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The Rapid Burster is a unique neutron star low-mass X-ray binary system, showing both thermonuclear v-I and accretion-driven Type-II X-ray bursts. Recent studies have demonstrated how coordinated observations of X-ray and radio variability can constrain jet properties of accreting neutron stars – particularly when the X-ray variability is dominated by discrete changes. We present a simultaneous very large array, Swift , and INTErnational Gamma-Ray Astrophysics Laboratory observing campaign of the Rapid Burster to investigate whether its jet responds to Type-II bursts. We observe the radio counterpart of the X-ray binary at its faintest-detected radio luminosity, while the X-ray observations reveal prolific, fast X-ray bursting. A time-resolved analysis reveals that the radio counterpart varies significantly between observing scans, displaying a fractional variability of |$38 \pm 5$|  per cent. The radio faintness of the system prevents the robust identification of a causal relation between individual Type-II bursts and the evolution of the radio jet. However, based on a comparison of its low-radio luminosity with archival Rapid Burster observations and other accreting neutron stars, and on a qualitative assessment of the X-ray and radio light curves, we explore the presence of a tentative connection between bursts and jet: i.e. the Type-II bursts may weaken or strengthen the jet. The former of those two scenarios would fit with magnetorotational jet models; we discuss three lines of future research to establish this potential relation between Type-II bursts and jets more confidently. [ABSTRACT FROM AUTHOR]

We report on the SRG /eROSITA detection of ultra-soft (⁠|$kT=47^{+5}_{-5}$|  eV) X-ray emission (L X = |$2.5^{+0.6}_{-0.5} \times 10^{43}$|  erg s−1) from the tidal disruption event (TDE) candidate AT 2022dsb ∼14 d before peak optical brightness. As the optical luminosity increases after the eROSITA detection, then the 0.2–2 keV observed flux decays, decreasing by a factor of ∼39 over the 19 d after the initial X-ray detection. Multi-epoch optical spectroscopic follow-up observations reveal transient broad Balmer emission lines and a broad He  ii 4686 Å emission complex with respect to the pre-outburst spectrum. Despite the early drop in the observed X-ray flux, the He  ii 4686  Å complex is still detected for ∼40 d after the optical peak, suggesting the persistence of an obscured hard ionizing source in the system. Three outflow signatures are also detected at early times: (i) blueshifted H α emission lines in a pre-peak optical spectrum, (ii) transient radio emission, and (iii) blueshifted Ly α absorption lines. The joint evolution of this early-time X-ray emission, the He  ii 4686 Å complex, and these outflow signatures suggests that the X-ray emitting disc (formed promptly in this TDE) is still present after optical peak, but may have been enshrouded by optically thick debris, leading to the X-ray faintness in the months after the disruption. If the observed early-time properties in this TDE are not unique to this system, then other TDEs may also be X-ray bright at early times and become X-ray faint upon being veiled by debris launched shortly after the onset of circularization. [ABSTRACT FROM AUTHOR]

We present an extensive radio monitoring campaign of the nuclear transient eRASSt J234402.9−352640 with the Australia Telescope Compact Array, one of the most X-ray luminous TDE candidates discovered by the SRG/eROSITA all-sky survey. The observations reveal a radio flare lasting >1000 d, coincident with the X-ray, UV, optical, and infrared flare of this transient event. Through modelling of the 10 epochs of radio spectral observations obtained, we find that the radio emission is well-described by an expanding synchrotron emitting region, consisting of a single ejection of material launched coincident with the optical flare. We conclude that the radio flare properties of eRASSt J234402.9−352640 are consistent with the population of radio-emitting outflows launched by non-relativistic tidal disruption events, and that the flare is likely due to an outflow launched by a tidal disruption event (but could also be a due to a new AGN accretion event) in a previously turned-off AGN. [ABSTRACT FROM AUTHOR]

