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Abstract:

We present the optical discovery and multiwavelength follow-up observations of AT 2024kmq, a likely tidal disruption event (TDE) associated with a supermassive (MBH ∼ 108 M⊙) black hole in a massive galaxy at z = 0.192. The optical light curve of AT 2024kmq exhibits two distinct peaks: an early fast (timescale 1 day) and luminous (M ≈ −20 mag) red peak, then a slower (timescale 1 month) blue peak with a higher optical luminosity (M ≈ −22 mag) and featureless optical spectra. The second component is similar to the spectroscopic class of “featureless TDEs” in the literature, and during this second component we detect highly variable, luminous (LX ≈ 1044 erg s−1), and hard (fν ∝ ν−1.5) X-ray emission. Luminous (1029 erg s−1 Hz−1 at 10 GHz) but unchanging radio emission likely arises from an underlying active galactic nucleus. The luminosity, timescale, and color of the early red optical peak can be explained by synchrotron emission, or alternatively by thermal emission from material at a large radius (R ≈ a few × 1015 cm). Possible physical origins for this early red component include an off-axis relativistic jet, and shocks from self-intersecting debris leading to the formation of the accretion disk. Late-time radio observations will help distinguish between the two possibilities.

Abstract:

We investigate close encounters between initially unbound black holes (BHs) in the gaseous discs of active galactic nuclei (AGNs), performing the first 3D non-isothermal hydrodynamical simulations of gas-assisted binary BH formation. We discuss a suite of 135 simulations, considering nine AGN disc environments and 15 BH impact parameters. We find that the gas distribution within the Hill sphere about an isolated embedded BH is akin to a spherically symmetric star with a low-mass convective envelope and a BH core, with large convective currents driving strong outflows away from the mid-plane. We find that Coriolis force acting on the outflow results in winds, analogous to cyclones, that counter-rotate with respect to the mid-plane flow within the Hill sphere. We confirm the existence of strong thermal blasts due to minidisc collisions during BH close encounters, as predicted in our previous 2D studies. We document binary formation across a wide range of environments, finding formation likelihood is increased when the gas mass in the Hill sphere is large, allowing for easier binary formation in the outer AGN disc. We provide a comprehensive overview of the supermassive black hole’s role in binary formation, investigating how binary formation in intermediate density environments is biased towards certain binary orientations. We offer two models for predicting dissipation by gas during close encounters, as a function of the ambient Hill mass alone, or with the periapsis depth. We use these models to motivate a prescription for binary formation likelihood that can be readily applied to Monte Carlo simulations of AGN evolution.

Abstract:

Separating resolved and unresolved sources in large imaging surveys is a fundamental step to enable downstream science, such as searching for extragalactic transients in wide-field time-domain surveys. Here we present our method to effectively separate point sources from the resolved, extended sources in the Dark Energy Spectroscopic Instrument (DESI) Legacy Surveys (LS). We develop a supervised machine learning model based on the Gradient Boosting algorithm XGBoost. The features input to the model are purely morphological and are derived from the tabulated LS data products. We train the model using ∼2 × 105 LS sources in the COSMOS field with HST morphological labels and evaluate the model performance on LS sources with spectroscopic classification from the DESI Data Release 1 (∼2 × 107 objects) and the Sloan Digital Sky Survey Data Release 17 (∼3 × 106 objects), as well as on ∼2 × 108 Gaia stars. A significant fraction of LS sources are not observed in every LS filter, and we therefore build a “Hybrid” model as a linear combination of two XGBoost models, each containing features combining aperture flux measurements from the “blue” (gr) and “red” (iz) filters. The Hybrid model shows a reasonable balance between sensitivity and robustness, and achieves higher accuracy and flexibility compared to the LS morphological typing. With the Hybrid model, we provide classification scores for ∼3 × 109 LS sources, making this the largest ever machine learning catalog separating resolved and unresolved sources. The catalog has been incorporated into the real-time pipeline of the La Silla Schmidt Southern Survey (LS4), enabling the identification of extragalactic transients within the LS4 alert stream.

Abstract:

We present the photometric and spectroscopic analysis of five Type Ibn supernovae (SNe): SN 2020nxt, SN 2020taz, SN 2021bbv, SN 2023utc, and SN 2024aej. These events share key observational features and belong to a family of objects similar to the prototypical Type Ibn SN 2006jc. The SNe exhibit rise times of approximately 10 days and peak absolute magnitudes ranging from −16.5 to −19 mag. Notably, SN 2023utc is the faintest Type Ibn SN discovered to date, with an exceptionally low r -band absolute magnitude of −16.4 mag. The pseudo-bolometric light curves peak at (1 − 10)×10 42 erg s −1 , with total radiated energies on the order of (1 − 10)×10 48 erg. Spectroscopically, these SNe display a relatively slow spectral evolution. The early spectra are characterised by a hot blue continuum and prominent He  I emission lines. The early spectra also show blackbody temperatures exceeding 10 000 K, with a subsequent decline in temperature during later phases. Narrow He  I lines, which are indicative of unshocked circumstellar material (CSM), show velocities of approximately 1000 km s −1 . The spectra suggest that the progenitors of these SNe underwent significant mass loss prior to the explosion, resulting in a He-rich CSM. Our light curve modelling yielded estimates for the ejecta mass ( M ej ) in the range 1 − 3 M ⊙ with kinetic energies ( E Kin ) of (0.1 − 1)×10 50 erg. The inferred CSM mass ranges from 0.2 to 1 M ⊙ . These findings are consistent with expectations for core collapse events arising from relatively massive envelope-stripped progenitors.