Xiang Liu

Active galactic nuclei (AGNs) are among the most powerful sources in the Universe, with their X-ray and radio emissions revealing the physics of black hole accretion and jet processes. We investigate the radio properties of Seyfert galaxies selected from the Swift-BAT 70-month hard X-ray survey, using radio observations from VLASS and RACS. From a parent sample of 752 unbeamed AGNs, we compile 604 Seyfert galaxies with measurements of radio luminosity, X-ray luminosity, and black hole mass. The sample spans X-ray Eddington ratios LX/LEdd from 10−7.5 to 1, with most sources ≳ 10−4. We first characterise the distribution of the X-ray–radio loudness, finding a broad, continuous range spanning about 4 orders of magnitude, with no clear evidence for a bimodal separation between radio-quiet and radio-loud populations. We also revisit the fundamental plane (FP) in an Eddington-scaled form and quantify its dependence on optical type and radio morphology. Subsamples by Seyfert type show consistent FP relations within uncertainties once a luminosity cut is applied, implying that optical classification does not significantly impact the radio–X-ray correlation. Radio morphology introduces secondary but systematic shifts, i.e., compact sources align with the FP for RIAF-dominated systems with compact jets, while extended and jet-like morphologies display excess radio luminosity at fixed LX/LEdd. This possibly reflects additional contributions from jets and/or star formation. We further discuss the implications for AGN unification, noting that orientation does not play a key role in shaping the FP.

This study conducts a comprehensive analysis of brightness and color variability in blazars, utilizing over 6 yr of quasi-simultaneous g -band and r -band data from 1149 sources in the Zwicky Transient Facility Data Release 22 (DR22), including 589 BL Lacs and 560 flat-spectrum radio quasars (FSRQs). We quantify the amplitude of variability and the fractional rms variability for each source and statistically assess the overall and short-term color behaviors across different subclasses; examine the distribution of brightness variability characteristics across different blazar types and investigate how the extent of variability correlates with color trends. We found BL Lacs tend to exhibit a bluer when brighter (BWB) trend, while FSRQs display a redder when brighter (RWB) trend; BL Lacs with negligible host-galaxy contamination exhibit a BWB trend fraction of 14.7% (68/462) compared to 2.3% (11/462) for RWB trend, while FSRQs show 8.8% (49/560) BWB trend versus 14.1% (79/560) RWB trend. By statistically investigating how color behavior depends on brightness state across different timescales, we find that brighter states in both BL Lacs and FSRQs are more likely to exhibit BWB trend. Our results also show that BL Lacs with a BWB trend exhibit higher variability than those with a RWB trend, whereas FSRQs with a RWB trend display significantly greater variability than those with a BWB trend. These results suggest that blazar color variability depends jointly on source type, brightness state, and variability amplitude, highlighting the complexity of color evolution in blazars.

Active galactic nuclei (AGNs) are characterized by strong temporal flux density variability across the electromagnetic spectrum, offering insights into the complex physical processes governing accretion and plasma outflows. To systematically investigate AGNs’ flux density variability in radio bands, a long-term program was initiated in late 2016: quasi-Simultaneous Multiwavelength Monitoring of gamma-ray-loud AGNs with the Nanshan 26 m radio telescope (SMMAN). This work presents the first data release of the SMMAN program, spanning over 8 yr from 2016 to 2024 with observations at 4.8 and 23.6 GHz. The SMMAN sample includes 131 northern ( δ > ∼ 0 ^∘ ) sources selected from the Fermi Large Area Telescope third source catalog. The characteristics of variability, spectral index, luminosity, and gamma-ray loudness factor are examined for different AGN classes within the sample. Target sources exhibit stronger variability at 23.6 GHz compared to 4.8 GHz, with BL Lacertae (BL Lac) objects being more variable than flat-spectrum radio quasars (FSRQs). BL Lacs generally have flatter radio spectra, while FSRQs, blazar candidates of uncertain type (BCUs), and radio galaxies (RDGs) span a wider range from flat to steep. FSRQs are more radio luminous than BL Lacs and other classes, with BCUs intermediate and RDGs generally fainter. FSRQs and BL Lacs have higher gamma-ray loudness factors than RDGs, while BCUs have intermediate values. The SMMAN dataset, incorporated with other historical and ongoing monitoring programs, will provide a unique opportunity to investigate the evolution of spectral energy distributions, search for quasiperiodic oscillations, and analyze supermassive black hole binary systems.

