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In the central region of the galaxy cluster, Abell 2199 (A 2199) resides the cD galaxy NGC 6166, which spatially coincides with the 3C 338 radio source. Lobes, jets, and a more detached southern structure (similar to a jet labelled as ridge) are seen at kiloparsec-scale images of 3C 338. This unusual radio morphology has led to the proposition of different hypotheses about its physical origin in the literature. In this work, we study the feasibility of a dynamical scenario where NGC 6166 moves around the X-ray inferred centre of A 2199 from the point of view of three-dimensional hydrodynamic simulations. The physical characteristics of the intracluster medium in which the jet propagates are constrained to those derived from X-ray observations in the vicinity of NGC 6166. Possible orbits for the jet inlet region are derived from the estimated radial velocity of NGC 6166, while the jet parameters are constrained by parsec-scale interferometric radio observations and the estimated jet power of 3C 338 obtained from radio and X-ray data. Our results show that the hypothesis of NGC 6166 has been moving around the centre of A 2199 during the last tens of million of years is compatible with the general radio morphology of 3C 338. Furthermore, the proposed dynamic scenario for the motion of NGC 6166 may be linked to gravitational perturbations induced by the passage of a sub-cluster of galaxies hundreds of millions of years ago. [ABSTRACT FROM AUTHOR]

In the period between 2009 and 2015, several very high-energy (VHE |$> 100$|  GeV) gamma-ray flaring events from the BL Lac object PKS 1424+240 were observed by the Cerenkov telescopes VERITAS and MAGIC. It had uncertain redshift (z) and using spectroscopical measurement, Paiano et al. (2017) found it to be |$z=0.604$|⁠. Using four different extragalactic background light (EBL) models and the photohadronic model, nine independently observed VHE gamma-ray spectra of PKS 1424+240 are analysed and a global |$\chi ^2$| fit is performed on all observations to estimate the best-fitting value for the redshift for each EBL model. Confidence levels (CL) intervals for the redshift are also estimated using all the EBL models. This method is tested by comparing our analysis with the observed value. It is shown that the photohadronic scenario provides an excellent description of all the observed spectra. It is found that the EBL model of Dominguez et al. (2011) is the one that provides the most restrictive limits on the redshift of PKS 1424+240, but in our analysis, |$z=0.604$| lies within the |$3\sigma$| CL interval of the EBL model of Saldana-Lopez et al. (2021). [ABSTRACT FROM AUTHOR]

A well-known calibrator source in radio astronomy, 3C 286 (⁠|$z=0.85$|⁠), is a compact steep-spectrum (CSS) radio source and spectroscopically classified as a narrow-line Seyfert 1 (NLS1) galaxy. It is also known for its damped Ly |$\alpha$| system from an intervening galaxy at |$z=0.692$| detected in both ultraviolet (UV) and radio spectra. In addition, despite being a misaligned active galactic nuclei (AGN), 3C 286 is also detected in |$\gamma$| -rays by Fermi. Thus, this unique object combines the characteristics of CSS sources, NLS1 galaxies, and |$\gamma$| -ray emitters with misaligned jets, providing an excellent laboratory for extending our knowledge of AGN disc–jet coupling. Despite its significance, 3C 286 has been rarely observed in X-rays. In this study, we present our deep XMM–Newton and Chandra observations of 3C 286. The results reveal that the X-ray spectrum can be well-described by models including an intervening absorber with redshift and column density consistent with previous UV and radio observations. The most important finding is that the spectrum cannot be described by a single power law, but a soft excess is required which is parametrized by a blackbody. Furthermore, we find evidence suggesting the presence of off-nuclear X-ray emission at a radius that corresponds to the location of the radio lobes. While further theoretical work is still needed, our findings offer new clues to understand the specific mechanism for |$\gamma$| -ray emission from this unique object. [ABSTRACT FROM AUTHOR]

The jets of radio AGN provide one of the most important forms of active galactic nuclei (AGN) feedback, yet considerable uncertainties remain about how they are triggered. Since the molecular gas reservoirs of the host galaxies can supply key information about the dominant triggering mechanism(s), here we present Atacama Large Millimeter/sub-millimeter Array CO(1-0) observations of a complete sample of 29 powerful radio AGN (⁠|$P_{1.4\,{\rm GHz}} \gt 10^{25}$|  W Hz |$^{-1}$| and |$0.05 \lt z \lt 0.3$|⁠) with an angular resolution of about 2–3 arcsec (corresponding to 2–8 kpc). We detect molecular gas with masses in the range |$10^{8.9} \lt M_{{\rm H}_2} \lt 10^{10.2}$| M |$_\odot$| in the early-type host galaxies of ten targets, while for the other 19 sources, we derive upper limits. The detection rate of objects with such large molecular masses – |$34\pm 9$|  per cent – is higher than in the general population of non-active early-type galaxies (ETGs: |$\lt $| 10 per cent). The kinematics of the molecular gas are dominated in most cases by rotating disc-like structures, with diameters up to 25 kpc. Compared with the results for samples of quiescent ETG in the literature, we find a larger fraction of more massive, more extended and less settled molecular gas structures. In most of the CO-detected sources, the results are consistent with triggering of the AGN as the gas settles following a merger or close encounter with a gas-rich companion. However, in a minority of objects at the centres of rich clusters of galaxies, the accretion of gas cooling from the hot X-ray haloes is a plausible alternative to galaxy interactions as a triggering mechanism. [ABSTRACT FROM AUTHOR]

In this work, we explore the connection of three jetted |$\gamma -$| loud AGNs classes: Compact steep-spectrum sources (CSS), Narrow-line Seyfert 1 (NLS1), and flat-spectrum radio quasars, through the modelling of the spectral energy distribution (SED). We selected two sources identified as CSS/NLS1 hybrids, PKS 2004-440 and 3C 286. Additionally, we included the source PKS 0440-00, initially classified as an FSRQ in the first Fermi -LAT catalogue, but recently reclassified as an NLS1. We present the results of their broadband SED modelling using a one-zone leptonic synchrotron-self Compton (SSC) + external Compton (EC) model. By exploring the parameter space and investigating the disc–jet connection in these sources, we analyse their classification in a model-dependent way. Our findings reveal that modeling PKS 2004-447 at relatively large angles, as expected for CSS, results in an SSC-dominated inverse Compton emission. In contrast, at low-observing angles, the inverse Compton emission is dominated by external photon fields. Both scenarios result in a jet with a low-radiative power. For 3C 286, we found that using a one-zone model limits the jet viewing angle to |$\sim 7^{\circ }$|⁠ , mainly due to its impact on the |$\gamma$| -ray emission. Our model results show a magnetically dominated jet, consistent with |$\gamma$| -CSS sources. Our results suggest that PKS 0440-00, can be classified as a powerful |$\gamma -$| NLS1, characterized by high accretion power and a jet dominated by bulk motion, similar to FSRQs. [ABSTRACT FROM AUTHOR]

We report on the discovery of one of the most extreme cases of high-frequency radio variability ever measured in active galactic nuclei (AGNs), observed on time-scales of days and exhibiting variability amplitudes of 3–4 orders of magnitude. These sources, all radio-weak narrow-line Seyfert 1 (NLS1) galaxies, were discovered some years ago at Aalto University Metsähovi Radio Observatory (MRO) based on recurring flaring at 37 GHz, strongly indicating the presence of relativistic jets. In subsequent observations with the Karl G. Jansky Very Large Array (JVLA) at 1.6, 5.2, and 9.0 GHz no signs of jets were seen. To determine the cause of their extraordinary behaviour, we observed them with the JVLA at 10, 15, 22, 33, and 45 GHz, and with the Very Long Baseline Array (VLBA) at 15 GHz. These observations were complemented with single-dish monitoring at 37 GHz at MRO, and at 15 GHz at Owens Valley Radio Observatory (OVRO). Intriguingly, all but one source either have a steep radio spectrum up to 45 GHz, or were not detected at all. Based on the 37 GHz data, the time-scales of the radio flares are a few days, and the derived variability brightness temperatures and variability Doppler factors are comparable to those seen in blazars. We discuss alternative explanations for their extreme behaviour, but so far no definite conclusions can be made. These sources exhibit radio variability at a level rarely, if ever, seen in AGN. They might represent a new type of jetted AGN, or a new variability phenomenon, and thus deserve our continued attention. [ABSTRACT FROM AUTHOR]