Here, we report on joint X-ray and radio monitoring of the neutron star low-mass X-ray binary SAX J1810.8−2609. Our monitoring covered the entirety of its |${\sim }\, 5\,$| month outburst in 2021, revealing a temporal correlation between its radio and X-ray luminosity and X-ray spectral properties consistent with a 'hard-only' outburst. During the outburst, the best-fitting radio position shows significant variability, suggesting emission from multiple locations on the sky. Furthermore, our 2023 follow-up observations revealed a persistent, unresolved, steep spectrum radio source |${\sim }\, 2\,$| yr after SAX J1810.8−2609 returned to X-ray quiescence. We investigated potential origins of the persistent emission, which included an unrelated background source, long-lasting jet ejection(s), and SAX J1810 as a transitional millisecond pulsar. While the chance coincidence probability is low (⁠|${\lesssim }\, 0.16~{{\ \rm per\ cent}}$|⁠), an unrelated background source remains the most likely scenario. SAX J1810.8−2609 goes into outburst every |${\sim }\, 5\,$| yr, so monitoring of the source during its next outburst at higher sensitivities and improved spatial resolutions (e.g. with the Karl G. Jansky Very Large Array or Square Kilometre Array) should be able to identify two components (if the persistent emission originates from a background source). If only one source is observed, this would be strong evidence that the persistent emission is local SAX J1810.8−2609, and future monitoring campaigns should focus on understanding the underlying physical mechanisms, as no neutron star X-ray binary has shown a persistent radio signal absent any simultaneous X-ray emission. [ABSTRACT FROM AUTHOR]

In this paper we significantly improve estimates of distance to the X-ray binary systems Cyg X-3 and GRS 1915+105. We report a highly accurate trigonometric parallax measurement for Cyg X-3 using the Very Long Baseline Array at 43 GHz, placing the source at a distance of 9.67 − 0.48 + 0.53 kpc. We also use Galactic proper motions and line-of-sight radial velocity measurements to determine three-dimensional (3D) kinematic distances to both systems, under the assumption that they have low peculiar velocities. This yields distances of 8.95 ± 0.96 kpc for Cyg X-3 and 9.4 ± 0.6 (statistical) ± 0.8 (systematic) for GRS 1915+105. The good agreement between parallax and 3D kinematic distances validates the assumption of low peculiar velocities, and hence small natal kicks, for both of the systems. For a source with a low peculiar velocity, given its parallax distance, Cyg X-3 should have a V LSR near −64 ± 5 km s−1. Our measurements imply a slightly higher inclination angle, and hence lower black hole mass, for GRS 1915+105 than found from previous work by Reid et al. and strengthen arguments from X-ray polarization that Cyg X-3 would be an ultraluminous X-ray source if viewed face-on. [ABSTRACT FROM AUTHOR]

Using multifrequency very long baseline interferometry (VLBI) observations, we probe the jet size in the optically thick hard state jets of two black hole X-ray binary (BHXRB) systems, MAXI J1820+070 and V404 Cygni. Because of optical depth effects, the phase-referenced VLBI core positions move along the jet axis of the BHXRB in a frequency-dependent manner. We use this 'core shift' to constrain the physical size of the hard state jet. We place an upper limit of 0.3 au on the jet size measured between the 15 and 5 GHz emission regions of the jet in MAXI J1820+070, and an upper limit of 1.0 au between the 8.4 and 4.8 GHz emission regions of V404 Cygni. Our limit on the jet size in MAXI J1820+070 observed in the low-hard state is a factor of 5 smaller than the values previously observed in the high-luminosity hard state (using time lags between multifrequency light curves), thus showing evidence of the BHXRB jet scaling in size with jet luminosity. We also investigate whether motion of the radio-emitting region along the jet axis could affect the measured VLBI parallaxes for the two systems, leading to a mild tension with the parallax measurements of Gaia. Having mitigated the impact of any motion along the jet axis in the measured astrometry, we find the previous VLBI parallax measurements of MAXI J1820+070 and V404 Cygni to be unaffected by jet motion. With a total time baseline of 8 yr, due to having incorporated 14 new epochs in addition to the previously published ones, our updated parallax measurement of V404 Cygni is 0.450 ± 0.018 mas (2.226 ± 0.091 kpc). [ABSTRACT FROM AUTHOR]

