Title:
Discovery and Follow-up Observations of ASASSN-19dj

Abstract:
We present the discovery and follow-up observations of ASASSN-19dj, 
a UV luminous tidal disruption event (TDE) discovered by 
the All-Sky Automated Survey for Supernovae (ASAS-SN). 
We show results from data spanning 42 days before until nearly a year after peak light. 
We compile survey data from ASAS-SN, ATLAS, Pan-STARRS, and ZTF, 
high-cadence optical photometry and spectroscopy, UV photometry from Swift, 
and X-ray observations from Swift and NICER. Initially remaining 
roughly constant in X-rays and slowing fading in the optical/UV, 
ASASSN-19dj suddenly reawakened ~200 days after peak, 
when X-ray flux increased by over an order of magnitude. 
We use Swift and NICER target-of-opportunity observations to study 
this extreme rebrightening and probe later stages of accretion. 
The combination of early-time observations, extreme UV brightness, 
and large X-ray rebrightening of ASASSN-19dj presents an exceptional opportunity 
to study, in detail, both the early- and late-time optical/UV and X-ray evolution of 
the same TDE.