We describe the latest iteration of upgrades (designated Phase III) to the Murchison Widefield Array (MWA), in the fourth paper in a series that covers the evolution of the telescope from design concept to initial operational facility, and through two major upgrades. As part of the Phase III upgrade of the MWA, we report the completion of work to design, build, and deploy a new fleet of digital receivers that further optimise the MWA for Epoch of Reionisation observations. These receivers complement existing receivers, such that the MWA now supports the full correlation of all 256 antenna tiles currently in the array. This step releases the MWA from the prior constraint of having to correlate only 128 of the 256 tiles at any given time, which means that the maximum instantaneous sensitivity of the MWA is doubled and the maximum number of interferometric baselines is approximately quadrupled. The upgrade is fundamentally enabled by the new MWAX correlator and various other improvements to the MWA sub-systems. In this paper we describe the new digital receivers and the other improvements that result in the Phase III system. A range of operational benefits arise from the upgrade and scientific flexibility is increased. We also comment on the transition from the MWA to the SKA-Low facility near the end of the decade, including a description of some unique science opportunities utilising joint MWA/SKA-Low data during the Science Verification phase of the SKA-Low Array Assembly 2 (AA2) period.

We present a comprehensive study of the faint counterjet in the radio galaxy 3C 84 using 15 years of 15 and 43 GHz data observed with the Very Long Baseline Array between 2009 and 2024. High-dynamic-range imaging reveals profound morphological and brightness asymmetry between the prominent approaching jet and the highly attenuated receding jet. The primary finding is that the counterjet provides a clean probe of subparsec external free-free absorption (FFA). The northern counterjet components N1 and N2 show strongly inverted 15-43 GHz spectra, whereas the approaching hotspot C3 is persistently optically thin, requiring a frequency-dependent absorbing screen in front of the receding flow. Taking the spectral index of alpha = -0.82 from C3, we infer that the FFA opacity tau 15 GHz changed from similar to 3.0 to similar to 1.9 for N1 and from similar to 3.7 to similar to 2.4 for N2, corresponding to emission measures of (1.7-3.3) & times; 109 pc cm-6 and electron densities of 104-105 cm-3 for plausible path lengths. Distinct opacity histories of N1 and N2 indicate a radially and azimuthally structured absorber, consistent with the ionized inner skin of a clumpy torus or torus-associated wind. Proper motions of C3, N1, and N2 imply a characteristic parsec-scale viewing angle of order 20 degrees and a pattern speed of similar to 0.45-0.52c under an assumption of bilateral symmetry. Evidence for 7-11 yr transverse quasi-periodic modulation is present, but remains tentative given the limited temporal baseline and possible core-reference systematics. The counterjet of 3C 84 therefore acts as a time-dependent tomographic probe of the environment of the inner active galactic nuclei and provides a useful template for linking jet propagation to circumnuclear gas on subparsec scales.

Context. Intermittent jet activity of active galactic nuclei (AGNs) is a common phenomenon, whereas significant jet reorientation during episodic jet activity in relatively young radio galaxies is rarely reported. The quasar 0954+556 at z = 0.903 is an intriguing source exhibiting an unusual radio jet structure with significantly different jet directions at kiloparsec (kpc) and parsec (pc) scales. At kpc scales, images from the Very Large Array (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 Very Long Baseline Array (VLBA) show a two-component structure with a projected separation of ∼360 pc in the north–south direction. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.

When a radio jet is partially optically thick in the launching region, its apparent compact core may display frequency-dependent positional shifts. High-precision astrometric measurements of core shifts enable astronomers to pinpoint the jet's origin and place tight constraints on the magnetic field. BL Lacertae, the archetypal BL Lac object, hosts a highly variable and well-collimated jet. To independently constrain its innermost core shifts, we conducted very long baseline interferometric (VLBI) observations at 8.4, 12.4, 15.2, 23.6, and 43.2 GHz. By exploiting a nearby (13.' 3) steep-spectrum calibrator (NVSS J220340+420839) through inverse phase-referencing VLBI astrometry, we detect nearly unbiased two-dimensional core shift measurements with state-of-the-art precisions of 5-30 mu as, which are significant at >3 sigma confidence. The core shift between 8.4 and 43.2 GHz reaches 250 mu as. The apparent core shifts scale with frequency as nu-1/k(r), implying the existence of an optically thick region in the upstream of the jet. The derived core-shift index, k(r)=1.18(-0.34)(+0.59), is consistent, within uncertainties, with the canonical k(r) = 1 expected under energy equipartition between the jet particle and magnetic field energy densities, while allowing for modest deviations given that BL Lacertae was captured in a flaring state.

