framework. The inferred magnetic flux reaching or exceeding the magnetically arrested disk threshold, together with evidence that magnetic energy release precedes the radio flares, supports a magnetically driven jet scenario. Overall, our results place magnetic field measurements, spectral evolution, and inner-jet structural changes on a common timeline, providing observational constraints on their coupled evolution during flares.

Context. Our previous work identified a class of SDSS quasars with multiple Gaia detections, classifying them as candidates for various astrophysical systems, such as quasar–star pairs, dual quasars, and gravitationally lensed quasars. In this paper we present a pilot very long baseline interferometry (VLBI) study targeting a radio-bright subsample and report the first high-resolution imaging results. Aims. By leveraging the milliarcsecond-scale resolution of VLBI and its precise astrometric coordination with Gaia , we aim to refine the classification of these multiple matched sources, search for potential dual active galactic nuclei (AGNs), and assess the efficacy of the combined Gaia -VLBI approach in resolving ambiguous quasar systems. Methods. We cross-matched the Strange QUasar Candidates With ABnormal Astrometric Characteristics (SQUAB) quasar sample with the FIRST and NVSS catalogs, identifying 18 radio-emitting sources. The three brightest were selected for dual-frequency (1.6 and 4.9 GHz) VLBA observations. We performed VLBI imaging at both Gaia positions, constructed spectral index maps, and estimated brightness temperatures to characterize the radio morphology and physical properties. Results. For the three target sources, our VLBI observations reveal compact radio structures consistent with single AGNs at the primary Gaia positions. No significant emission is detected at the secondary Gaia locations. These results support the interpretation of the sources as quasar−star pairs, in line with earlier studies. Conclusions. This pilot study demonstrates the value of radio-VLBI high-resolution follow-ups on Gaia -selected quasar systems with multiple counterparts, showing how they can unambiguously reveal the true nature of these systems and help remove contaminants from dual AGN candidate samples.

Context. Compact symmetric objects (CSOs) trace the earliest phases of radio-galaxy growth; however, robust classification is difficult when radio cores are weak or invisible. Aims. We aim to develop and test a Gaia +VLBI approach that utilizes the high-precision optical astrometry of Gaia together with the high-resolution imaging of very long baseline interferometry (VLBI) to reliably locate the central engine and classify CSOs. Methods. We analysed 40 literature CSO candidates by overlaying Gaia Data Release 3 (DR3) positions on VLBI maps and by examining spectral index distributions, whole-source variability, and hotspot kinematics over up to 25 years. A source is classified as a CSO when the Gaia centroid lies between two steep-spectrum lobes; alignment with one end of the radio structure indicates a core–jet source. Results. Our method yields 20 confirmed CSOs, ten core–jet sources, and ten ambiguous cases affected by significant optical–radio positional offsets or limited data. The confirmed CSOs show low integrated variability, hotspot advance speeds typically < 0.5 c , where c denotes the speed of light (with a few mildly relativistic cases), and kinematic ages of ≈20 − 2000 yr. Five nearby CSOs show systematic Gaia –VLBI offsets despite the CSO-like morphology, likely reflecting host-galaxy environments and Gaia astrometric systematics. We find a clear dichotomy with radio power: high-power CSOs ( P _1.4 GHz > 10 ^26.5 W Hz ⁻¹ ) tend to be larger and host faster hotspots, while many low-power systems remain sub-kiloparsec and environmentally confined. Conclusions.Gaia +VLBI registration is a powerful method for CSO classification, especially where radio cores are faint. The observed power-size-velocity-age relations support distinct multiple evolutionary tracks, with high-power CSOs plausibly growing into large radio galaxies, while low-power CSOs appear confined by their host galaxy environments. Taken together, our results indicate that CSO evolution is shaped not only by intrinsic jet power, but also by host–galaxy environment and the duty cycle of the central engine. High-sensitivity observations of low-power CSOs will be crucial to map the full diversity of formation channels and evolutionary pathways of radio galaxies.

