Classification of ASASSN-20hx as a Tidal Disruption Event Candidate
ATel #13893; J. T. Hinkle, B. J. Shappee (IfA, Hawai'i), T. W.-S. Holoien (Carnegie Observatories), K. Auchettl (Univ. of Melbourne; DARK), K. Z. Stanek, C. S. Kochanek, P. Vallely, S. Bose (Ohio State), J. L. Prieto (Diego Portales; MAS), T. de Jaeger (UC Berkeley), D. Gruen, J. Myles (SLAC/KIPAC), T. Jeltema (UC Santa Cruz)
on 24 Jul 2020; 18:32 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Benjamin Shappee (shappee@hawaii.edu)
Subjects: Optical, Ultra-Violet, X-ray, Transient, Tidal Disruption Event
ASASSN-20hx (AT 2020ohl) was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014, Kochanek et al. 2017) on UT 2020-07-10.34 at g = 16.7 mag (ATel #13891). The position of the transient was consistent with the nucleus of the host galaxy (NGC 6297), which has an SDSS redshift of z = 0.01671.
Follow-up spectra with LRIS on the Keck I telescope were obtained on 2020-07-17 and 2020-07-22. Each showed a strong blue continuum and broad emission features possibly consistent with Balmer lines, He II 4686, and Bowen fluorescent emission at the host galaxy redshift.
Four epochs of Neil Gehrels Swift observations from 2020-07-19 to 2020-07-23 all show luminous UV emission (UVW2 ~ -18.98 +/- 0.04 (AB mag) and blue UV-optical colors (UVW2 - V ~ -0.9 mag).
Since an archival 2018 Swift XRT epoch, the X-ray flux at the location of the transient has increased by nearly an order of magnitude. Assuming a blackbody spectrum with a typical TDE temperature of kT = 0.05 keV, this corresponds to an absorbed [0.3-10 keV] flux of ~2.4e-12 erg/s/cm^2 and a luminosity of 1.5e42 erg/s at the distance of the host galaxy (73 Mpc).
We classify ASASSN-20hx as a TDE candidate based on the location in the center of the host galaxy, a slow rise in ASAS-SN photometry, luminous UV and X-ray emission, roughly constant blue colors, and the detection of a blue spectrum with broad emission lines. The source shows no prior variability in ASAS-SN or CRTS data.
The host galaxy is in the northern Transiting Exoplanet Survey Satellite (TESS) continuous viewing zone. Based on the ASAS-SN light curve, the first ~10 days of the rise of this transient were observed by TESS.
We have triggered Swift and NICER follow-up. We encourage further follow-up of this source, in particular high S/N spectroscopy and multi-band imaging.
