Discovery and Early Evolution of ASASSN-19bt, the First TDE Detected by TESS

Kavli Affiliate: Michael M. Fausnaugh

| First 5 Authors: Thomas W. -S. Holoien, Patrick J. Vallely, Katie Auchettl, K. Z. Stanek, Christopher S. Kochanek

| Summary:

We present the discovery and early evolution of ASASSN-19bt, a tidal
disruption event (TDE) discovered by the All-Sky Automated Survey for
Supernovae (ASAS-SN) at a distance of $dsimeq115$ Mpc and the first TDE to be
detected by TESS. As the TDE is located in the TESS Continuous Viewing Zone,
our dataset includes 30-minute cadence observations starting on 2018 July 25,
and we precisely measure that the TDE begins to brighten $sim8.3$ days before
its discovery. Our dataset also includes 18 epochs of Swift UVOT and XRT
observations, 2 epochs of XMM-Newton observations, 13 spectroscopic
observations, and ground data from the Las Cumbres Observatory telescope
network, spanning from 32 days before peak through 37 days after peak.
ASASSN-19bt thus has the most detailed pre-peak dataset for any TDE. The TESS
light curve indicates that the transient began to brighten on 2019 January 21.6
and that for the first 15 days its rise was consistent with a flux $propto
t^2$ power-law model. The optical/UV emission is well-fit by a blackbody SED,
and ASASSN-19bt exhibits an early spike in its luminosity and temperature
roughly 32 rest-frame days before peak and spanning up to 14 days that has not
been seen in other TDEs, possibly because UV observations were not triggered
early enough to detect it. It peaked on 2019 March 04.9 at a luminosity of
$Lsimeq1.3times10^{44}$ ergs s$^{-1}$ and radiated $Esimeq3.2times10^{50}$
ergs during the 41-day rise to peak. X-ray observations after peak indicate a
softening of the hard X-ray emission prior to peak, reminiscent of the
hard/soft states in X-ray binaries.

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