Neutron stars accreting from OB supergiants are often divided between persistently and transiently accreting systems, called supergiant X-ray binaries (SgXBs) and supergiant fast X-ray transients (SFXTs). This dichotomy in accretion behaviour is typically attributed to systematic differences in the massive stellar wind, binary orbit, or magnetic field configuration, but direct observational evidence for these hypotheses remains sparse. To investigate their stellar winds, we present the results of pilot 100-GHz observations of one SFXT and one SgXB with the Northern Extended Millimetre Array. The SFXT, IGR J18410-0535, is detected as a point source at 63.4 ± 9.6 μJy, while the SgXB, IGR J18410-0535 remains undetected. Radio observations of IGR J18410-0535 imply a flat or inverted low-frequency spectrum, arguing for wind emission and against non-thermal flaring. Due to the uncertain SFXT distance, however, the observations do not necessarily imply a difference between the wind properties of the SFXT and SgXB. We compare the mm constraints with other HMXBs and isolated OB supergiants, before considering how future mm campaigns can constrain HMXB wind properties by including X-ray measurements. Specifically, we discuss caveats and future steps to successfully measure wind mass-loss rates and velocities in HMXBs with coordinated mm, radio, and X-ray campaigns. [ABSTRACT FROM AUTHOR]

A tidal disruption event (TDE) occurs when a star is destroyed by a supermassive black hole. Broad-band radio spectral observations of TDEs trace the emission from any outflows or jets that are ejected from the vicinity of the supermassive black hole. However, radio detections of TDEs are rare, with <20 published to date, and only 11 with multi-epoch broad-band coverage. Here we present the radio detection of the TDE AT2020vwl and our subsequent radio monitoring campaign of the outflow that was produced, spanning 1.5 yr post-optical flare. We tracked the outflow evolution as it expanded between 1016 and 1017 cm from the supermassive black hole, deducing it was non-relativistic and launched quasi-simultaneously with the initial optical detection through modelling the evolving synchrotron spectra of the event. We deduce that the outflow is likely to have been launched by material ejected from stream-stream collisions (more likely), the unbound debris stream, or an accretion-induced wind or jet from the supermassive black hole (less likely). AT2020vwl joins a growing number of TDEs with well-characterized prompt radio emission, with future timely radio observations of TDEs required to fully understand the mechanism that produces this type of radio emission in TDEs. [ABSTRACT FROM AUTHOR]

Tracking the motions of transient jets launched by low-mass X-ray binaries (LMXBs) is critical for determining the moment of jet ejection, and identifying any corresponding signatures in the accretion flow. However, these jets are often highly variable and can travel across the resolution element of an image within a single observation, violating a fundamental assumption of aperture synthesis. We present a novel approach in which we directly fit a single time-dependent model to the full set of interferometer visibilities, where we explicitly parametrize the motion and flux density variability of the emission components, to minimize the number of free parameters in the fit, while leveraging information from the full observation. This technique allows us to detect and characterize faint, fast-moving sources, for which the standard time binning technique is inadequate. We validate our technique with synthetic observations, before applying it to three Very Long Baseline Array (VLBA) observations of the black hole candidate LMXB MAXI J1803−298 during its 2021 outburst. We measured the proper motion of a discrete jet component to be 1.37 ± 0.14 mas h−1, and thus we infer an ejection date of MJD |$59348.08_{-0.06}^{+0.05}$|⁠ , which occurs just after the peak of a radio flare observed by the Australia Telescope Compact Array (ATCA) and the Atacama Large Millimeter/Sub-Millimeter Array (ALMA), while MAXI J1803−298 was in the intermediate state. Further development of these new VLBI analysis techniques will lead to more precise measurements of jet ejection dates, which, combined with dense, simultaneous multiwavelength monitoring, will allow for clearer identification of jet ejection signatures in the accretion flow. [ABSTRACT FROM AUTHOR]

