PS18kh: A New Tidal Disruption Event with a Non-axisymmetric Accretion Disk

Abstract

We present the discovery of PS18kh, a tidal disruption event discovered at the center of SDSS J075654.53+341543.6 ( d  ≃ 322 Mpc) by the Pan-STARRS Survey for Transients. Our data set includes pre-discovery survey data from Pan-STARRS, the All-sky Automated Survey for Supernovae, and the Asteroid Terrestrial-impact Last Alert System as well as high-cadence, multiwavelength follow-up data from ground-based telescopes and Swift , spanning from 56 days before peak light until 75 days after. The optical/UV emission from PS18kh is well-fit as a blackbody with temperatures ranging from T  ≃ 12,000 K to T  ≃ 25,000 K and it peaked at a luminosity of L  ≃ 8.8 × 10 43 erg s −1 . PS18kh radiated E  = (3.45 ± 0.22) × 10 50 erg over the period of observation, with (1.42 ± 0.20) × 10 50 erg being released during the rise to peak. Spectra of PS18kh show a changing, boxy/double-peaked H α emission feature, which becomes more prominent over time. We use models of non-axisymmetric accretion disks to describe the profile of the H α line and its evolution. We find that at early times the high accretion rate leads the disk to emit a wind which modifies the shape of the line profile and makes it bell-shaped. At late times, the wind becomes optically thin, allowing the non-axisymmetric perturbations to show up in the line profile. The line-emitting portion of the disk extends from r in  ∼ 60 r g to an outer radius of r out  ∼ 1400 r g and the perturbations can be represented either as an eccentricity in the outer rings of the disk or as a spiral arm in the inner disk.