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

TL;DR

This paper searches for and characterizes luminous, long-duration ambiguous nuclear transients (ANTs) in ZTF data, identifying 11 candidates, analyzing their properties, and estimating their occurrence rate, suggesting they may be related to tidal disruptions.

Contribution

The study systematically searches for long-duration ANTs, classifies seven as likely driven by SMBH accretion, and estimates their volumetric rate, expanding understanding of these rare transients.

Findings

Identified 11 ANTs with diverse durations and luminosities.

All ANTs show mid-infrared dust echoes, indicating dust presence.

Estimated a volumetric rate of ≥3×10⁻¹¹ Mpc⁻³ yr⁻¹ for ANTs.

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, seven 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 AGN. 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 $\sim50$ per cent purity against variable active galactic nuclei. We calculate a volumetric rate of $\gtrsim 3\times10^{-11}$\,Mpc$^{-1}$\,yr$^{-1}$, consistent with the events being caused by tidal disruptions of intermediate and high-mass stars.