Space very long baseline interferometry (Space VLBI) employs antennas on space-based platforms, such as satellites, to extend its baseline beyond the Earth’s atmosphere. This significantly increases the effective baseline length and dramatically enhances the angular resolution of radio astronomical observations. As a key component of the fourth phase of China’s Lunar Exploration Program (CLEP), the Queqiao-2 relay satellite carries the Lunar Orbital VLBI Experiment (LOVEX) payload, which utilizes a 4.2-m reflector antenna. This initiative aims to establish a VLBI station in lunar orbit to perform joint observations with ground-based telescopes. The LOVEX payload integrates a VLBI feed assembly, an X-band cryogenic receiver, a cryocooler control unit, a frequency conversion and data acquisition backend, and a passive hydrogen maser. It has a total mass of 32.777 kg and an average power consumption of 220 W under nominal conditions. The payload supports simultaneous reception of X-band left- and right-hand circularly polarized (LCP and RCP) signals. The complete system noise temperature, incorporating contributions from the cosmic background, feed horn, cryogenic receiver, and other components, is maintained below 100 K. Furthermore, the antenna efficiency of the system exceeds 35

We observed a newly-discovered Galactic black hole X-ray binary Swift J1727.8$-$1613 with the European Very Long Baseline Interferometry Network (EVN) at 5 GHz. The observation was conducted immediately following a radio quenching event detected by the Karl G. Jansky Very Large Array (VLA). The visibility amplitude evolution over time reveals a large-amplitude radio flare and is consistent with an ejection event. The data can be interpreted either as a stationary component (i.e., the radio core) and a moving blob, or as two blobs moving away from the core symmetrically in opposite directions. The initial angular separation speed of the two components was estimated to 30 mas d^{-1}. We respectively fitted a single circular Gaussian model component to each of 14 sliced visibility datasets. For the case of including only European baselines, during the final hour of the EVN observation, the fitted sizes exhibited linear expansion, indicating that the measured sizes were dominated by the angular separation of the two components. The 6-h EVN observation took place in a rising phase of an even larger 4-day-long radio flare, implying that the ejection events were quite frequent and therefore continuous radio monitoring is necessary to correctly estimate the power of the transient jet. Combined with X-ray monitoring data, the radio quenching and subsequent flares/ejections were likely driven by instabilities in the inner hot accretion disk.

The Lunar Orbital VLBI Experiment (LOVEX) is a scientific component of the Chinese Lunar Exploration Project (CLEP) Chang’E-7. The spaceborne component of LOVEX is implemented onboard the relay satellite QueQiao-2, which was launched on 20 March 2024, and later placed into an elliptical selenocentric orbit. The LOVEX-specific payload consists of an X-band cryogenic receiver, a hydrogen maser frequency standard, and VLBI data formatting and acquisition electronics. Several components of the QueQiao-2 nominal onboard instrumentation, such as the 4.2-m antenna, the data storage device, and the downlink communication system, contribute to the overall spaceborne VLBI instrumentation. This allows us to form a space radio telescope capable of co-observing with Earth-based radio telescopes in VLBI mode. In this space VLBI system, the length of the baseline extends up to approximately 380000 km. This paper presents the LOVEX scientific objectives, architecture, instrumentation, prelaunch tests, in-flight verification and calibration, and the first in-flight detections of interferometric response (“fringes”) achieved through observations of the quasar AO 0235+164 and the Chang’E-6 orbital module, positioned at the Sun-Earth Lagrange point L2. These initial results demonstrate the successful performance of LOVEX, verifying its capability for both astronomical and spacecraft tracking observations at ultra-long VLBI baselines.