Active Galactic Nuclei (AGNs) are characterized by strong temporal flux density variability across the electromagnetic spectrum, offering insights into the complex physical processes governing accretion and plasma outflows. To systematically investigate AGNs flux density variability in radio bands, a long-term program was initiated in late 2016: quasi-Simultaneous Multiwavelength Monitoring of gamma-ray-loud AGNs with the Nanshan 26-m radio telescope (SMMAN). This work presents the first data release of the SMMAN program, spanning over eight years from 2016 to 2024 with observations at 4.8 and 23.6 GHz. The SMMAN sample includes 131 northern ($δ>\sim0^{\circ}$) sources selected from the Fermi Large Area Telescope third source catalog. The characteristics of variability, spectral index, luminosity, and $γ$-ray loudness factor are examined for different AGN classes within the sample. Target sources exhibit stronger variability at 23.6 GHz compared to 4.8 GHz, with BL Lac objects being more variable than flat-spectrum radio quasars (FSRQs). BL Lacs generally have flatter radio spectra, while FSRQs, blazar candidates of uncertain type (BCUs), and radio galaxies (RDGs) span a wider range from flat to steep. FSRQs are more radio-luminous than BL Lacs and other classes, with BCUs intermediate and RDGs generally fainter. FSRQs and BL Lacs have higher $γ$-ray loudness factors than RDGs, while BCUs have intermediate values. The SMMAN dataset, incorporated with other historical and ongoing monitoring programs, will provide a unique opportunity to investigate the evolution of spectral energy distributions, search for quasi-periodic oscillations, and analyze supermassive black hole binary systems.

We present X-ray intraday variability, cross-correlated variability, and power spectrum density analysis of the binary black hole blazar candidate OJ 287. The X-ray pointed observations of the source were carried out on eight occasions by the EPIC-pn camera on board the XMM-Newton satellite from 2005 November to 2022 November. The good time intervals range between 3.6 and 24.1 hr. Three energy bands—0.2–2 keV (soft), 2–10 keV (hard), and 0.2–10 keV (total)—have been used to estimate variability. Low amplitude variations are observed in four, five, and six light curves in soft, hard, and total energy bands, respectively. Only two observation IDs have shown variation in the all energy bands. The discrete correlation function of the light curves in soft and hard energy bands peaks at zero lag, suggesting that the emission in both bands was cospatial and came from the same population of leptons. Red noise dominates the power spectral densities of variable light curves. According to our flux and spectrum investigations, both particle acceleration and synchrotron cooling mechanisms contribute significantly to the emission from this blazar.

Intermittent jet activity of AGNs is a common phenomenon, whereas significant jet reorientation during episodic jet activity in relatively young radio galaxies are rarely reported. The quasar 0954+556 at redshift of 0.903 is an intriguing source exhibiting an unusual radio jet structure with significantly different jet directions at kpc and pc scales. At kpc scales, images from the VLA exhibit a bright core, a linear jet extending 24 kpc to the northwest, and a discrete jet component 16 kpc to the northeast. At pc scales, images from the VLBA show a two-component structure with a projected separation of 360 pc in the north-south direction. The peculiar structure of 0954+556 might result from jet reorientation. Here, our aim was to investigate the possible mechanism via multiscale and multifrequency deep radio images. We performed VLA and VLBA observations of 0954+556. Together with some existing data in the NRAO data archive, we made multiple VLA images at 1.4-22 GHz and VLBA images at 1.7-43 GHz for various image analyses of the jet structure. We identified the location of the radio core at pc scales, detected the faint counter-jets at both pc and kpc scales for the first time, and revealed a diffuse emission region connecting pc and kpc scale forward jets. Our spectral index distribution and spectral aging analysis indicate that 0954+556 might undergo at least two episodes of jet activity during the current AGN phase. Moreover, pc scale polarization maps display a well-resolved spine-sheath polarization structure. It seems that the jet direction of 0954+556 changed significantly during intermittent jet activity. This may explain the different jet orientations and spectral ages observed from kpc to pc scales. The research provides a strong case that AGN jet direction might change rapidly on timescales of one million years.