The coupling between radio and X-ray luminosity is an important diagnostic tool to study the connection between the accretion inflow and jet outflow for low-mass X-ray binaries (LMXBs). The radio/X-ray correlation for individual neutron star (NS) LMXBs is scattered, whereas for individual black hole (BH) LMXBs a more consistent correlation is generally found. Furthermore, jet quenching is observed for both types of LMXBs, but it is unclear whether jets in NS-LMXBs quench as strongly as those in BH-LMXBs. While additional soft X-ray spectral components can be detected in NS-LMXB spectra due to the presence of the NS surface, disentangling the individual X-ray spectral components has thus far not been considered when studying the radio/X-ray coupling. Here we present eleven epochs of Swift /XRT observations matched with quasi-simultaneous archival radio observations of the 2009 November outburst of Aql X-1. We decompose the thermal and Comptonized spectral components in the Swift /XRT spectra, with the aim of studying whether the presence of additional thermal emission affects the coupling of the radio/X-ray luminosity. We find that there is no evidence of a significant thermal contribution in Swift /XRT spectra that could cause scatter in the radio/X-ray coupling. To explore the role of potential spectral degeneracies in the X-ray models and consider the improvements from including hard X-rays, we perform joint fits with quasi-simultaneous RXTE /PCA spectra. Follow-up research using more sensitive, broad-band X-ray observations and densely sampled near-simultaneous radio observations is required to study this in more detail. [ABSTRACT FROM AUTHOR]

We present a high time resolution, multifrequency linear polarization analysis of very large array (VLA) radio observations during some of the brightest radio flaring (⁠|${\sim } 1 \,{\rm Jy}\,$|⁠) activity of the 2015 outburst of V404 Cygni. The VLA simultaneously captured the radio evolution in two bands (each with two 1 GHz base-bands), recorded at 5/7 GHz and 21/26 GHz , allowing for a broadband polarimetric analysis. Given the source's high flux densities, we were able to measure polarization on time-scales of |${\sim }13\,$| min, constituting one of the highest temporal resolution radio polarimetric studies of a black hole X-ray binary outburst to date. Across all base-bands, we detect variable, weakly linearly polarized emission (⁠|${\lt } 1{{ \rm per\ cent}}$|⁠) with a single, bright peak in the time-resolved polarization fraction, consistent with an origin in an evolving, dynamic jet component. We applied two independent polarimetric methods to extract the intrinsic electric vector position angles and rotation measures from the 5 and 7 GHz base-band data and detected a variable intrinsic polarization angle, indicative of a rapidly evolving local environment or a complex magnetic field geometry. Comparisons to the simultaneous, spatially-resolved observations taken with the Very Long Baseline Array at 15.6 GHz , do not show a significant connection between the jet ejections and the polarization state. [ABSTRACT FROM AUTHOR]

The ROSAT -selected tidal disruption event (TDE) candidate RX J133157.6−324319.7 (J1331) was detected in 1993 as a bright [0.2–2 keV flux of (1.0 ± 0.1) × 10−12 erg s−1 cm−2], ultra-soft (kT  = 0.11 ± 0.03 keV) X-ray flare from a quiescent galaxy (z  = 0.051 89). During its fifth all-sky survey (eRASS5) in 2022, Spectrum-Roentgen-Gamma (SRG)/ eROSITA detected the repeated flaring of J1331, where it had rebrightened to an observed 0.2–2 keV flux of (6.0 ± 0.7) × 10−13 erg s−1 cm−2, with spectral properties (kT  = 0.115 ± 0.007 keV) consistent with the ROSAT -observed flare ∼30 yr earlier. In this work, we report on X-ray, ultraviolet, optical, and radio observations of this system. During a pointed XMM observation ∼17 d after the eRASS5 detection, J1331 was not detected in the 0.2–2 keV band, constraining the 0.2–2 keV flux to have decayed by a factor of ≳40 over this period. Given the extremely low probability (∼5 × 10−6) of observing two independent full TDEs from the same galaxy over a 30 yr period, we consider the variability seen in J1331 to be likely caused by two partial TDEs involving a star on an elliptical orbit around a black hole. J1331-like flares show faster rise and decay time-scales [ |$\mathcal {O}(\mathrm{d})$| ] compared to standard TDE candidates, with negligible ongoing accretion at late times post-disruption between outbursts. [ABSTRACT FROM AUTHOR]