We present a new approach for stably evolving general relativistic magnetohydrodynamic (GRMHD) simulations in regions where the magnetization |$\sigma =b^2/\rho c^2$| becomes large. GRMHD codes typically struggle to evolve plasma above |$\sigma \approx 100$| in simulations of black hole accretion. To ensure stability, GRMHD codes will inject mass density artificially to the simulation as necessary to keep the magnetization below a ceiling value |$\sigma _{\rm max}$|⁠. We propose an alternative approach where the simulation transitions to solving the equations of general relativistic force-free electrodynamics (GRFFE) above a magnetization |$\sigma _{\rm trans}$|⁠. We augment the GRFFE equations in the highly magnetized region with approximate equations to evolve the decoupled field-parallel velocity and plasma energy density. Our hybrid scheme is explicit and easily added to the framework of standard-volume GRMHD codes. We present a variety of tests of our method, implemented in the GRMHD code koral , and we show results from a 3D hybrid GRMHD + GRFFE simulation of a magnetically arrested disc (MAD) around a spinning black hole. Our hybrid MAD simulation closely matches the average properties of a standard GRMHD MAD simulation with the same initial conditions in low magnetization regions, but it achieves a magnetization |$\sigma \approx 10^6$| in the evacuated jet funnel. We present simulated horizon-scale images of both simulations at 230 GHz with the black hole mass and accretion rate matched to M87*. Images from the hybrid simulation are less affected by the choice of magnetization cut-off |$\sigma _{\rm cut}$| imposed in radiative transfer than images from the standard GRMHD simulation. [ABSTRACT FROM AUTHOR]

We present the results of seven years of multicolour photometric monitoring of a sample of 31 |$\gamma$| -ray bright blazars using the RINGO3 polarimeter on the Liverpool Telescope from 2013–2020. We explore the relationships between simultaneous observations of flux in three optical wavebands along with Fermi |$\gamma$| -ray data in order to explore the radiation mechanisms and particle populations in blazar jets. We find significant correlations between optical and |$\gamma$| -ray flux with no detectable time lag, suggesting leptonic emission processes in the jets of these sources. Furthermore, we find the spectral behaviour against optical and |$\gamma$| -ray flux for many sources is best fit logarithmically. This is suggestive of a transition between bluer-/redder-when-brighter into stable-when-brighter behaviour during high activity states; a behaviour that might be missed in poorly sampled data, resulting in apparent linear relationships. [ABSTRACT FROM AUTHOR]

In the absence of supplementary heat, the radiative cooling of halo gas around massive galaxies (Milky Way mass and above) leads to an excess of cold gas or stars beyond observed levels. Active galactic nucleus jet-induced heating is likely essential, but the specific properties of the jets remain unclear. Our previous work concludes from simulations of a halo with |$10^{14} \,\mathrm{ M}_\odot$| that a successful jet model should have an energy flux comparable to the free-fall energy flux at the cooling radius and should inflate a sufficiently wide cocoon with a long enough cooling time. In this paper, we investigate three jet modes with constant fluxes satisfying the criteria, including high-temperature thermal jets, cosmic ray (CR)-dominant jets, and widely precessing kinetic jets in |$10^{12}-10^{15}\, {\rm M}_{\odot }$| haloes using high-resolution, non-cosmological magnetohydrodynamic simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model, conduction, and viscosity. We find that scaling the jet energy according to the free-fall energy at the cooling radius can successfully suppress the cooling flows and quench galaxies without violating observational constraints. On the contrary, if we scale the energy flux based on the total cooling rate within the cooling radius, strong interstellar medium cooling dominates this scaling, resulting in a jet flux exceeding what is needed. Among the three jet types, the CR-dominant jet is most effective in suppressing cooling flows across all surveyed halo masses due to enhanced CR pressure support. We confirm that the criteria for a successful jet model work across a wider range, encompassing halo masses of |$10^{12}-10^{15} {\rm M_\odot }$|⁠. [ABSTRACT FROM AUTHOR]

At optical/ultraviolet energies, blazars display an underlying thermal (unpolarized) contribution from the accretion disc, torus, and line-emitting regions, diluting the polarized emission from the jet-component. Optical polarimetry can be used to disentangle the thermal and non-thermal components, and place constraints on the particle populations and acceleration mechanisms responsible for the non-thermal emission. We present the results of a linear optical spectropolarimetric observing campaign of 18 blazars (six BL Lac-type objects and 12 flat-spectrum radio quasars) undertaken with the Southern African Large Telescope between 2016 and 2022. This was done to observe these systems during flaring states, as well as long-term monitoring of PKS1510-089, AP Lib, and PKS 1034-293. The observations traced the frequency dependence of the degree and angle of polarization, as well as changes in the spectral line strengths. We investigated possible correlations between the polarization and other observed characteristics for the sources. While an indication of correlation was found between the frequency dependence and the average level of polarization for some sources, a correlation was not found for the population as a whole. These results highlight that continuous observations and in-depth modelling of polarization and its frequency dependence is required to obtain a more holistic view of TeV blazars. [ABSTRACT FROM AUTHOR]

Jets from active nuclei may supply the heating which moderates cooling and accretion from the circum-galactic medium. While steady overpressured jets can drive a circulatory flow, lateral energy transfer rarely exceeds 3 per cent of jet power, after the initial bow shock has advanced. Here, we explore if pulses in high-pressure jets are capable of sufficient lateral energy transfer into the surrounding environment. We answer this by performing a systematic survey of numerical simulations in an axisymmetric hydrodynamic mode. Velocity pulses along low Mach jets are studied at various overpressures. We consider combinations of jet velocity pulse amplitude and frequency. We find three flow types corresponding to slow, intermediate, and fast pulsations. Rapid pulsations in light jets generate a series of travelling shocks in the jet. They also create ripples which propagate into the ambient medium while a slow convection flow brings in ambient gas which is expelled along the jet direction. Long period pulses produce slowly evolving patterns which have little external effect, while screeching persists as in non-pulsed jets. In addition, rapid pulses in jets denser than the ambient medium generate a novel breathing cavity analogous to a lung. Intermediate period pulses generate a series of bows via a bellows action which transfer energy into the ambient gas, reaching power efficiencies of over 30 per cent when the jet overpressure is sufficiently large. This may adequately inhibit galaxy gas accretion. In addition, such pulses enhance the axial out-flow of jet material, potentially polluting the circum-galactic gas with metal-enriched interstellar gas. [ABSTRACT FROM AUTHOR]

Supermassive black hole binaries (SMBHBs) are natural by-products of galaxy mergers and are expected to be powerful multimessenger sources. They can be powered by the accretion of matter and then radiate across the electromagnetic spectrum, much like normal active galactic nuclei (AGNs). Current electromagnetic observatories have a good chance of detecting and identifying these systems in the near future. However, precise observational indicators are needed to distinguish individual AGNs from SMBHBs. In this paper, we propose a novel electromagnetic signature from SMBHBs: non-thermal emission produced by the interaction between the jets ejected by the black holes. We study close SMBHBs, which accrete matter from a circumbinary disc and the mini-discs formed around each hole. Each black hole ejects a magnetically dominated jet in the direction of its spin through the Blandford–Znajek mechanism. We argue that in such a situation, the interaction between the jets can trigger strong magnetic reconnection events, where particles are accelerated and emit non-thermal radiation. Depending on whether the jets are aligned or misaligned, this radiation can have different periodicities. We model the evolution of the particles accelerated during the dual jet interaction and calculate their radiative output, obtaining spectra and providing estimates for the variability time-scales. We finally discuss how this emission compares with that of normal AGNs. [ABSTRACT FROM AUTHOR]