We present an investigation of the compact structure of the active galactic nucleus 2021+317 based on multiepoch very long baseline interferometry (VLBI) observations at 15, 22, and 43 GHz in the period from 2013 through 2024. The VLBI images show a core–jet structure extended to the south, with two stationary components in the northern region, one of which is likely to be the core of the source. We also detected two new moving jet components (S4 and S5) in the observations of 2021. Based on these observational findings, we analyzed two distinctive jet models involving one or another stationary component mentioned above as the jet core. One model assumes a moderate bulk motion velocity, a wider viewing angle, and a lower Doppler factor, with the magnetic field energy density significantly dominating over the nonthermal particle energy density. The other model involves a higher bulk motion velocity, a narrower viewing angle, and a higher Doppler factor, with an even greater dominance of magnetic field energy in the core. The position angle of the jet ridgeline rotates counterclockwise over the observed period. The apparent kinematics of the jet components is more consistent with a model of the precessing jet, which has recently completed the first half of the precession cycle. Our results provide constraints on the dynamic evolution of the jet and its interaction with the surrounding medium.

Understanding the formation mechanisms of stellar-mass black hole X-ray binaries (BHXBs) remains a fundamental challenge in astrophysics. The natal kick velocities imparted during black hole (BH) formation provide crucial constraints on these formation channels. In this work, we present a new-epoch very long baseline interferometry (VLBI) observation of the Galactic BHXB AT2019wey carried out in 2023. Combining with archival VLBI data from 2020, we successfully measure the proper motion of AT2019wey over a 3 yr timescale, namely, 0.78 ± 0.12 mas yr ^−1 in R.A. and −0.42 ± 0.07 mas yr ^−1 in decl. Employing the measured proper motion, we estimate its peculiar velocity and the potential kick velocity (PKV) through Monte Carlo simulations, incorporating uncertainties of its distance and radial velocity. The estimated PKV distributions and height above the Galactic plane suggest that AT2019wey’s BH likely formed through a supernova explosion rather than direct collapse.

We conducted a detailed analysis of the jet structure and dynamics of the source 0241+622 on milliarcsecond (mas) scales. We stacked images from multiple epochs to better recover the crosssection of the jet. By analyzing the relationship between jet width and distance, we observed that the jet exhibits a parabolic shape from the core, spanning a region from 0.12 to 6.1 mas. This structure suggests the acceleration and collimation processes of the jet. Beyond 18 mas from the core, the jet adopts a conical shape, and the expansion speed of the jet becomes faster within the range from 4500 to 6500 mas. We obtained the core shift of this source using five pairs of data from VLBA at 1.6 GHz to 43 GHz. Based on previous studies, through proper motion analysis of the jet components, we estimated the angle between the jet and the line of sight to be approximately 65.7°, so 1 mas corresponds to 0.95 pc (de-projected distance). We then obtained the velocity field of the source within 3.14 mas from the central black hole and found that the jet exhibits accelerated motion within this range. At approximately 6.1 mas from the core, we observed that the jet width begins to decrease, which we identified as possibly corresponding to the Bondi radius of this source. The reduction in jet width may be related to changes in the external environmental pressure, particularly within the Bondi radius, indicating that the jet dynamics and collimation characteristics are strongly influenced by the surrounding medium conditions.

The Queqiao-2 relay satellite, launched in March 2024, is equipped with an interferometric antenna with a diameter of 4.2 m, presenting an excellent opportunity to extend the baseline length of VLBI. By integrating the lunar orbit VLBI antenna with the ground VLBI antenna, the baseline length can be extended to approximately 380000 km, approximately 100 times longer than the ground baseline length, thereby further enhancing the orbit determination accuracy. This article derives a model for measuring the time delay of the Earth-Moon VLBI within the framework of relativity theory. It analyzes the orbit determination accuracy of relay satellite through simulations and real data. Furthermore, it explores the application of Earth-Moon baseline VLBI data in precisely determining the orbit of deep space probes, such as Mars, through simulation analysis. The analysis reveals that the orbit determination accuracy of the relay satellite in a 24-h elliptical lunar orbit is approximately 30 m. Compared with traditional ground-based ranging and VLBI data orbit determination method, incorporating 2 h of daily Earth-Moon VLBI delay data can enhance the orbit determination accuracy of Mars probes from 27 to 12 km prior to Mars capture. Additionally, the orbit determination accuracy of asteroid probes during transfer orbit is improved from 10 to 6 km. The simulation analysis results confirm the potential of Earth-Moon VLBI technology in enhancing the orbit determination accuracy of deep space probes.