There is a great technological need for the development of devices capable of producing wide-angle and efficient, transmissive optical beam steering. In this article, variable pitch phase diffraction gratings using electroconvection in liquid crystals are considered for applications in electrically controllable beam-steerers. The devices contain thin layers of nematic liquid crystal mixtures of negative dielectric anisotropy, which are doped with different concentrations of an ionic species to determine the effect of conductivity on the beam-steering characteristics. The devices are seen to act as continuously variable diffraction gratings, where the pitch increases with the frequency of the applied electric field. The second order diffraction peak is found to be the most efficient, with absolute efficiency ranging from 5% to 15%, and continuous steering from 4° to 8° within 20 μm spaced devices. Devices with thinner layers of liquid crystal were also examined; these demonstrated wider angle steering but had substantial losses of efficiency due to lowering of optical contrast. Suggestions for possible methodologies for optimizing such devices are discussed. [ABSTRACT FROM AUTHOR]

We present a targeted search for low-frequency (144–215 MHz) fast radio burst (FRB) emission from five repeating FRBs using 23.3 h of archival data taken with the Murchison Widefield Array (MWA) voltage capture system (VCS) between 2014 September and 2020 May. This is the first time that the MWA VCS has been used to search for FRB signals from known repeaters, which enables much more sensitive FRB searches than previously performed with the standard MWA correlator mode. We performed a standard single-pulse search with a temporal and spectral resolution of |$400\, \mu$| s and 10 kHz, respectively, over a |$100\, \text{pc}\, \text{cm}^{-3}$| dispersion measure (DM) range centred at the known DM of each studied repeating FRB. No FRBs exceeding a 6σ threshold were detected. The fluence upper limits in the range of 32–1175 and 36–488 Jy ms derived from 10 observations of FRB 20190711A and four observations of FRB 20201124A, respectively, allow us to constrain the spectral indices of their bursts to ≳−1 if these two repeaters were active during the MWA observations. If free–free absorption is responsible for our non-detection, we can constrain the size of the absorbing medium in terms of the electron temperature T to |${\lt} 1.00\times (T/10^4\,\text{K})^{-1.35}\, \text{pc}$|⁠ , |${\lt} 0.92\times (T/10^4\,\text{K})^{-1.35}\, \text{pc}$|⁠ , and |${\lt} [0.22\!-\!2.50]\times (T/10^4\,\text{K})^{-1.35}\, \text{pc}$| for FRB 20190117A, FRB 20190711A, and FRB 20201124A, respectively. However, given that the activities of these repeaters are not well characterized, our non-detections could also suggest they were inactive during the MWA observations. [ABSTRACT FROM AUTHOR]