Radio galaxies harbouring jetted active galactic nuclei are a frequent target of very long baseline interferometry (VLBI) because they play an essential role in our exploration of how jets form and propagate. Hence, only a few have not yet been detected with VLBI; Fornax A was one of the most famous examples. Here we present the first detection of the compact core region of Fornax A with VLBI. At 8.4 GHz the faint core is consistent with an unresolved point source. We constrained its flux density to be S0 = 47.5 − 62.3 mJy and its diameter to be D0min ≤ 70 μas. The high values of the measured brightness temperature (TB ≳ 1011 K) imply that the observed radiation is of non-thermal origin, likely associated with the synchrotron emission from the active galactic nucleus. We also investigated the possibility of a second radio source being present within the field of view. Adding a second Gaussian component to the geometrical model fit does not significantly improve the quality of the fit, and we therefore, conclude that our detection corresponds to the compact core of Fornax A. Analysis of the non-trivial closure phases provides evidence for the detection of a more extended flux density, on the angular scale of ∼4000 μas. Finally, the fractional circular polarisation of the core is consistent with zero, with a conservative upper limit being mcirc ≤ 4%. [ABSTRACT FROM AUTHOR]

Context. Extreme TeV BL Lacs are a class of blazars with unique spectral and temporal features that are not easily reproducible using standard one-zone models based on single shock acceleration. To account for their peculiar properties, we elaborated a two-step acceleration model in which a recollimation shock and the subsequent downstream turbulence energize non-thermal electrons. Aims. We applied the model to a sample of extreme TeV BL Lacs with well-characterized spectral energy distributions. Since we used several sources, we automatized the exploration of the parameter space. This allowed us to derive the parameter distributions and study the correlations among them. Methods. We numerically solved a system of two coupled nonlinear differential equations to obtain the non-thermal particles and turbulence spectra. We calculated the spectral energy distribution via the synchrotron self-Compton emission model. The automatization of the parameter space exploration is possible through a Markov chain Monte Carlo (MCMC) ensemble sampler, in our case emcee. Results. We derived well-defined posterior distributions for the parameters, showing that the model is well constrained by available data and demonstrating the suitability of our method. The cross-correlations among some of the physical parameters are not trivial. Therefore, we conclude that MCMC sampling is a key instrument for characterizing the complexity of our multiparameter phenomenological model. [ABSTRACT FROM AUTHOR]

We present a multi-wavelength analysis, from the radio to the X-ray band, of the redshift z = 6.44 VIK J2318−31 radio-loud quasi-stellar object, one of the most distant currently known of this class. The work is based on newly obtained observations (uGMRT, ATCA, and Chandra) as well as dedicated archival observations that have not yet been published (GNIRS and X-shooter). Based on the observed X-ray and radio emission, its relativistic jets are likely young and misaligned from our line of sight. Moreover, we can confirm, with simultaneous observations, the presence of a turnover in the radio spectrum at νpeak ∼ 650 MHz that is unlikely to be associated with self-synchrotron absorption. From the near-infrared spectrum we derived the mass of the central black hole, $ M_{\mathrm{BH}}=8.1^{+6.8}_{-5.6} \times 10^8\,{M_{\odot}} $ , and the Eddington ratio, $ \lambda_{\mathrm{EDD}} = 0.8^{+0.8}_{-0.6} $ , using broad emission lines as well as an accretion disc model fit to the continuum emission. Given the high accretion rate, the presence of a ∼8 × 108 M⊙ black hole at z = 6.44 can be explained by a seed black hole (∼104 M⊙) that formed at z ∼ 25, assuming a radiative efficiency ηd ∼ 0.1. However, by assuming ηd ∼ 0.3, as expected for jetted systems, the mass observed would challenge current theoretical models of black hole formation. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is often considered as the primary particle acceleration mechanism in a magnetized blazar zone environment. The majority of radiation in the reconnection layer comes from plasmoids and their mergers. In particular, plasmoid mergers can produce strong multiwavelength flares and major variations in synchrotron polarization signatures. However, radiative properties of plasmoid mergers have not been well explored due to difficulties in tracking the merging processes. Here we use an image processing method that combines the magnetic vector potential and density to identify isolated and merging plasmoids. We find that this method can clearly distinguish radiation contributions from isolated plasmoids, merging plasmoids, and the primary current sheet of reconnection. This new method enables us to study the radiative properties of plasmoids and mergers statistically. Our results show that isolated plasmoids have similar emissivity regardless of their sizes, and they generally have non-zero polarization degree (PD) due to their quasi-circular shape. Flares due to plasmoid mergers have relative amplitudes that are antiproportional to the size ratio of the plasmoids participating in the mergers. Finally, only mergers between plasmoids of comparable sizes (width ratio ≲5) can lead to significant spectral hardening and polarization angle (PA) variations; the amplitude of the PA variations is between 0 and 180° and has a mean value of 90°. Our analyses on 2D simulations can pave the way for future analyses and machine learning techniques on radiative properties of 3D magnetic reconnection simulations. [ABSTRACT FROM AUTHOR]

We present rest-frame optical JWST Near-infrared Spectrograph (NIRSpec) integral field unit (IFU) spectroscopy of TN J1338−1942 at z  = 4.1, one of the most luminous galaxies in the early universe with powerful extended radio jets. Previous observations showed evidence for strong, large-scale outflows based on its large (∼150 kpc) halo detected in Ly α, and high-velocity [O  ii ] emission features detected in ground-based spectroscopy. Our NIRSpec/IFU observations spatially resolve the emission line properties across the host galaxy. We find at least five concentrations of strong line emission, coinciding with discrete continuum features previously detected in imaging from Hubble Space Telescope and JWST , over an extent of ∼2 arcsec (∼15 kpc). The spectral diagnostics unambiguously trace active galactic nuclei (AGN) activity plus interaction between the interstellar medium and the radio jet as the dominant mechanisms for the ionization state and kinematics of the gas in the system. A secondary region of very high ionization lies at roughly 5 kpc away from the nucleus, and within the context of an expanding cocoon enveloping the radio lobe, this may be explained by strong shock-ionization of the entrained gas. However, it could also signal the presence of a second obscured AGN, which may offer an explanation for an intriguing outflow feature seen perpendicular to the radio axis. The presence of a dual supermassive black holes system in this galaxy would support that large galaxies in the early Universe quickly accumulated their mass through the merging of smaller units, at the centres of large overdensities. The inferred black hole mass to stellar mass ratio of 0.01−0.1 for TNJ1338 points to a more rapid assembly of black holes compared to the stellar mass of galaxies at high redshifts, consistent with other recent observations. [ABSTRACT FROM AUTHOR]

The detection of quasi-periodic oscillations (QPOs) in blazar light curves is crucial for understanding their nature and fundamental physical processes. PKS J2156−0037 (z = 0.495), a blazar monitored by the Owens Valley Radio Observatory (OVRO) 40-m radio telescope, is the subject of this study. We searched for the presence of QPOs in the publicly available 15 GHz light curve of PKS J2156−0037, spanning 2008 January to 2020 January. We employed four well-established methods (the Jurkevich method, the generalized Lomb–Scargle periodogram, the weighted wavelet Z -transform, and the redfit method) that offer complementary strengths for robust periodicity detection. A consistent and statistically significant QPO signal was detected with a periodicity of around 611 d (at least 4.26σ local significance) and a global significance exceeding 99.83 per cent (or 2.92σ) based on Monte Carlo simulations with 105 simulated light curves (assuming underlying red-noise processes). This is the first reported detection of such a variability feature in this object. The possible physical mechanisms responsible for radio QPOs in blazars are discussed briefly. [ABSTRACT FROM AUTHOR]

