MeerKAT

Abstract:

Abstract Radio continuum surveys provide samples of active galactic nuclei (AGN) and star forming galaxies (SFGs) to high redshifts, free of biases due to dust obscuration. However, radio detected sources require multi-wavelength counterparts to understand their intrinsic properties (e.g. redshift, stellar mass) and to study the evolution of star formation and AGN activity. In this work we present host galaxy counterparts for the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey continuum Data Release 1 in regions with the best ancillary data (totalling 7.5 sq. deg). We combine statistical cross-matching and visual inspection to identify Ks-band selected host galaxies, and additionally combine multiple radio components into single physical objects, where needed. This results in a combined radio catalogue of ~66 000 sources, with host counterparts and redshifts identified for ~95 per cent of sources in the COSMOS field, ~91 per cent in XMM-LSS and ~90 per cent in CDFS-DEEP. This includes a significant fraction of sources with spectroscopic redshifts within the COSMOS field (~50 per cent), with ~30 and ~20 per cent in the XMM-LSS and CDFS-DEEP fields respectively. Using the cross-matched catalogue, we make an initial identification of radio-excess and star forming galaxies based on comparisons of the radio luminosities to host star formation rates. Using this split as a proxy for radio loud AGN or SFGs, we present expectations for the redshift distributions of these sources, finding broad agreement with those from deep radio luminosity functions and simulated catalogues.

Abstract:

ABSTRACT Swift J1727.8–1613 is a newly discovered transient low-mass X-ray binary harbouring a stellar-mass ($\sim 10\,\mathrm{ M}_\odot$) black hole. We present state-resolved VLT/X-Shooter optical spectroscopy of its 2023 outburst, sampling the luminous hard-to-soft and late soft-to-hard transitions. During the onset of the brightest radio flare, He ii flux rises relative to adjacent epochs, with reduced peak-to-peak separation and full-width-half-maximum, consistent with enhanced irradiation shifting line emissivity to larger radii. We detect no contemporaneous change in the line base tracing the inner disc. The most dramatic change occurs at the onset of the dim-hard state, when strong, broad (higher-order) Balmer lines appear in absorption, and He ii remains in emission, but becomes highly asymmetric. While the hardening of the X-ray spectrum likely promotes the reappearance of an underlying disc photosphere, the kinematic alignment between the Balmer absorption ($v_w\sim -750\, \mathrm{km\, s^{-1}}$) and the suppressed blue peak of He ii suggests a unified origin in a massive, cool ($T\lesssim 10^{4}\, \mathrm{K}$) accretion disc wind. Radiative transfer simulations demonstrate that such asymmetric He ii profiles are naturally produced in a rotating and accelerating outflow. Using the Sobolev approximation, we estimate the wind mass-loss rate to be $\dot{M}_w\gtrsim 10^{-9}\, \mathrm{ M}_\odot \, \mathrm{yr^{-1}}$, comparable to the instantaneous accretion rate and a significant fraction of the secular mass-transfer rate from the donor. If persistent at quiescent-level X-ray luminosities, this outflow could strongly impact the system’s secular evolution.

Abstract:

Abstract Fast X-ray transients (FXTs) are brief, luminous bursts of soft X-ray emission whose physical origins remain uncertain. The Einstein Probe (EP) mission has recently enabled prompt discovery of these events, providing opportunities for rapid multi-wavelength follow-up. We present a coordinated radio observing campaign targeting 20 FXTs detected by the EP in 2024. The core consists of 59 epochs with the Allen Telescope Array (ATA), sampling post-burst timescales from ~1 to ~65 days and reaching typical 3σ sensitivities of 0.6–1.5 mJy across 1–8 GHz. We additionally incorporate higher-sensitivity observations from MeerKAT, the VLA, ATCA, eMERLIN, and AMI-LA. Two FXTs—EP240315a and EP241021a—have radio counterparts. For EP241021a, AMI-LA monitoring at 15.5 GHz reveals a light curve peaking at ~30 days with Fν ≈ 1.0 mJy; equipartition analysis implies a Newtonian equipartition energy of ~3 × 1049 erg and a mildly relativistic on-axis solution with Γon ≈ 1.3. Multi-frequency modelling of EP240315a, combining new VLA measurements with published MeerKAT, ATCA, and eMERLIN data, indicates highly relativistic early emission (Γ ≳ 3) that decelerates with time, consistent with jetted outflow. The detections and ATA non-detections span luminosities overlapping the brightest GRB and relativistic tidal disruption event (TDE) afterglows, suggesting FXTs comprise a heterogeneous population including both relativistic and non-relativistic explosions. Under a GRB-like redshift prior, our 3σ ATA limits correspond to observer-frame specific-luminosity thresholds of ~1032–1033 erg s−1 Hz−1; for nearby events (z ≲ 0.2), Lν ≲ 1030–31 erg s−1 Hz−1. Planned ATA upgrades and next-generation arrays (SKA, DSA, ngVLA) will enable sensitive, population-level radio studies of the FXT radio sky.