Tidal disruption events (TDEs) occur when a star passes too close to a supermassive black hole and is destroyed by tidal gravitational forces. Radio observations of TDEs trace synchrotron emission from outflowing material that may be ejected from the inner regions of the accretion flow around the supermassive black hole or by the tidal debris stream. Radio detections of TDEs are rare, but provide crucial information about the launching of jets and outflows from supermassive black holes and the circumnuclear environment in galaxies. Here, we present the radio detection of the TDE AT2020opy, including three epochs of radio observations taken with the Karl G. Jansky Very Large Array, MeerKAT, and upgraded Giant Metrewave Radio telescope. AT2020opy is the most distant thermal TDE with radio emission reported to date, and from modelling the evolving synchrotron spectra we deduce that the host galaxy has a more dense circumnuclear medium than other thermal TDEs detected in the radio band. Based on an equipartition analysis of the synchrotron spectral properties of the event, we conclude that the radio-emitting outflow was likely launched approximately at the time of, or just after, the initial optical flare. We find no evidence for relativistic motion of the outflow. The high luminosity of this event supports that a dense circumnuclear medium of the host galaxy produces brighter radio emission that rises to a peak more quickly than in galaxies with lower central densities. [ABSTRACT FROM AUTHOR]

We present analysis of comprehensive radio observations of the black hole V404 Cyg during its 2015 outburst. These data represent the best ever coverage of jet production and particle acceleration from any black hole. We report for the first time a clear and near-linear flux–rms correlation in the radio flux densities. Investigation of individual flares reveals in nearly all cases the peak corresponds to the transition from optically thick to thin to synchrotron emission, but an extended phase of particle acceleration is required in contrast to simple impulsive injection models. The largest radio flare is preceded by a phase of optical oscillations and followed one day later by a smaller but optically thin flare, likely due to ejecta interacting with the interstellar medium. Comparing the radio emission to contemporaneous X-ray and optical data, we find that the X-ray and radio measurements are correlated on all time-scales from seconds to one day. Correlation with the optical flux densities is weak at short time-scales, but becomes significant on time-scales greater than a few hours. We evaluate the physical conditions (size, magnetic field, and internal energy) associated with 86 individual radio flares, which in turn allows us to place a lower limit on the kinetic feedback over the 15 d of intense activity. If this energy was deposited locally to the source, as implied by the failure to detect jets on angular scales larger than milliarcsec, then we predict that a nova-like shell could have been formed. [ABSTRACT FROM AUTHOR]

We use the Very Long Baseline Array to conduct high precision astrometry of a sample of 33 compact, flat spectrum, variable radio sources in the direction of the Galactic plane (Becker et al. 2010). Although Becker et al. (2010) ruled out a few potential scenarios for the origin of the radio emission, the study could not rule out that these sources were black hole X-ray binaries (BHXBs). Most known BHXBs are first detected by X-ray or optical emission when they go into an outburst, leaving the larger quiescent BHXB population undiscovered. In this paper, we attempt to identify any Galactic sources amongst the Becker et al. (2010) sample by measuring their proper motions as a first step to finding quiescent BHXB candidates. Amongst the 33 targets, we could measure the proper motion of six sources. We find that G32.7193-0.6477 is a Galactic source and are able to constrain the parallax of this source with a 3σ significance. We found three strong Galactic candidates, G32.5898-0.4468, G29.1075-0.1546, and G31.1494-0.1727, based purely on their proper motions, and suggest that G29.1075-0.1546 is also likely Galactic. We detected two resolved targets for multiple epochs (G30.1038+0.3984 and G29.7161-0.3178). We find six targets are only detected in one epoch and have an extended structure. We cross-match our VLBA detections with the currently available optical, infrared, and X-ray surveys, and did not find any potential matches. We did not detect 19 targets in any VLBA epochs and suggest that this could be due to limited uv -coverage, drastic radio variability, or faint, extended nature of the sources. [ABSTRACT FROM AUTHOR]