We observed a newly discovered Galactic black hole X-ray binary Swift J1727.8–1613 with the European VLBI Network (EVN) at 5 GHz. The observation was conducted immediately following a radio quenching event detected by the Karl G. Jansky Very Large Array. The visibility amplitude evolution over time reveals a large-amplitude radio flare and is consistent with an ejection event. The data can be interpreted either as a stationary component (i.e., the radio core) and a moving blob, or as two blobs moving away from the core symmetrically in opposite directions. The initial angular separation speed of the two components was estimated to 30 mas day −1 . We respectively fitted a single circular Gaussian model component to each of 14 sliced visibility data sets. For the case of including only European baselines, during the final hour of the EVN observation, the fitted sizes exhibited linear expansion, indicating that the measured sizes were dominated by the angular separation of the two components. The 6 hr EVN observation took place in a rising phase of an even larger 4 day long radio flare, implying that the ejection events were quite frequent and therefore continuous radio monitoring is necessary to correctly estimate the power of the transient jet. Combined with X-ray monitoring data, the radio quenching and subsequent flares/ejections were likely driven by instabilities in the inner hot accretion disk.

The Lunar Orbital VLBI Experiment (LOVEX) aims to utilize the 4.2-m diameter antenna on the Queqiao-2 relay satellite of the Chang’E-7 mission, which is equipped with an X-band cryogenic receiver, an H-maser, and a VLBI data acquisition backend, thereby forming a space radio telescope in lunar orbit. The lunar orbital telescope will collaborate with Earth-based telescopes to conduct VLBI observations, thus forming a lunar-Earth space VLBI network. The length of the baseline will extend up to approximately 380000 km, which will be the longest VLBI baseline to date. This paper introduces the LOVEX VLBI data correlation system. The system is capable of generating VLBI delay models for both the lunar orbital and Earth-based telescopes; it also performs the initial clock search within a large clock offset window ranging from −10 ms to +10 ms for the lunar-Earth baseline leveraging both wideband blazar signals and spacecraft (SC) differential one-way ranging (DOR) signals—a distinctive feature of the system. To fulfill the scientific objectives of astrophysics, astrometry, and orbit determination of SC, the data correlation system outputs visibilities in various formats. Additionally, the system can directly output the VLBI residual delay and delay rate of SC after bandwidth synthesis. Anticipating the inclusion of more Earth-based stations in the future, the system is designed with the capability to correlate digital signals observed by 10 stations, with a bandwidth of 512 MHz and dual-polarization, running on an off-the-shelf central processing unit (CPU) + graphics processing unit (GPU) cluster. This system was applied to both the ground verification system and the first LOVEX observation experiment, and successfully detected VLBI fringes for signals from blazars and SC after fringe fitting.

We present an investigation of the compact structure of the AGN 2021+317 based on multi-epoch Very Long Baseline Interferometry (VLBI) observations at 15, 22, and 43 GHz in the period from 2013 through 2024. The VLBI images show a core-jet structure extended to the south, with two stationary components in the northern region, one of which likely to be the core of the source. We also detected two new moving jet components (S4 and S5) in the observations of 2021. Based on these observational findings, we analyzed two distinctive jet models, involving one or another stationary component mentioned above as the jet core. One model assumes a moderate bulk motion velocity, a wider viewing angle, and a lower Doppler factor, with the magnetic field energy density significantly dominating over non-thermal particle energy density. The other model involves a higher bulk motion velocity, a narrower viewing angle, and a higher Doppler factor, with an even greater dominance of magnetic field energy in the core. The position angle of the jet ridge line rotates counter-clockwise over the observed period. The apparent kinematics of the jet components is more consistent with a model of the precessing jet, which has recently completed the first half of the precession cycle. Our results provide constraints on the dynamic evolution of the jet and its interaction with the surrounding medium.

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.

