Professor Rob Fender

Abstract:

Abstract Multi-wavelength observations of low-mass X-ray binaries (LMXBs) during bright outbursts reveal many details about the coupling of their inflows and outflows. However, only high angular resolution radio observations are able to resolve and track the motion and variability of individual jet ejecta. We present the results of our intensive VLBI campaign on the black-hole low-mass X-ray binary (LMXB) Swift J1727.8-1613 during its 2023-2024 outburst. We observed the repeated quenching and re-establishment of the highly-extended continuous core jet during several transitions between hard-intermediate and soft-intermediate states, and the repeated ejection of transient jets. Using time-dependent visibility model fitting, we tracked the motion of nine discrete jet knots, obtaining some of the most precise measurements of transient jet proper motions and ejection dates in an LMXB. These ejecta were only detectable for a short time with VLBI, and some showed rapid intra-observation flux density variability that was not captured in image reconstructions. For the first time, we use time-dependent visibility modelling to fit a piecewise model for the jet knot flux densities, allowing us to create complex, non-parametric light curves of their intra-observation variability. We observed the launching of multiple ejecta across several state transitions, however, we could not identify a consistent signature of jet ejection in the available X-ray intensity or hardness data. We constrained the intrinsic speeds and bulk Lorentz factors of the jet knots, finding that Swift J1727.8-1613 launched both mildly relativistic (βΓ<1) and highly relativistic (βΓ>2) ejecta throughout its outburst. We used their proper motions to constrain a posterior distribution for the maximum inclination angle of the jet axis, which had 50th, 84th, and 99th percentiles of 40°, 50°, and 66°, respectively. These unique observations of the repeated ejection of transient jets by a single LMXB reveal that fixed parameters such as black-hole mass, black-hole spin, and spin-orbit misalignment do not uniquely determine the varying properties of transient jets, particularly their speeds and Lorentz factors.

Abstract:

Accretion and relativistic jet formation in black holes occur across a wide range of their masses, revealing qualitative and quantitative similarities that connect the entire population of black holes. In March 2026, researchers from all areas across the black hole mass spectrum met in Oxford to educate, explore and forge new research directions.

Abstract:

Abstract We report the discovery and characterisation of MKT J032848.4–271904.6, a new radio transient identified in the LADUMA field using SARAO Science Data Processor SDP UHF-band images from approximately one year of MeerKAT observations. Using the Transient Pipeline TraP and advanced filtering techniques, we identified a number of candidate variable radio sources in the field. All but one show variability consistent with refractive interstellar scintillation, leaving MKT J032848.4–271904.6 as the sole source exhibiting intrinsic variability. In addition, MKT J032848.4–271904.6 shows intrinsic variability at 0.816 GHz, with 13 radio detections across 41 epochs and a peak flux density of 1.041 ± 0.043 mJy. We associate this emission with a low-mass M-dwarf star LP 888–63, located 23 pc from the Sun and identified as a companion to the white dwarf binary system LAWD 14 WD 0326–273. LP 888–63 displays active flaring behaviour across the electromagnetic spectrum including multi-band optical data from MeerLICHT. TESS photometry reveals a periodic modulation in the blended light curve of 5.780 ± 0.507 days, consistent with rotational variability of a mid-M dwarf. Archival ESO spectra reveal Hα emission, confirming magnetic activity. These findings highlight the capability of MeerKAT’s SDP imaging and facilities like MeerLICHT for real-time detection and characterisation of stellar transients.