Axions provide a natural and well-motivated dark matter candidate, with the capability to convert directly to photons in the presence of an electromagnetic field. A particularly compelling observational target is the conversion of dark matter axions into photons in the magnetospheres of highly magnetised neutron stars, which is expected to produce a narrow spectral peak centerd at the frequency of the axion mass. We point the MeerKAT radio telescope towards the isolated neutron star $\mathrm{J}0806.4-4123$ for 10-hours of observation and obtain the radio spectra in the frequency range 769–1051 MHz. By modeling the conversion process of infalling axion dark matter (DM), we then compare these spectra to theoretical expectations for a given choice of axion parameters. Whilest finding no signal above $5\sigma$ in the data, we provide a unique constraint on the Primakoff coupling of axion DM, $g_{\mathrm{a}\gamma \gamma }\lesssim 9.3\times {10}^{-12}{\mathrm{GeV}}^{-1}$ at the 95% confidence level, in the mass range $3.18–4.35\mu \mathrm{eV}$. This result serves the strongest constraint in the axion mass range $4.20–4.35\mu \mathrm{eV}$.

Measuring the proper motion of the emission component in radio-quiet quasars (RQQs) could help to distinguish between the origins of the radio emission and to understand whether the jet production mechanism is the same in radio-loud quasars (RLQs) and RQQs. PG 1351+640 is one of the few RQQs suitable for proper motion studies: it has two compact components on milli-arcsecond scales, a flat-spectrum core and a steep-spectrum jet; both components are ≳2 mJy at 5 GHz and are well suited for Very Long Baseline Array (VLBA) observations. We compare recent VLBA observations with that made seventeen years ago and find no significant change in the core-jet separation between 2005 and 2015 (a proper motion of 0.003 mas yr⁻¹). However, the core-jet separation increased significantly between 2015 and 2022, inferring a jet proper motion velocity of 0.063 mas yr⁻¹, which corresponds to an apparent transverse velocity of 0.37 c. The result suggests that the jet of the RQQ PG 1351+640 is mildly relativistic and oriented at a relatively small viewing angle.

Disk winds and jets are ubiquitous in active galactic nuclei (AGN), and how these two components interact remains an open question. We study the radio properties of the radio-intermediate quasar III Zw 2. We detect two jet knots, J1 and J2, on parsec scales that move at a mildly apparent superluminal speed of 1.35c. Two γ-ray flares were detected in III Zw 2 in 2009–2010, corresponding to the primary radio flare in late 2009 and the secondary radio flare in early 2010. The primary 2009 flare was found to be associated with the ejection of J2. The secondary 2010 flare occurred at a distance of ∼0.3 pc from the central engine, probably resulting from the collision of the jet with the accretion disk wind. The variability characteristics of III Zw 2 (periodic radio flares, unstable periodicity, multiple quasiperiodic signals and the possible harmonic relations between them) can be explained by the global instabilities of the accretion disk. These instabilities originating from the outer part of the warped disk propagate inward and can lead to modulation of the accretion rate and consequent jet ejection. At the same time, the wobbling of the outer disk may also lead to oscillations of the boundary between the disk wind and the jet tunnel, resulting in changes in the jet–wind collision site. Object III Zw 2 is one of the few cases observed with jet–wind interactions, and the study in this paper is of general interest for gaining insight into the dynamic processes in the nuclear regions of AGN.