Active galactic nuclei (AGNs) play an integral role in galaxy formation and evolution by influencing galaxies and their environments through radio jet feedback. Historically, interpreting observations of radio galaxies and quantifying radio jet feedback has been challenging due to degeneracies between their physical parameters. In particular, it is well established that different combinations of jet kinetic power and environment density can yield indistinguishable radio continuum properties, including apparent size and Stokes I luminosity. We present an approach to breaking this degeneracy by probing the line-of-sight environment with Faraday rotation. We study this effect in simulations of three-dimensional relativistic magnetohydrodynamic AGN jets in idealized environments with turbulent magnetic fields. We generate synthetic Stokes I emission and Faraday rotation measure (RM) maps, which enable us to distinguish between our simulated sources. We find enhanced RMs near the jet head and lobe edges. We show that increasing the environment density and the average cluster magnetic field strength broadens the distribution of Faraday rotation measure values. We study the depolarization properties of our sources, finding that the hotspot regions depolarize at lower frequencies than the lobes. We quantify the effect of depolarization on the RM distribution, finding that the frequency at which the source is too depolarized to measure the RM distribution accurately is a probe of environmental properties. This technique offers a range of new opportunities for upcoming surveys, including probing radio galaxy environments and determining more accurate estimates of the AGN feedback budget. [ABSTRACT FROM AUTHOR]

Strong AGN heating provides an alternative means for the disruption of cluster cool cores (CCs) to cluster mergers. In this work, we present a systematic Chandra study of a sample of 108 nearby (z < 0.1) galaxy clusters, to investigate the effect of AGN heating on CCs. About 40 per cent of clusters with small offsets between the BCG and the X-ray centre (≤50 kpc) have small CCs. For comparison, 14 of 17 clusters with large offsets have small CCs, which suggests that mergers or sloshing can be efficient in reducing the CC size. Relaxed, small CC clusters generally have weak radio AGNs (⁠|$P_{1.4\rm GHz}\lt 10^{23}$|  W Hz−1), and they show a lack of systems hosting a radio AGN with intermediate radio power (⁠|$2\times 10^{23}\lt P_{1.4\rm GHz}\lt 2\times 10^{24}$|  W Hz−1). We found that the strongest circumnuclear (<1 kpc) X-ray emission only exists in clusters with strong radio AGN. The duty cycle of relaxed, small CC clusters is less than half of that for large CC clusters. It suggests that the radio activity of BCGs is affected by the properties of the surrounding gas beyond the central ∼10 kpc, and strong radio AGNs in small X-ray CCs fade more rapidly than those embedded in large X-ray CCs. A scenario is also presented for the transition of large CCs and coronae due to radio AGN feedback. We also present a detailed analysis of galaxy cluster 3C 129.1 as an example of a CC remnant possibly disrupted by radio AGN. [ABSTRACT FROM AUTHOR]

Radiative cooling and active galactic nucleus heating are thought to form a feedback loop that regulates the evolution of low-redshift cool-core galaxy clusters. Numerical simulations suggest that the formation of multiphase gas in the cluster core imposes a floor on the ratio of cooling time (t cool) to free-fall time (t ff) at min(t cool/ t ff) ≈ 10. Observations of galaxy clusters show evidence for such a floor, and usually the cluster cores with min(t cool/ t ff) ≲ 30 contain abundant multiphase gas. However, there are important outliers. One of them is Abell 2029 (A2029), a massive galaxy cluster (M 200 ≳ 1015 M⊙) with min(t cool/ t ff) ∼ 20, but little apparent multiphase gas. In this paper, we present high-resolution 3D hydrodynamic adaptive mesh refinement simulations of a cluster similar to A2029 and study how it evolves over a period of 1–2 Gyr. Those simulations suggest that A2029 self-regulates without producing multiphase gas because the mass of its central black hole (⁠|${\sim} 5 \times 10^{10} \, \mathrm{ M}_\odot$|⁠) is great enough for Bondi accretion of hot ambient gas to produce enough feedback energy to compensate for radiative cooling. [ABSTRACT FROM AUTHOR]

There is still a limited number of high-redshift (z > 3) active galactic nuclei (AGNs) whose jet kinematics have been studied with very long baseline interferometry (VLBI). Without a dedicated proper motion survey, regularly conducted astrometric VLBI observations of bright radio-emitting AGN with sensitive arrays can be utilized to follow changes in the jets, by means of high-resolution imaging and brightness distribution modelling. Here, we present a first-time VLBI jet kinematic study of NVSS J080518 + 614423 (z = 3.033) and NVSS J165844 − 073918 (z = 3.742), two flat-spectrum radio quasars that display milliarcsecond-scale jet morphology. Archival astrometric observations carried out mainly with the Very Long Baseline Array, supplemented by recent data taken with the European VLBI Network, allowed us to monitor changes in their radio structure in the 7.6−8.6 GHz frequency band, covering almost two decades. By identifying individual jet components at each epoch, we were able to determine the apparent proper motion for multiple features in both sources. Apparent superluminal motions range |$(1-14)\, c$|⁠ , and are found to be consistent with studies of other high-redshift AGN targets. Using the physical parameters derived from the brightness distribution modelling, we estimate the Doppler-boosting factors (δ ≈ 11.2 and δ ≈ 2.7), the Lorentz factors (Γ ≈ 7.4 and Γ ≈ 36.6), and the jet viewing angles (θ ≈ 4.4° and θ ≈ 8.0°), for NVSS J080518 + 614423 and NVSS J165844 − 073918, respectively. The data revealed a stationary jet component with negligible apparent proper motion in NVSS J165844 − 073918. [ABSTRACT FROM AUTHOR]

Identifying methods to discover dual active galactic nucleus (AGN) has proven to be challenging. Several indirect tracers have been explored in the literature, including X/S-shaped radio morphologies and double-peaked (DP) emission lines in the optical spectra. However, the detection rates of confirmed dual AGN candidates from the individual methods remain extremely small. We search for binary black holes (BBH) in a sample of six sources that exhibit both X-shaped radio morphology and DP emission lines using the Very Long Baseline Array (VLBA). Three out of the six sources show dual VLBA compact components, making them strong candidates for BBH sources. In addition, we present deep uGMRT images revealing the exquisite details of the X-shaped wings in three sources. We present a detailed precession modeling analysis of these sources. The black hole separations estimated from the simplistic geodetic precession model are incompatible with those estimated from emission line offsets and the VLBA separations. However, precession induced by a non-coplanar secondary black hole is a feasible mechanism for explaining the observed X-shaped radio morphologies and the black hole separations estimated from other methods. The black hole separations estimated from the double-peaked emission lines agree well with the VLBA compact component separations. Future multifrequency VLBA observations will be critical in ruling out or confirming the BBH scenario in the three galaxies with dual component detections. [ABSTRACT FROM AUTHOR]

