MeerKAT

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.

Abstract:

Abstract Blue asymmetries (“Blueshifts”) in the C iv 1550Å emission line are common in luminous quasars. If they are formed in winds, how much energy, momentum and mass do those winds transport? We address this question by considering how much mass must be supplied through the line-forming region to maintain a given density and ionization state. Using a combination of 1D analytic and 2D numerical models, we find that for blueshifted C iv lines to form in a wind, the wind must have mass outflow rates of ~50fV times the accretion rate, where fV ≤ 1 is the volume filling factor accounting for clumping. Our results therefore disfavour line formation in a smooth disc wind and point towards one of two scenarios: either the wind is clumpy, with required clumping factors suggestively close to those in hot star winds; alternatively, if the mass is instead swept up from the ambient medium, the wind need not be clumpy and MHD and radiative winds can provide the original source of momentum and energy. The power of the outflow depends on the square of the terminal velocity of the flow, v∞. If the wind is also the BAL outflow, with v∞ ~ 10, 000 km s−1, the wind power is significant and important for feedback. There are various caveats which moderate our conclusions, motivating i) a better theoretical understanding of wind driving and clump formation physics and ii) improved observational constraints on the physical conditions where the lines are formed.

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.