The Tidal Disruption Event at 2018hyz II: Light-curve modelling of a partially disrupted star

AT 2018hyz (= ASASSN-18zj) is a tidal disruption event (TDE) located in the nucleus of a quiescent E+A galaxy at a redshift of z = 0.04573, first detected by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present optical+UV photometry of the transient, as well as an X-ray spectrum and radio upper limits. The bolometric light curve of AT 2018hyz is comparable to other known TDEs and declines at a rate consistent with a t-5/3 at early times, emitting a total radiated energy of E = 9 × 1050 erg. An excess bump appears in the UV light curve about 50 d after bolometric peak, followed by a flattening beyond 250 d. We detect a constant X-ray source present for at least 86 d. The X-ray spectrum shows a total unabsorbed flux of ~4 × 10-14 erg cm -2 s-1 and is best fit by a blackbody plus power-law model with a photon index of r = 0.8. A thermal X-ray model is unable to account for photons > 1 keV, while a radio non-detection favours inverse-Compton scattering rather than a jet for the non-thermal component. We model the optical and UV light curves using the Modular Open-Source Fitter for Transients (MOSFiT) and find a best fit for a black hole of 5.2 × 106 M0 disrupting a 0.1 M0 star; the model suggests the star was likely only partially disrupted, based on the derived impact parameter of ß = 0.6. The low optical depth implied by the small debris mass may explain how we are able to see hydrogen emission with disc-like line profiles in the spectra of AT2018hyz (see our companion paper).