A substantial fraction of quasars display broad absorption lines (BALs) in their rest-frame ultraviolet spectra. While the origin of BALs is thought to be related to the accretion disc wind, it remains unclear whether the observed ratio of BAL to non-BAL quasars is a result of orientation. We conducted observations of 48 BAL quasars and the same number of non-BAL quasars at 322 MHz using the Giant Metrewave Radio Telescope. Combined with previous flux measurements ranging from MHz to GHz frequencies, we compared continuum radio spectra between the two quasar groups. These data offer insights into low-frequency radio properties that have been difficult to investigate with previous observations only at GHz frequencies. Our results show that 73 ± 13 per cent of the BAL quasars exhibit steep or peaked spectra, a higher proportion than the 44 ± 14 per cent observed in the non-BAL quasars. In contrast, there are no discernible differences between the two quasar groups in the radio luminosity, peak frequency, and spectral index distributions of sources with steep or peaked spectra and sources with flat or inverted spectra. Generally, as the jet axis and line of sight become closer to parallel, quasars exhibit flat or inverted spectra rather than steep or peaked spectra. Therefore, these results suggest that BAL quasars are more frequently observed farther from the jet axis than non-BAL quasars. However, given that a certain proportion of BAL quasars exhibit flat or inverted spectra, more than the simple orientation scenario is required to elucidate the radio properties of BAL quasars. [ABSTRACT FROM AUTHOR]

In this work, we study the co-evolution of central black holes (BHs) and host galaxies by utilizing an advanced iteration of the DELPHI semi-analytical model of galaxy formation and evolution. Based on dark matter halo merger trees spanning the redshift range from z  = 20 to z  = 4, it now incorporates essential components such as gas heating and cooling, cold and hot BH accretion, jet and radiative AGN feedback. We show how different BH growth models impact quasar and galaxy observables at z ≥ 5, providing predictions that will help discriminate between super-Eddington and Eddington-limited accretion models: despite being both consistent with observed properties of SMBHs and their host galaxies at z ∼ 5–7, they become very clearly distinguishable at higher redshift and in the intermediate mass regime. We find that the super-Eddington model, unlike the Eddington-limited scenario, predicts a gap in the BH mass function corresponding to the intermediate-mass range 104 M⊙ < M bh < 106 M⊙. Additionally, it predicts black holes up to two orders of magnitude more massive for the same stellar mass at z  = 9. The resulting velocity dispersion–BH mass relation at z ≥ 5 is consistent with local measurements, suggesting that its slope and normalization are independent of redshift. Depending on the Eddington ratio, we also model the emergence of AGN jets, predicting their duty cycle across as a function of BH mass and their potential impact on the observed number density distribution of high-redshift AGN in the hard X-ray band. [ABSTRACT FROM AUTHOR]

Jets are endemic to both Galactic solar mass and extragalactic supermassive black holes. A recent 86 GHz image of M 87 shows a jet emerging from the accretion ring around a black hole, providing the first direct observational constraint on the kinematics of the jet-launching region in any black hole jetted system. The very wide (∼280 μas), highly collimated, limb-brightened cylindrical jet base is not predicted in current numerical simulations. The emission was shown to be consistent with that of a thick-walled cylindrical source that apparently feeds the flow that produces the bright limbs of the outer jet at an axial distance downstream of 0.4 mas < z < 0.65 mas. The analysis here applies the conservation laws of energy, angular momentum, and magnetic flux to the combined system of the outer jet, the cylindrical jet, and the launch region. It also uses the brightness asymmetries of the jet and counterjet to constrain the Doppler factor. The only global solutions have a source that is located < 34 μas from the event horizon. This includes the Event Horizon Telescope annulus of emission and the regions interior to this annulus. The axial jet begins as a magnetically dominated flow that spreads laterally from the launch radius (< 34 μas). It becomes super-magnetosonic before it reaches the base of the cylindrical jet. The flow is ostensibly redirected and collimated by a cylindrical nozzle formed in a thick accretion disk. The flow emerges from the nozzle as a mildly relativistic (0.3c < v < 0.4c) jet with a significant protonic kinetic energy flux. [ABSTRACT FROM AUTHOR]

The radio galaxy M 87 is a variable very-high energy (VHE) gamma-ray source, exhibiting three major flares, which were reported in 2005, 2008, and 2010. Despite extensive studies, the origin of the VHE gamma-ray emission is not yet fully understood. In this study, we investigate the VHE gamma-ray spectrum of M 87 during states of high gamma-ray activity, utilizing 20.2 h of H.E.S.S. observations. Our findings indicate a preference for a curved spectrum, characterized by a log-parabola model with extra-galactic background light (EBL) model above 0.3 TeV at the 4σ level, compared to a power-law spectrum with EBL. We investigate the degeneracy between the absorption feature and the EBL normalization and derive upper limits on EBL models that are mainly sensitive in the wavelength range of 12.4 μm–40 μm. [ABSTRACT FROM AUTHOR]

We performed three-dimensional numerical simulations of magnetized relativistic jets propagating in a uniform density environment in order to study the effect of the entrainment and the consequent deceleration, extending a previous work in which magnetic effects were not present. As in previous papers, our aim is to understand the connection between the jet properties and the resulting Fanaroff-Riley classification. We considered jets with different low densities, and therefore low power, and different magnetizations. We find that lower magnetization jets effectively decelerate to sub-relativistic velocities and may then result in an FR I morphology on larger scales. Conversely, in the higher magnetization cases, the entrainment and consequent deceleration are substantially reduced. [ABSTRACT FROM AUTHOR]

Highly collimated relativistic jets are a defining feature of certain active galactic nuclei (AGN), yet their formation mechanism remains elusive. Previous observations and theoretical models have proposed that the ambient medium surrounding the jets could exert pressure, playing a crucial role in shaping the jets. However, a direct observational confirmation of such a medium has been lacking. In this study, we present very long baseline interferometric (VLBI) observations of 3C 84 (NGC 1275), located at the center of the Perseus Cluster. Through monitoring observations with the Very Long Baseline Array (VLBA) at 43 GHz, a jet knot was detected to have been ejected from the sub-parsec scale core in the late 2010s. Intriguingly, this knot propagated in a direction significantly offset from the parsec-scale jet direction. To delve deeper into the matter, we employed follow-up VLBA 43 GHz observations, tracing the knot's trajectory until the end of 2022. We discovered that the knot abruptly changed its trajectory in the early 2020s, realigning itself with the parsec-scale jet direction. Additionally, we present results from an observation of 3C 84 with the Global VLBI Alliance (GVA) at 22 GHz, conducted near the monitoring period. By jointly analyzing the GVA 22 GHz image with a VLBA 43 GHz image observed about one week apart, we generated a spectral index map, revealing an inverted spectrum region near the edge of the jet where the knot experienced deflection. These findings suggest the presence of a dense, cold ambient medium characterized by an electron density exceeding ∼105 cm−3, which guides the jet's propagation on parsec scales and significantly contributes to the overall shaping of the jet. [ABSTRACT FROM AUTHOR]

An increasing number of observations and simulations suggests that low-power (< 1044 erg s−1) jets may be a significant channel of feedback produced by active galactic nuclei (AGN), but little is known about their actual effect on their host galaxies from the observational point of view. We targeted four luminous type 2 AGN hosting moderately powerful radio emission (∼1044 erg s−1), two of which and possibly a third are associated with jets, with optical integral field spectroscopy observations from the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) to analyze the properties of their ionized gas as well as the properties and effects of ionized outflows. We combined these observations with Very Large Array (VLA) and e-MERLIN data to investigate the relations and interactions between the radio jets and host galaxies. We detected ionized outflows as traced by the fast bulk motion of the gas. The outflows extended over kiloparsec scales in the direction of the jet, when present. In the two sources with resolved radio jets, we detected a strong enhancement in the emission-line velocity dispersion (up to 1000 km s−1) perpendicular to the direction of the radio jets. We also found a correlation between the mass and the energetics of this high-velocity dispersion gas and the radio power, which supports the idea that the radio emission may cause the enhanced turbulence. This phenomenon, which is now being observed in an increasing number of objects, might represent an important channel for AGN feedback on galaxies [ABSTRACT FROM AUTHOR]

