Search

We present a search for luminous long-duration ambiguous nuclear transients (ANTs) similar to the unprecedented discovery of the extreme ambiguous event AT2021lwx with a |$\gt 150$|  d rise time and luminosity |$10^{45.7}$|  erg s |$^{-1}$|⁠. We use the Lasair transient broker to search Zwicky Transient Facility (ZTF) data for transients lasting more than one year and exhibiting smooth declines. Our search returns 59 events, 7 of which we classify as ANTs assumed to be driven by accretion onto supermassive black holes. We propose the remaining 52 are stochastic variability from regular supermassive black hole accretion rather than distinct transients. We supplement the seven ANTs with three nuclear transients in ZTF that fail the light curve selection but have clear single flares and spectra that do not resemble typical active galactic nucleus. All of these 11 ANTs have a mid-infrared flare from an assumed dust echo, implying the ubiquity of dust around the black holes giving rise to ANTs. No events are more luminous than AT2021lwx, but one (ZTF19aamrjar) has twice the duration and a higher integrated energy release. On the other extreme, ZTF20abodaps reaches a luminosity close to AT2021lwx with a rise time |$\lt 20$|  d and that fades smoothly in |$\gt 600$|  d. We define a portion of rise-time versus flare amplitude space that selects ANTs with |$\sim 50$|  per cent purity against variable AGNs. We calculate a volumetric rate of |$\gtrsim 3\times 10^{-11}$|  Mpc |$^{-1}$|  yr |$^{-1}$|⁠ , consistent with the events being caused by tidal disruptions of intermediate and high-mass stars. [ABSTRACT FROM AUTHOR]

Aims. Stripped envelope (SE) supernovae are explosions of stars that have somehow lost most of their outer envelopes. We present the discovery and analyse the observations of the Type Ib supernova 2019odp (a.k.a. ZTF19abqwtfu) covering epochs within days of the explosion to late nebular phases at 360 d post-explosion. Methods. Our observations include an extensive set of photometric observations and low- to medium-resolution spectroscopic observations, both covering the complete observable time range. We analysed the data using analytic models for the recombination cooling emission of the early excess emission and the diffusion of the peak light curve. We expanded on existing methods to derive oxygen mass estimates from nebular phase spectroscopy, and briefly discuss progenitor models based on this analysis. Results. Our spectroscopic observations confirm the presence of He in the supernova ejecta and we thus (re)classify SN 2019odp as a Type Ib supernova. From the pseudo-bolometric light curve, we estimate a high ejecta mass of Mej ∼ 4 − 7 M⊙. The high ejecta mass, large nebular [O I]/[Ca II] line flux ratio (1.2 − 1.9), and an oxygen mass above ⪆0.5 M⊙ point towards a progenitor with a pre-explosion mass higher than 18 M⊙. Whereas a majority of analysed SE supernovae in the literature seem to have low ejecta masses, indicating stripping in a binary star system, SN 2019odp instead has parameters that are consistent with an origin in a single massive star. The compact nature of the progenitor (≲10 R⊙) suggests that a Wolf-Rayet star is the progenitor. [ABSTRACT FROM AUTHOR]