Our previous work identified a class of SDSS quasars exhibiting multiple Gaia detections, classifying them as candidates for various astrophysical systems such as quasar-star pairs, dual quasars, and gravitationally lensed quasars. In this paper, we present a pilot VLBI study targeting a radio-bright subsample and report the first high-resolution imaging results. By leveraging the milliarcsecond-scale resolution of VLBI and its precise astrometric coordination incorporating with Gaia, we aim to refine the classification of these multiple matched sources, search for potential dual AGNs, and assess the efficacy of the combined Gaia-VLBI approach in resolving ambiguous quasar systems. We cross-matched the SQUAB quasar sample with the FIRST and NVSS catalogs, identifying 18 radio-emitting sources. The three brightest were selected for dual-frequency (1.6 and 4.9 GHz) VLBA observations. We performed VLBI imaging at both Gaia positions, constructed spectral index maps, and estimated brightness temperatures to characterize the radio morphology and physical properties. For the three target sources, our VLBI observations reveal compact radio structures consistent with single AGN at the primary Gaia positions. No significant emission is detected at the secondary Gaia locations. These results support the interpretation of the sources as quasar$-$star pairs, in line with earlier studies. This pilot study demonstrates the value of radio-VLBI high-resolution follow-ups on Gaia-selected quasar systems with multiple counterparts, showing how they can unambiguously reveal the true nature of these systems and help remove contaminants from dual AGN candidate samples.

High-redshift (z > 3) γ-ray blazars are rare, but they are crucial for our understanding of jet evolution, γ-ray production and propagation, and the growth of supermassive black holes in the early Universe. A new analysis of Fermi-LAT data reveals a significant (5σ), spectrally soft (Γ ≃ 3.0) γ-ray source in a specific 4 month epoch, cospatial with PKS 0201+113 (z = 3.64). Monitoring of PKS 0201+113 at 15 GHz by the Owens Valley Radio Observatory 40 m telescope from 2008 to 2023 shows a prominent flare that dominates the radio light curve. The maximum of the radio flare coincides with the γ-ray flare, strongly suggesting an association (p-value = 0.023) between the γ-ray and the radio sources. PKS 0201+113 is only the third γ-ray blazar to be identified with z > 3.5, and it is the first such object to be identified by the detection of quasi-simultaneous γ-ray and radio flares. The jet properties of this peculiar blazar have been investigated. A detailed study of a two-zone leptonic model is presented that fits the broadband spectral energy distribution. An alternative scenario is also briefly discussed.

Context. Understanding the mechanisms that launch and shape powerful relativistic jets from supermassive black holes (SMBHs) in high-redshift active galactic nuclei (AGNs) is crucial for probing the co-evolution of SMBHs and galaxies over cosmic time. Aims. We focus on the high-redshift (z = 3.396) blazar OH 471 to explore the jet launching mechanism in the early Universe. Methods. Using multi-frequency radio monitoring observations and high-resolution Very Long Baseline Interferometry (VLBI) imaging over three decades, we studied the milliarcsecond structure and long-term variability of OH 471. Results. Our spectral modeling of the radio flux densities revealed a synchrotron self-absorbed spectrum, indicating strong magnetic fields within the compact core. By applying the flux freezing approximation, we estimated the magnetic flux carried by the jet. We found that it reaches or exceeds theoretical predictions for jets powered by black hole spin energy via the Blandford-Znajek mechanism. This implies that OH 471 is in a magnetically arrested disk (MAD) state, where the magnetic flux accumulated near the horizon regulates the accretion flow, allowing for an efficient extraction of black hole rotational energy. Conclusions. Our study demonstrates the dominance of MAD accretion in powering the prominent radio flares and relativistic jets observed in the radio-loud AGN named OH 471. Statistical studies of larger samples of high-redshift AGNs will shed light on the role of MAD accretion in launching and accelerating the earliest relativistic jets.