A sample of 17 Fanaroff–Riley type I (FR I) radio galaxies constructed from the 3CR catalogue, which is characterized by edge-darkened radio structures, is studied. The optical core luminosities derived from Hubble Space Telescope observation are used to estimate the Eddington ratios which are found to be below 10−3.4 for this sample. This is supported by the Baldwin–Phillips–Terlevich optical diagnostic diagrams derived with the spectroscopic observation of Telescopio Nazionale Galileo, suggesting that these sources are of low-ionization nuclear emission-line regions. It implies that the accretion in these FR I sources can be modelled as advection-dominated accretion flows (ADAFs). Given the low accretion rate, the predicted jet power with a fast-spinning black hole (BH) a  = 0.95 in the Blandford–Znajek mechanics is lower than the estimated one for almost all the sources in our sample. Such powerful jets indicate the presence of magnetically arrested discs (MAD) in the inner region of the ADAF, in the sense that the magnetic fields in the inner accretion zone are strong. Moreover, we show that, even in the MAD scenario, the BH spins in the sample are most likely moderate and/or fast with a ≳ 0.5. [ABSTRACT FROM AUTHOR]

The BL Lac object PKS 0735+178 was in its historic γ-ray brightness state during 2021 December. This period also coincides with the detection of a neutrino event IC 211208A, which was localized close to the vicinity of PKS 0735+178. We carried out detailed γ-ray timing and spectral analysis of the source in three epochs: (a) quiescent state (E 1), (b) moderate-activity state (E 2), and (c) high-activity state (E 3) coincident with the epoch of neutrino detection. During the epoch of neutrino detection (E 3), we found the largest variability amplitude of 95 per cent. The γ-ray spectra corresponding to these three epochs are well fit by the power-law model and the source is found to show spectral variations with a softer when brighter trend. In epoch E 3, we found the shortest flux doubling/halving time of 5.75 h. Even though the spectral energy distribution in the moderate-activity state and in the high-activity state could be modelled by the one-zone leptonic emission model, the spectral energy distribution in the quiescent state required an additional component of radiation over and above the leptonic component. Here, we show that a photomeson process was needed to explain the excess γ-ray emission in the hundreds of GeV that could not be accounted for by the synchrotron self-Compton process. [ABSTRACT FROM AUTHOR]

The issue of radiation mechanisms triggered in 1950–60s the first applications of plasma physics to understand the nature of radio galaxies. This interplay has steadily intensified during the past five decades due to the premise of in-situ acceleration of relativistic electrons occurring in the lobes of radio galaxies. This article briefly traces the chain of these remarkable developments, largely from an observational perspective. We recount several observational and theoretical milestones established along the way and the lessons drawn from them. We also present a new observational clue about in-situ acceleration of the relativistic particles radiating in the lobes of radio galaxies, gleaned by us from the very recently published sensitive radio observations of a tailed radio source in the galaxy cluster Abell 1033. [ABSTRACT FROM AUTHOR]

We investigate the nature of low-luminosity radio-loud active galactic nuclei (RLAGN) selected from the LOFAR Two-metre Sky Survey (LoTSS) first data release (DR1). Using optical, mid-infrared, and radio data, we have conservatively selected 55 radiative AGN candidates from DR1 within the redshift range 0.03 < z < 0.1. We show using high-frequency Karl G. Jansky Very Large Array (VLA) observations that 10 out of 55 objects show radio emission on scales >1–3 kpc, 42 are compact at the limiting resolution of 0.35 arcsec (taking an upper limit on the projected physical size, this corresponds to less than 1 kpc), and three are undetected. The extended objects display a wide range of radio morphologies: two-jet (5), one-jet (4), and double-lobed (1). We present the radio spectra of all detected radio sources which range from steep to flat/inverted and span the range seen for other compact radio sources such as compact symmetric objects (CSOs), compact steep spectrum (CSS) sources, and gigahertz peaked-spectrum (GPS) sources. Assuming synchrotron self-absorption (SSA) for flat/inverted radio spectrum sources, we predict small physical sizes for compact objects to range between 2 and 53 pc. Alternatively, using free–free absorption (FFA) models, we have estimated the free electron column depth for all compact objects, assuming a homogeneous absorber. We find that these objects do not occupy a special position on the power/linear size (P − D) diagram but some share a region with radio-quiet quasars (RQQs) and the so-called 'FR0' sources in terms of radio luminosity and linear size. [ABSTRACT FROM AUTHOR]

A minimalist approach to the linear stability problem in fluid dynamics is developed that ensures efficiency by utilizing only the essential elements required to find the eigenvalues for given boundary conditions. It is shown that the problem is equivalent to a single first-order ordinary differential equation, and that studying the argument of the unknown complex function in the eigenvalue space is sufficient to find the dispersion relation. The method is applied to a model for relativistic magnetized astrophysical jets. [ABSTRACT FROM AUTHOR]

We present six case studies from a comprehensive mass range (1-109 M⊙) of astrophysical objects, each of which possess jets, emit high-energy gamma radiation and in some instances spatially identifiable ultra-high-energy cosmic rays (UHECRs). All sources are strong candidates for UHECR emission, if not already known to emit them. We surmise that wakefield acceleration in conjunction with the magnetorotational instability of the accretion disc explains both structural properties of the jets and details in their emission signals, such as correlations in neutrino and gamma-ray bursts, and in the case of blazars, anticorrelations in flux and spectral index. Furthermore, our model predicts an upper bound for the energy of UHECRs emitted from a source given the mass of its central compact object and total jet luminosity. To provide context for our model predictions, we quantitatively compare them with observational data, however, we have not accounted for the GZK limit and assumed universal values for several model parameters (e.g. jet-spreading index, p) that likely differ between sources. Since the accretion and acceleration mechanisms are independent of mass, aside from determining maximum values, blazars (∼109 M⊙), radio galaxies (∼ 108 M⊙), Seyfert galaxies (∼ 106 M⊙ ), starburst galaxies (∼ 103 M⊙ ), even microquasars (1-10 M⊙) interestingly exhibit the same physics. Other radiation bands, such as X-ray, ultraviolet, or radio, may harbour additional information, but we chose not to focus on them for brevity. However, such an endeavour may open the door to a new multimessenger approach for understanding these objects.

This review explores the field of X-shaped radio galaxies (XRGs), a distinctive subset of winged radio sources that are identified by two pairs of jetted lobes which aligned by a significant angle, resulting in an inversion-symmetric structure. These lobes, encompassing active (primary) and passive (secondary) phases, exhibit a diverse range of properties across the multiple frequency bands, posing challenges in discerning their formation mechanism. The proposed mechanisms can broadly be categorized into those related either to a triaxial ambient medium, into which the jet propagates, or to a complex, central AGN mechanism, where the jet is generated. The observed characteristics of XRGs as discovered in the most substantial sample to date, challenge the idea that there is universal process at work that produces the individual sources of XRGs. Instead, the observational and numerical results rather imply the absence of an universal model and infer that distinct mechanisms may be at play for the specific sources. By scrutinizing salient and confounding properties, this review intends to propose the potential direction for future research to constrain and constrict individual models applicable to XRGs. [ABSTRACT FROM AUTHOR]

