The Curious Case of ASASSN-20hx: A Slowly Evolving, UV- and X-Ray-Luminous, Ambiguous Nuclear Transient

Abstract

We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from −30 to 275 days relative to the peak UV/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of L = (3.15 ± 0.04) × 1043 erg s−1. The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to L x ∼ 1.5 × 1042 erg s−1 and then slowly declined over time. The X-ray emission is well fit by a power law with a photon index of Γ ∼ 2.3–2.6. Both the optical and near-infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events and active galactic nuclei, it cannot be definitively classified with current data.

The ASAS-SN bright supernova catalogue – V. 2018–2020

“…In addition to studying supernovae (SNe; e.g., Bose et al 2018Bose et al , 2019Hoeflich et al 2021;Chen et al 2022), ASAS-SN obtains data for a broad range of transients, multi-messenger searches, and variable sources. For transients, these include tidal disruption events (TDEs; e.g., Holoien et al 2019b,c, 2020, Hinkle et al 2021, Payne et al 2022, novae (e.g., Kawash et al 2021bKawash et al , 2022, dwarf novae (e.g., Kawash et al 2021a), and changing look or other active galactic nuclei (AGN; e.g., Neustadt et al 2020, Hinkle et al 2022, Holoien et al 2022. There are multi-messenger searches associated with both neutrino (e.g., IceCube Collaboration et al 2018, Necker et al 2022) and gravitational wave (e.g., de Jaeger et al 2022) events.…”

High-energy Neutrino Production from AGN Disk Transients Impacted by the Circum-disk Medium

“…These energetic events are predicted to have high event-rate densities in AGN disks and may contribute a considerable fraction of the diffuse neutrino background. Recently, a few potential AGN disk transients were detected although their origins were still unclear (Hinkle et al 2022;Holoien et al 2022), and more AGN disk transients might be recorded in previous optical survey projects. Lightcurves of AGN disk transients could have similar peak brightness, peak time, and evolution pattern with those of TDEs (Zhu et al 2021d;Grishin et al 2021;Ren et al 2022) so that some AGN disk transients could be wrongly identified as TDEs.…”

Four Years of Type Ia Supernovae Observed by TESS: Early-time Light-curve Shapes and Constraints on Companion Interaction Models

“…With its large field of view, TESS observes a large number of bright SNe, averaging six type Ia SN light curves for every sector. TESS has also made important contributions to extragalactic transient studies more broadly: Vallely et al (2021) used TESS data to measure rise times of 22 core-collapse SNe from 2018-2020, while high-cadence TESS light curves have enabled new investigations of tidal disruption events (Holoien et al 2019), gamma-ray bursts (Smith et al 2021), and active galactic nuclei (Burke et al 2020;Weaver et al 2020;Payne et al 2021;Hinkle et al 2022;Payne et al 2022;Hinkle et al 2023).…”