We present radio [1.3 GHz MeerKAT, 4–8 GHz Karl G. Jansky Very Large Array (VLA), and 15.5 GHz Arcminute Microkelvin Imager Large Array (AMI-LA)] and X-ray (Swift and MAXI) data from the 2019 outburst of the candidate Black Hole X-ray Binary (BHXB) EXO 1846−031. We compute a Hardness–Intensity diagram, which shows the characteristic q-shaped hysteresis of BHXBs in outburst. EXO 1846−031 was monitored weekly with MeerKAT and approximately daily with AMI-LA. The VLA observations provide sub-arcsecond-resolution images at key points in the outburst, showing moving radio components. The radio and X-ray light curves broadly follow each other, showing a peak on ∼MJD 58702, followed by a short decline before a second peak between ∼MJD 58731–58739. We estimate the minimum energy of these radio flares from equipartition, calculating values of E min ∼ 4 × 1041 and 5 × 1042 erg, respectively. The exact date of the return to 'quiescence' is missed in the X-ray and radio observations, but we suggest that it likely occurred between MJD 58887 and 58905. From the Swift X-ray flux on MJD 58905 and assuming the soft-to-hard transition happened at 0.3–3 per cent Eddington, we calculate a distance range of 2.4–7.5 kpc. We computed the radio:X-ray plane for EXO 1846−031 in the 'hard' state, showing that it is most likely a 'radio-quiet' BH, preferentially at 4.5 kpc. Using this distance and a jet inclination angle of θ = 73°, the VLA data place limits on the intrinsic jet speed of βint = 0.29 c , indicating subluminal jet motion. [ABSTRACT FROM AUTHOR]

Strongly magnetized (B ≥ 1012 G) accreting neutron stars (NSs) are prime targets for studying the launching of jets by objects with a solid surface; while classical jet-launching models predict that such NSs cannot launch jets, recent observations and models argue otherwise. Transient Be/X-ray binaries (BeXRBs) are critical laboratories for probing this poorly explored parameter space for jet formation. Here, we present the coordinated monitoring campaigns of three BeXRBs across four outbursts: giant outbursts of SAX 2103.5+4545, 1A 0535+262, and GRO J1008–57, as well as a Type-I outburst of the latter. We obtain radio detections of 1A 0535+262 during ten out of twenty observations, while the other targets remained undetected at typical limits of 20–50  |$\mu$| Jy. The radio luminosity of 1A 0535+262 positively correlates with its evolving X-ray luminosity, and inhabits a region of the LX – LR plane continuing the correlation observed previously for the BeXRB Swift J0243.6+6124. We measure a BeXRB LX – LR coupling index of β  = 0.86 ± 0.06 (⁠|$L_R \propto L_X^\beta$|⁠), similar to the indices measured in NS and black hole low-mass X-ray binaries. Strikingly, the coupling's LR normalization is ∼275 and ∼6.2 × 103 times lower than in those two comparison samples, respectively. We conclude that jet emission likely dominates during the main peak of giant outbursts, but is only detectable for close-by or super-Eddington systems at current radio sensitivities. We discuss these results in the broader context of X-ray binary radio studies, concluding that our results suggest how supergiant X-ray binaries may host a currently unidentified additional radio emission mechanism. [ABSTRACT FROM AUTHOR]

Centaurus X–4 (Cen X–4) is a relatively nearby neutron star low-mass X-ray binary that showed outbursts in 1969 and 1979, but has not shown a full outburst since. Due to its proximity and sustained period of quiescence, it is a prime target to study the coupling between accretion and jet ejection in quiescent neutron star low-mass X-ray binaries. Here, we present four MeerKAT radio observations at 1.3 GHz of Cen X–4, combined with NICER and Swift X-ray monitoring. During the first and most sensitive observation, Cen X–4 was in a fully quiescent X-ray state. The three later and shorter observations targeted a brief period of faint X-ray activity in 2021 January, which has been referred to as a 'mis-fired' outburst. Cen X–4 is not detected in any of the four MeerKAT observations. We place these radio non-detections on the X-ray–radio luminosity diagram, improving the constraints on the correlation between the two luminosities from earlier quiescent radio studies. We confirm that Cen X–4 is radio fainter than the transitional millisecond pulsar PSR J1023+0038 at the same X-ray luminosity. We discuss the radio behaviour of accreting neutron stars at low X-ray luminosity more generally and finally comment on future observing campaigns. [ABSTRACT FROM AUTHOR]