Previous studies on the fitting of spectral energy distributions (SEDs) often apply the external-Compton process to interpret the high-energy peak of low-synchrotron-peaked (LSP) BL Lac objects (LBLs), despite the lack of strong broad emission lines observed for LBLs. In this work, we collect quasi-simultaneous multiwavelength data of 15 LBLs from the Fermi fourth LAT AGN catalogue (4LAC). We propose an analytical method to assess the necessity of external photon fields in the framework of one-zone scenario. Following derived analytical results, we fit the SEDs of these LBLs with the conventional one-zone leptonic model and study their jet physical properties. Our main results can be summarized as follows. (1) We find that most LBLs cannot be fitted by the one-zone synchrotron-self-Compton (SSC) model. This indicates that external photons play a crucial role in the high-energy emission of LBLs, therefore we suggest that LBLs are masquerading BL Lacs. (2) We suggest that the γ-ray emitting regions of LBLs are located outside the broad-line region and within the dusty torus. (3) By extending the analytical method to all types of LSPs in Fermi -4LAC (using historical data), we find that the high-energy peaks of some flat spectrum radio quasars and blazar candidates of unknown types can be attributed to the SSC emission, implying that the importance of external photons could be minor. We suggest that the variability time-scale may help distinguish the origin of the high-energy peak. [ABSTRACT FROM AUTHOR]

The blazar sequence, including negative correlations between radiative luminosity L rad and synchrotron peak frequency ν, and between Compton dominance Y and ν, is widely adopted as a phenomenological description of spectral energy distributions (SEDs) of blazars, although its underlying cause is hotly debated. In particular, these correlations turn positive after correcting Doppler boosting effect. In this work, we revisit the phenomenological and intrinsic blazar sequence with three samples, which are historical sample (SEDs are built with historical data), quasi-simultaneous sample (SEDs are built with quasi-simultaneous data) and Doppler factor corrected sample (a sample with available Doppler factors), selected from literature. We find that phenomenological blazar sequence holds in historical sample, but does not exist in quasi-simultaneous sample, and intrinsic correlation between L rad and ν becomes positive in Doppler factor corrected sample. We also analyse if the blazar sequence still exists in subclasses of blazars, i.e. flat-spectrum radio quasars and BL Lacertae objects, with different values of Y. To interpret these correlations, we apply a simple scaling model, in which physical parameters of the dissipation region are connected to the location of the dissipation region. We find that the model generated results are highly sensitive to the chosen ranges and distributions of physical parameters. Therefore, we suggest that even though the simple scaling model can reproduce the blazar sequence under specific conditions that have been fine-tuned, such results may not have universal applicability. Further consideration of a more realistic emission model is expected. [ABSTRACT FROM AUTHOR]

ASASSN-20hx, a.k.a AT2020ohl, is an ambiguous nuclear transient, which was discovered in the nearby galaxy NGC6297 by the All-Sky Automated Survey for Supernovae. We have investigated the evolution of AT2020ohl using a multiwavelength data set to explain the geometry of the system and the energy radiated by it between X-ray and radio wavelengths. Our X-ray, UV/optical, and radio observations of the object jointly clarify the association of AT2020ohl with the nuclear activity of NGC6297. We detected radio counterpart of AT2020ohl 111 and 313 d after the discovery in Jansky Very Large Array X-band with flux densities 47 ± 14 and 34 ± 3 |$\mu$| Jy, respectively. Using multiwavelength data analysis, we nullify the possibility of associating any stellar disruption process with this event. We found some evidence showing that the host galaxy is a merger remnant, so the possibility of a binary supermassive black hole (SMBH) system cannot be ruled out. The central SMBH has a mass of ∼1.2 × 107 M⊙. We propose the accretion disc activity as the origin of AT2020ohl – it is either due to disc accretion event on to the central SMBH or due to the sudden accretion activity in a pre-existing accretion disc of the system during the interaction of two SMBHs which became gravitationally bound during a merger process. However, we also admit that with the existing data set, it is impossible to say definitively, among these two probabilities, which one is the origin of this nuclear transient. [ABSTRACT FROM AUTHOR]

Double lobe radio sources associated with active galactic nuclei represent one of the longest studied groups in radio astronomy. A particular subgroup of double radio sources comprises the compact symmetric objects (CSOs). CSOs are distinguished by their prominent double structure and subkpc total size. It has been argued that the vast majority of high-luminosity CSOs (CSO 2s) represent a distinct class of active galactic nuclei with its own morphological structure and lifecycle. In this work, we present theoretical considerations regarding CSO 2s. We develop a semi-analytic evolutionary model, inspired by the results of large-scale numerical simulations of relativistic jets, that reproduces the features of the radio source population. We show that CSO 2s may be generated by finite energy injections and propose stellar tidal disruption events as a possible cause. We find that tidal disruption events of giant branch stars with masses ≳1 M⊙ can fuel these sources and discuss possible approaches to confirming this hypothesis. We predict that if the tidal disruption scenario holds, CSO 2s with sizes less than 400 pc should outnumber larger sources by more than a factor of 10. Our results motivate future numerical studies to determine whether the scenarios we consider for fuelling and source evolution can explain the observed radio morphologies. Multiwavelength observational campaigns directed at these sources will also provide critical insight into the origins of these objects, their environments, and their lifespans. [ABSTRACT FROM AUTHOR]

In this work, we use hydrodynamical simulations to explore the effects of kinetic active galactic nuclei (AGN) jet feedback on the progression and outcome of the major merger of two isolated, gas-rich galaxies. We present simulations that use the moving-mesh code arepo to follow the progression of the merger through the first passage and up to the final coalescence, modelling the black holes at the centres of both of the merging galaxies using our prescription for black hole accretion via an α-disc and feedback in the form of a spin-driven jet. We find that the jets drive large-scale, multiphase outflows which launch large quantities of cold gas out to distances greater than 100 kpc and with velocities that reach |$\sim 2500 \, {\rm km \, s^{-1}}$|⁠. Gas in the outflows that decelerates, cools, and falls back on the galaxies can provide a rich source of fuel for the black hole, leading to intense episodes of jet activity in which the jet can become significantly misaligned. The presence of AGN jets affects the growth of the stellar component: star formation is moderately suppressed at all times during the merger and the peak of the star formation rate, attained during the final coalescence of the galaxies, is reduced by a factor of ∼2. Analysis of simulations such as these will play a central role in making precise predictions for multimessenger investigations of dual radio-AGN, which next-generation observational facilities such as LISA, Athena and SKA will make possible. [ABSTRACT FROM AUTHOR]

Blazars are a highly variable subclass of active galactic nuclei that have been observed to vary significantly during a single night. This intranight variability remains a debated phenomenon, with various mechanisms proposed to explain the behaviour including jet energy density evolution or system geometric changes. We present the results of an intranight optical monitoring campaign of four blazars: TXS 0506+056, OJ287, PKS 0735+178, and OJ248 using the Carlos Sánchez Telescope. We detect significant but colourless behaviour in OJ287 and both bluer- and redder-when-brighter colour trends in PKS 0735+178. Additionally, the g band shows a lag of |$\sim 10\, \mathrm{min}$| with respect to the r, i, z s bands for PKS 0735+178 on 2023 January 17. This unexpected hard lag in PKS 0735+178 is not in accordance with the standard synchrotron shock cooling model (which would predict a soft lag) and instead suggests the variability may be a result of changes in the jet's electron energy density distribution, with energy injection from Fermi acceleration processes into a post-shocked medium. [ABSTRACT FROM AUTHOR]

Variability studies of active galactic nuclei are a powerful diagnostic tool in understanding the physical processes occurring in disc-jet regions, unresolved by direct imaging with currently available techniques. Here, we report the first attempt to systematically characterize intra-night optical variability (INOV) for a sample of seven apparently radio-quiet narrow-line Seyfert 1 galaxies (RQNLSy1s) that had shown recurring flaring at 37 GHz in the radio observations at Metsähovi Radio Observatory, indicating the presence of relativistic jets in them, but no evidence for relativistic jets in the recent radio observations of Karl G. Jansky Very Large Array at 1.6, 5.2, and 9.0 GHz. We have conducted a total of 28 intra-night sessions, each lasting ≥3 h for this sample, resulting in an INOV duty cycle (⁠|$\overline{\mathrm{ DC}} ~\sim$| 20 per cent) similar to that reported for γ-ray-NLSy1s (DC ∼ 25 per cent–30 per cent), that display blazar-like INOV. This in turn infers the presence of relativistic jet in our sample sources. Thus, it appears that even lower mass (M BH ∼ 106 M⊙) RQNLSy1 galaxies can maintain blazar-like activities. However, we note that the magnetic reconnection in the magnetosphere of the black hole can also be a viable mechanism to give rise to the INOV from these sources. [ABSTRACT FROM AUTHOR]

