Hintze Centre for Astrophysical Surveys

Abstract:

We present ultraviolet to infrared observations of the extraordinary Type IIn supernova 2023zkd (SN 2023zkd). Photometrically, it exhibits persistent and luminous precursor emission spanning ∼4 yr preceding discovery (Mr ≈ −15 mag, 1500 days in the observer frame), followed by a secondary stage of gradual brightening in its final year. Post-discovery, it exhibits two photometric peaks of comparable brightness (Mr ≲ −18.7 mag and Mr ≈ −18.4 mag, respectively) separated by 240 days. Spectroscopically, SN 2023zkd exhibits highly asymmetric and multicomponent Balmer and He I profiles that we attribute to ejecta interaction with fast-moving (1000–2000 km s−1) He-rich polar material and slow-moving (∼400 km s−1) equatorially distributed H-rich material. He II features also appear during the second light curve peak and evolve rapidly. Shock-driven models fit to the multiband photometry suggest that the event is powered by interaction with ∼5–6 M⊙ of CSM, with 2–3 M⊙ associated with each light curve peak, expelled during mass-loss episodes ∼3–4 yr and ∼1–2 yr prior to explosion. The observed precursor emission, combined with the extreme mass-loss rates required to power each light curve peak, favors either super-Eddington accretion onto a black hole or multiple long-lived eruptions from a massive star to luminosities that have not been previously observed. We consider multiple progenitor scenarios for SN 2023zkd, and find that the brightening optical precursor and inferred explosion properties are most consistent with a massive (MZAMS ≥ 30 M⊙) and partially stripped He star undergoing an instability-induced merger with a black hole companion.

Abstract:

We present high angular resolution (0.19 arcsec or pc) Atacama Large Millimeter/submillimeter Array observations of the CO(3–2) line emission of the galaxy NGC 4751. The data provide evidence for the presence of a central supermassive black hole (SMBH). Assuming a constant mass-to-light ratio (), we infer a SMBH mass M and a F160W filter stellar M/L, where the first uncertainties are statistical and the second systematic. Assuming a linearly spatially varying , we infer M and , where R is the galactocentric radius. We also present SMBH mass estimates using the Jeans Anisotropic Modelling (JAM) method and Very Large Telescope Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) stellar kinematics. Assuming a cylindrically aligned velocity ellipsoid (JAM), we infer M, and while assuming a spherically aligned velocity ellipsoid (JAM), we infer M. The SMBH mass assuming a constant is statistically consistent with that of JAM, whereas the mass assuming a linearly varying is consistent with both JAM and JAM (within the uncertainties). Our derived masses are larger than (and inconsistent with) one previous stellar dynamical measurement using the Schwarzschild orbit-superposition method and the same SINFONI kinematics.

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.