Hintze Centre for Astrophysical Surveys

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.

Abstract:

ABSTRACT We present the discovery and multiwavelength follow-up of gamma-ray bursts (GRB) 250818B, detected by the Space Variable Objects Monitor (SVOM) and localized by the Gravitational-wave Optical Transient Observer. We compile X-ray, optical/near-infrared, and radio data into broad-band light curves and spectral energy distributions. Despite being initially reported as short, the public prompt light-curve figure suggests a duration of $\sim 5$ s, arguing against a secure short-duration classification. The afterglow is unusually bright relative to short-GRB samples: in X-rays and optical it is broadly consistent with the long-GRB population, while in the radio it is among the most luminous events in the short-GRB comparison sample and remains within the long-GRB range. MeerKAT detects the source at 3.1 GHz, while the Atacama Large Millimetre/submillimetre Array provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe ii, Mg ii, Mg i), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and refreshed (energy-injection) emission, well described by a two-component jet with core isotropic-equivalent kinetic energy $E_{0}\sim 8\times 10^{52}$ erg, ambient number density $n_0\sim 12\, \mathrm{cm^{-3}}$, jet half-opening angle $\theta _j\simeq 0.13$ rad ($\sim 7.4^\circ$), and electron index $p\simeq 1.67$. We do not identify a host conclusively: the data admit either a faint LS DR10 galaxy at $\sim 4\, {\rm arcsec}$ ($\sim 34$ kpc) as an offset candidate, or a fainter, near-coincident uncatalogued host. GRB 250818B highlights the power of rapid counterpart identification in the SVOM era, while uncertain prompt classification and inconclusive host association still limit progenitor interpretation.