The WAT (wide-angle-tail) radio galaxies ESO 137-006 and 3C338 show radio filaments including some extending between the two radio tails. If, in analogy to the Parker solar wind model, jets carry a powerful relativistic electric current, the continuity equation shows that slow return current responding to a temporal variation in jet power would inevitably lead to a localized short-lived charging-up, i.e. a temporary patch of electric field. Progenitors of the observed radio filaments are the relic jets left by repeated episodes of nuclear activity and pushed sideways by the ram pressure, though remaining connected to the broad radio lobes. The Debye shielding distance, for energetic particles with a p −2-distribution of momentum p , allows a very large Debye scale at the highest energies, approaching the radio galaxy scale, with a huge total energy content. Here, we propose that discharges dissipating the transient patches of electric fields through the filaments illuminate them. Our proposal is based on gradient drift currents within the bipolar jets, allowing for drift speeds approaching the speed of light. We sketch a scenario how a discharge current sometimes gets manifested as a radio filament connecting opposite lobes/tails of a WAT radio galaxy. [ABSTRACT FROM AUTHOR]

The core-shift method is a powerful method to estimate the physical parameters in relativistic jets from active galactic nuclei. The classical approach assumes a conical geometry of jets and a constant plasma speed. However, recent observations have shown that neither may hold close to the central engine, where the plasma in the jet is effectively accelerating and the jet geometry is quasi-parabolic. We modify the classical core-shift method to account for these jet properties. We show that the core-shift index may assume values in the range 0.8−1.2 or 0.53−0.8 depending on the jet geometry and viewing angle, and indices close to both values are indeed observed. We obtain expressions to estimate the jet magnetic field and the total magnetic flux in a jet. We show that the obtained magnetic field value can be easily recalculated down to the gravitational radius scale. For M87 and NGC 315, these values are in good agreement with those obtained by different methods. [ABSTRACT FROM AUTHOR]

Compact radio active galactic nuclei (AGNs) are thought to be young radio AGN at the early stage of AGN evolution, thus are ideal laboratory to study the high-energy emission throughout the evolution of radio AGN. In this work, we report for the first time the detection of the high-energy cutoff (E cut), a direct indicator of thermal coronal radiation, of X-ray emission in Mrk 348 (z  = 0.015), a young radio galaxy classified as compact symmetric object. With a 100 ks Nuclear Spectroscopic Telescope Array exposure, we find that the high-energy cutoff (E cut) is firmly detected (⁠|$218^{+124}_{-62}$| keV). Fitting with various Comptonization models indicates the presence of a hot corona with temperature kT e = 35–40 keV. These strongly support the corona origin for its hard X-ray emission. The comparison in the E cut – spectra index Γ plot of Mrk 348 with normal large-scale radio galaxies (mostly Fanaroff–Riley type II) yields no difference between them. This suggests the corona properties in radio sources may not evolve over time (i.e. from the infant stage to mature stage), which is to-be-confirmed with future sample studies of young radio AGNs. [ABSTRACT FROM AUTHOR]

We conducted multi-epoch, multifrequency parsec-scale studies on the gigahertz-peaked spectrum quasar PKS 0858 − 279 with the Very Long Baseline Array (VLBA). Our observations on 2005 November 26 elucidated a weak core, characterized by an inverted spectrum, and a distinctly bent jet that exhibited a notable bright feature in its Stokes I emission. Through comprehensive analysis of polarization and spectral data, we inferred the formation of a shock wave within this feature, stemming from interactions with a dense cloud in the ambient medium. In this paper, Very Long Baseline Interferometry- Gaia astrometry further reinforces the core identification. With a deep analysis of six additional VLBA epochs spanning from 2007 to 2018, we observed that while the quasar's parsec-scale structure remained largely consistent, there were discernible flux density changes. These variations strongly imply the recurrent ejection of plasma into the jet. Complementing our VLBA data, RATAN-600 observations of the integrated spectra suggested an interaction between standing and travelling shock waves in 2005. Moreover, our multi-epoch polarization analysis revealed a drastic drop in rotation measure values from 6000 to 1000 rad m−2 within a single year, attributable to diminishing magnetic fields and particle density in an external cloud. This change is likely instigated by a shock in the cloud, triggered by the cloud's interaction with the jet, subsequently prompting its expansion. Notably, we also observed a significant change in the magnetic field direction of the jet, from being perpendicular post its observed bend to being perpendicular prior to the bend – an alteration possibly induced by the dynamics of shock waves. [ABSTRACT FROM AUTHOR]

We present the study of multimessenger signatures of massive black hole (MBH) binaries residing in the centres of galaxy merger remnants. In particular, we first focus on the gravitational wave background (GWB) produced by an ensemble of MBH binary inspirals in the frequency range probed by the Pulsar Timing Array (PTA) experiments. The improved estimates of the characteristic strain were obtained with the inclusion of environmental effects on the MBH binary orbital decay within the galaxy merger remnants, added in post-processing to the semi-analytical model of galaxy formation and evolution SHARK. Secondly, we explore two, intriguing in terms of the MBH binary evolution studies, hypotheses aiming to explain the origins of X-shaped radio galaxies – a peculiar type of objects with double lobe structures, constituting approximately 6–10 per cent of known radio loud galaxies. The two considered scenarios involve either an abrupt change in the jet direction after an MBH merger (a spin-flip) or an unresolved close binary, where each of the two components produces a jet. We find that the estimated GWB amplitude at the reference frequency |$f_0=1 \, {\rm yr}^{-1}$| is in the range of |$A_{\rm { yr^{-1}}} = 1.20\times 10^{-15}{\!-\!}1.46\times 10^{-15}$|⁠ , which is 50 per cent lower than the strain of the signal detected by the PTA experiments. We also show that the spin-flip scenario considered in gas-poor mergers reproduces the observed properties of X-shaped radio galaxies well in terms of flip angle, redshift, and luminosity distributions. [ABSTRACT FROM AUTHOR]

The correlation between optical and γ-ray flux variations in blazars reveals a complex behaviour. In this study, we present our analysis of the connection between changes in optical and γ-ray emissions in the blazar Ton 599 over a span of approximately 15 yr, from 2008 August to 2023 March. Ton 599 reached its highest flux state across the entire electromagnetic spectrum during the second week of 2023 January. To investigate the connection between changes in optical and γ-ray flux, we have designated five specific time periods, labelled as epochs A, B, C, D, and E. During periods B, C, D, and E, the source exhibited optical flares, while it was in its quiescent state during period A. The γ-ray counterparts to these optical flares are present during periods B, C, and E; however, during period D, the γ-ray counterpart is either weak or absent. We conducted a broad-band spectral energy distribution (SED) fitting by employing a one-zone leptonic emission model for these epochs. The SED analysis unveiled that the optical–ultraviolet emission primarily emanated from the accretion disc in quiescent period A, whereas synchrotron radiation from the jet dominated during periods B, C, D, and E. Diverse correlated patterns in the variations of optical and γ-ray emissions, like correlated optical and γ-ray flares, could be accounted for by changes in factors such as the magnetic field, bulk Lorentz factor, and electron density. On the other hand, an orphan optical flare could result from increased magnetic field and bulk Lorentz factor. [ABSTRACT FROM AUTHOR]