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

Kavli Affiliate: Michael Fausnaugh

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| Summary:

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 peak
UV/optical emission using high-cadence, multi-wavelength spectroscopy and
photometry. From Transiting Exoplanet Survey Satellite (TESS) 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 $L =
(3.15 pm 0.04) times 10^{43}$ 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 $L_{x} sim 1.5 times 10^{42}$ 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 $Gamma sim 2.3 – 2.6$. 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 (TDEs) and active galactic nuclei (AGNs), it cannot be
definitively classified with current data.

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