A Luminous X-Ray Transient in SDSS J143359.16+400636.0: A Likely Tidal Disruption Event

We present the discovery of a luminous X-ray transient, serendipitously detected by Swift's X-ray Telescope on 2020 February 5, located in the nucleus of the galaxy SDSS J143359.16+400636.0 at z = 0.099 (luminosity distance D-L = 456 Mpc). The transient was observed to reach a peak luminosity of similar to 10(44) erg s(-1) in the 0.3-10 keV X-ray band, which was similar to 20 times more than the peak optical/UV luminosity. Optical, UV, and X-ray light curves from the Zwicky Transient Facility and Swift show a decline in flux from the source consistent with t(-5/3), and observations with NuSTAR and Chandra show a soft X-ray spectrum with photon index Gamma = 2.9 +/- 0.1. The X-ray/UV properties are inconsistent with well-known active galactic nucleus properties and have more in common with known X-ray tidal disruption events (TDEs), leading us to conclude that it was likely a TDE. The broadband spectral energy distribution can be described well by a disk blackbody model with an inner disk temperature of K, with a large fraction (>40%) of the disk emission upscattered into the X-ray band. An optical spectrum taken with Keck/LRIS after the X-ray detection reveals LINER line ratios in the host galaxy, suggesting low-level accretion onto the supermassive black hole prior to the event, but no broad lines or other indications of a TDE were seen. The stellar velocity dispersion implies that the mass of the supermassive black hole powering the event is log(M-BH/M-circle dot) = 7.41 +/- 0.41, and we estimate that at peak the Eddington fraction of this event was similar to 50%. This likely TDE was not identified by wide-field optical surveys or optical spectroscopy, indicating that more events like this would be missed without wide-field UV or X-ray surveys.