To TDE or not to TDE: The luminous transient ASASSN-18jd with TDE-like and AGN-like qualities

We present the discovery of ASASSN-18jd (AT 2018bcb), a luminous optical/ ultraviolet(UV)/X-ray transient located in the nucleus of the galaxy 2MASX J22434289- 1659083 at z = 0.1192. Over the year after discovery, Swift UltraViolet and Optical Telescope (UVOT) photometry shows the UV spectral energy distribution of the transient to be well modelled by a slowly shrinking blackbody with temperature T ∼ 2.5 × 104 K, a maximum observed luminosity of Lmax = 4.5+0.6 -0.3 × 1044 erg s-1, and a radiated energy of E = 9.6+1.1 -0.6 × 1051 erg. X-ray data from Swift X-Ray Telescope (XRT) and XMM-Newton show a transient, variable X-ray flux with blackbody and power-law components that fade by nearly an order of magnitude over the following year. Optical spectra show strong, roughly constant broad Balmer emission and transient features attributable to He II, NIII-V, OIII, and coronal Fe. While ASASSN-18jd shares similarities with tidal disruption events (TDEs), it is also similar to the newly discovered nuclear transients seen in quiescent galaxies and faint active galactic nuclei (AGNs).