Hintze Centre for Astrophysical Surveys

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 Remnant radio galaxies, whose lobes are no longer replenished by jets from the active galactic nucleus (AGN), offer key constraints on AGN duty cycles and the timescales of radio jets. We present a spectral-ageing study of 14 candidate remnant radio galaxies in the XMM−LSS field, combining new broad-band data from the MeerKAT MIGHTEE (L-band) and uGMRT superMIGHTEE (band-3 and band-4) surveys with complementary observations from LOFAR, GMRT, and JVLA, covering 144 MHz−1.5 GHz. Spectral modeling confirms 12 sources as genuine remnants, while two are reclassified as active, emphasising the importance of sensitive, multi-frequency coverage for robust remnant identification. Pixel-based spectral age maps yield results (~3-43 Myr) broadly consistent with integrated estimates, revealing relatively short spectral ages (~8-42 Myr). These ages likely reflect enhanced inverse-Compton losses at higher redshifts (0.35 ≤ z ≤ 2.85; median z = 1.25) and possible rapid lobe expansion in low-density environments. The ratios of remnant to total source ages (tOFF/ts) span 0.04-0.83, indicating that the sample traces a broad range of evolutionary stages. Our findings reveal a previously underrepresented population of faint, rapidly fading remnants, suggesting that the remnant phase may be shorter and more dynamic than previously thought. This study highlights the crucial role of MIGHTEE and superMIGHTEE surveys in reliably classifying genuine remnants and provides a framework for constraining AGN life cycles in preparation for forthcoming SKA surveys.

Abstract:

We report multiwavelength studies of the transient EP250916a, detected by the Einstein Probe on 2025 September 16. Located at low Galactic latitude, the source exhibited a rapid X-ray brightening, reaching an unabsorbed 0.5–10 keV flux of (6.4 ± 0.1) × 10−10 erg cm−2 s−1, followed by a plateau and a two-stage decay lasting over 40 days. Swift/X-Ray Telescope (XRT) monitoring shows a persistently hard spectrum (Γ ≈ 1.6–2.2) with only modest softening during decay, while a Nuclear Spectroscopic Telescope Array (NuSTAR) observation confirms a hard-state continuum extending up to 70 keV. Timing analysis of XMM-Newton data reveals a weak quasiperiodic oscillation (QPO) at ∼13 Hz. No other coherent pulsations or thermonuclear bursts are detected. Broadband spectral modeling favors a nonthermal power-law continuum with partial-covering absorption and shows no significant thermal disk component. Optical imaging obtained with Nordic Optical Telescope, Las Campanas Observatory, and GaiaDR3 identifies two faint sources within the 2″ Swift/XRT positional uncertainty. A MeerKAT observation at 1.28 GHz yielded no radio counterpart, with a 3σ upper limit of 60 μJy beam−1. The combination of a long-lasting outburst, a hard nonthermal X-ray spectrum, a weak QPO detection, the absence of coherent timing features, and faint potential optical counterparts disfavors a stellar-flare or extragalactic origin and supports an accreting compact-object scenario. Comparisons with similar faint, hard-state transients place EP250916a within a growing population of low-luminosity, hard-state black hole X-ray binary candidates.

Abstract:

Abstract Tidal disruption event (TDE) light curves are increasingly used to infer the masses of quiescent supermassive black holes (MBH), offering a powerful probe of low-mass black hole demographics independent of host-galaxy scaling relations. However, most semi-analytic TDE models invoke assumptions aligned with a full stellar disruption, despite theoretical expectations that partial disruptions dominate the TDE population. In this work we test the robustness of current TDE models using three repeating partial TDEs (rpTDEs), in which the multiple flares produced by the same surviving stellar core must yield consistent black hole masses. We present spectroscopic observations establishing AT 2023adr as a rpTDE, making it the third such spectroscopically confirmed event. We independently model the flares of the three rpTDEs; 2020vdq, 2022dbl, and 2023adr, applying fallback-accretion fits, stream–stream collision scaling relations, luminosity-based empirical relations, accretion disc models, and cooling-envelope fits. After accounting for statistical and model-specific systematics, we find that all TDE models generally return self-consistent MBH values between flares, and are broadly consistent with host-galaxy MBH proxies, recovering MBH to within 0.3–0.5 dex. However, the convergence of fallback models towards unphysical stellar masses and impact parameters reveals limitations in existing fallback model grids. We also show that light curve coverage, particularly in the near-ultraviolet, is critical for constraining model parameters. This has direct implications for interpreting the thousands of TDE light curves expected from upcoming surveys such as the Rubin Observatory’s Legacy Survey of Space and Time, where from MOSFiT simulations, we find that MBH may be underestimated on average by 0.1 – 0.5 dex depending on peak light curve coverage and the availability of follow-up data.

Abstract:

Abstract Radio emission during X-ray binary outbursts is dominated by synchrotron radiation from relativistic jets, but is usually studied through total-intensity diagnostics such as flux density, spectra, variability, and proper motion. Radio spectropolarimetry provides a complementary probe of the magneto-ionic plasma through Faraday rotation and depolarisation. When the Faraday rotating material is local to the source, these effects can constrain the jet plasma composition and mass content, but this approach is rarely applied to transient jetted sources. We present MeerKAT L-band spectropolarimetry of the black hole X-ray binary Swift J1727 during its 2023 outburst, focusing on the brightest radio flaring interval, when relativistic jets were being launched intermittently. Using multiple spectropolarimetric techniques, we identify transient Faraday-complex structure coincident with the major radio flares. The close temporal association with the flaring activity, together with the stability of the foreground Faraday screen, favours an origin local to the jet rather than in the ISM or in a separate local screen external to the emitting plasma. Since internal Faraday rotation is suppressed in a pure electron–positron plasma, the data favour a dominant electron–proton component. Interpreting the characteristic Faraday thickness as internal rotation, and anchoring the magnetic-field and size scales with synchrotron self-absorption arguments, we infer a characteristic Faraday-rotating mass of order Mrot ~ 1021 g, corresponding to only a small fraction, frot ~ 10−3, of the accreted mass available during the flare. These results show that time-domain spectropolarimetry can turn transient Faraday complexity into a diagnostic of jet composition, mass loading, and plasma evolution in X-ray binary outbursts, and potentially other transient jetted sources.