Accretion of black holes at near-Eddington or super-Eddington rates represents the most powerful episode driving black hole growth, potentially occurring across various types of objects. However, the physics governing accretion and jet–disk coupling in such states remains unclear, primarily due to the difficulty in detecting associated jets, which may emit extremely weakly or exhibit episodic behavior. Only a few near/super-Eddington systems have demonstrated radio activity, and it remains uncertain whether jets exist and what their properties are in super-Eddington active galactic nuclei (AGNs) and ultraluminous X-ray sources. This uncertainty stems mainly from the complex radio emission mix, which includes contributions from jets, star formation activity, photoionized gas, accretion disk wind, and coronal activity. In this work, we conducted high-resolution, very long baseline interferometry observations to investigate jets in the highly accreting narrow-line Seyfert I system I Zw 1. Our observations successfully revealed small-scale jets (with a linear size of ∼45 pc) at both 1.5 and 5 GHz, based on the high radio brightness temperature, radio morphology, and spectral index distribution. Additionally, the parsec-scale jet observed in I Zw 1 displays a knotted morphology reminiscent of other sources accreting at similar rates. In summary, the high accretion rates and jet properties observed in the AGN I Zw 1 may support the AGN/X-ray binary analogy in this extreme state.

We present a multifrequency polarimetric study for the quasar 1604+159. The source was observed at the L band with the American Very Long Baseline Array and the L, X, and U bands with the Very Large Array. These observations provide different resolutions from mas to arcsec, enabling us to probe the morphology and magnetic field from tens of parsec to hundreds of kiloparsec scale. We detect a symmetrical Fanaroff-Riley Class I-like structure. The source has several lobes and bulges, forming a cocoon shape. The polarization is normal to the edges of the structure with high fractional polarization up to similar to 60%. Two hotspots are observed at the eastern and western sides of the source, located symmetrically relative to the core. The flux density ratio (>1.5) between the two hotspots suggests the Doppler beaming effect exists at a large scale. The polarized emission in the hotspots also shows a symmetrical structure with an oblique direction from the jet direction. In general, the jet propagates in a collimating structure with several bends. Polarization is also detected perpendicular to the local jet from similar to 100 mas to similar to 1 ''. The jet shows strong polarized intensity and high fractional polarization at the bending edges. We discuss the possible origins of the observed structure and magnetic field.

We have collected and analyzed the limited the Very Large Array(VLA)archival data of J1458+4121,which is a possible candidate of neutrino event IC-220624A,based on which we observed the source first time with the Very Long Baseline Array(VLBA)at L and C bands.Through data processing and analysis,we obtained the VLBA images of J1458+4121.In addition,its more precise coordinate(α=14∶58∶20.772,δ=41∶21∶01.911)was obtained by phase reference technique.The total flux density observed by VLA decreases from 1.4 to 8.4 GHz,so its radio spectrum may be power-law.However,the total flux density observed by VLBA shows an increasing trend from 1.5 to 5 GHz,which means an inversted spectrum in the GHz band.Therefore,it is highly likely that J1458+4121 is a young radio source,and its neutrino production may be caused by the emergence of a new jet component.The results of this paper are beneficial to the follow-up study of J1458+4121,and extend the focus on neutrino origin from bright blazars to other types of active galactic nuclei.

M60, an elliptical galaxy located 16.5 Mpc away, has an active nucleus with a very low luminosity and an extremely low accretion rate. Its central supermassive black hole (SMBH) has a mass of M _BH ∼ 4.5 × 10 ^9 M _⊙ and a Schwarzschild radius corresponding to R _S ∼ 5.4 μ as. To investigate the nature of its innermost radio nucleus, data from the Very Long Baseline Array (VLBA) at 4.4 and 7.6 GHz were reduced. The VLBA images reveal a compact component with total flux densities of ∼20 mJy at both frequencies, a size of ≤0.27 mas (99.7% confidence level), about 0.022 pc (50 R _S ) at 7.6 GHz, and a brightness temperature of ≥6 × 10 ^9 K. This suggests that the observed centiparsec-scale compact core could be attributed to a nonthermal jet base or an advection-dominated accretion flow (ADAF) with nonthermal electrons. The extremely compact structure also supports the presence of an SMBH in the center. Our results indicate that M60 is a promising target for broadband very long baseline interferometry observations at millimeter wavelengths to probe ADAF scenarios and tightly constrain the potential photon ring (about 28 μ as) around its SMBH.