A systematically-selected sample of luminous, long-duration, ambiguous nuclear transients

Abstract

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 > 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 < 20 d and that fades smoothly in > 600 d. We define a
portion of rise-time versus flare amplitude space that selects ANTs with ∼ 50 per cent purity against variable AGNs. We calculate
a volumetric rate of ≥ 3 × 10⁻¹¹ Mpc⁻¹ yr⁻¹, consistent with the events being caused by tidal disruptions of intermediate and
high-mass stars

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Identifiers

ORCID for P. Wiseman: orcid.org/0000-0002-3073-1512

ORCID for M. Sullivan: orcid.org/0000-0001-9053-4820

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