Type-C quasiperiodic oscillations (QPOs) in black hole X-ray transients typically manifest in the low-hard and hard-intermediate states. This study presents a detailed spectral and temporal analysis of the black hole candidate Swift J1727.8−1613 using NICER observations from 2023 August and September, with a focus on the first flare period. We detected Type-C QPOs whose centroid frequency increased from 0.33 Hz to 2.63 Hz. An additional increase in frequency was observed when the outburst entered a flare period. The time-averaged spectra, along with the rms and phase-lag spectra of the QPOs, were jointly fitted using the time-dependent Comptonization model vkompthdk to examine the geometry of the corona during this flare. Correlations between spectral and temporal properties suggest that the detected type-C QPOs are primarily modulated by Lense–Thirring precession. Leveraging simultaneous radio observations that indicate discrete jet ejections, we propose a scenario to describe the coevolution of the disk–corona–jet during a flare (∼3 days). This scenario is partially supported for the first time by polarization data in the soft gamma-ray band from the IBIS instrument on board the International Gamma-Ray Astrophysics Laboratory. A phenomenological analysis of the corona scenario was also conducted.

Context. The nearby elliptical galaxy M87 contains one of only two supermassive black holes whose emission surrounding the event horizon has been imaged by the Event Horizon Telescope (EHT). In 2018, more than two dozen multi-wavelength (MWL) facilities (from radio to γ-ray energies) took part in the second M87 EHT campaign. Aims. The goal of this extensive MWL campaign was to better understand the physics of the accreting black hole M87*, the relationship between the inflow and inner jets, and the high-energy particle acceleration. Understanding the complex astrophysics is also a necessary first step towards performing further tests of general relativity. Methods. The MWL campaign took place in April 2018, overlapping with the EHT M87* observations. We present a new, contemporaneous spectral energy distribution (SED) ranging from radio to very high-energy (VHE) γ-rays as well as details of the individual observations and light curves. We also conducted phenomenological modelling to investigate the basic source properties. Results. We present the first VHE γ-ray flare from M87 detected since 2010. The flux above 350 GeV more than doubled within a period of ≈36 hours. We find that the X-ray flux is enhanced by about a factor of two compared to 2017, while the radio and millimetre core fluxes are consistent between 2017 and 2018. We detect evidence for a monotonically increasing jet position angle that corresponds to variations in the bright spot of the EHT image. Conclusions. Our results show the value of continued MWL monitoring together with precision imaging for addressing the origins of high-energy particle acceleration. While we cannot currently pinpoint the precise location where such acceleration takes place, the new VHE γ-ray flare already presents a challenge to simple one-zone leptonic emission model approaches, and it emphasises the need for combined image and spectral modelling.

S-type asymptotic giant branch (AGB) stars are defined by their carbon-to-oxygen (C/O) ratio approaching unity. However, observations of circumstellar molecules in these stars are limited and challenging. In this study, we used the James Clerk Maxwell Telescope to search for cyanide (CN) in 12 selected S-type AGB stars, with only 4 of these stars displaying CN spectral line signals. The column density and fractional abundance of CN were calculated assuming local thermodynamic equilibrium, and the average CN-to-hydrogen-cyanide (HCN) abundance ratio of the four sources is about 0.036. We investigate, for the first time, the relationship between CN and HCN fractional abundances in all types of AGB stars, finding that the ratio is similar in O-rich and S-type stars, while those in C-rich stars are quite different. This result supports the idea that the level of HCN photodissociation plays a crucial role in determining the CN abundance in the circumstellar envelopes of AGB stars at various stages of their evolution.

It is believed that dual active galactic nuclei (AGN) form during galaxy mergers. Studying dual-AGN emission can provide valuable insights into galaxy merging and evolution. To investigate parsec-scale radio emission properties, we observed eight radio components of four selected dual-AGN systems using the Very Long Baseline Array (VLBA) at 5 GHz in multiple-phase-center mode. Among them, two compact radio components, labeled J0051+0020B and J2300-0005A, were detected clearly on parsec scales for the first time. However, the radio emission of the other six components was resolved out in the high-resolution images. We provide the values or upper limits of the brightness temperature and radio emission power, and analyze the emission origins in detail for each target. Based on their physical properties reported in this work and in the literature, we suggest that the radio emission in J0051+0020B and J2300-0005A originates primarily from compact jets, while the other six sources show more complex emission mechanisms. In addition, our VLBA observations suggest that the systematic X-ray deficit in our dual-AGN sample is likely attributed to the tidally induced effect and possibly a viewing angle effect.