The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole

Abstract

When a star passes within the tidal radius of a supermassive black hole, it will be torn apart1. For a star with the mass of the Sun (M ⊙) and a non-spinning black hole with a mass <108 M ⊙, the tidal radius lies outside the black hole event horizon2 and the disruption results in a luminous flare3–6. Here we report observations over a period of ten months of a transient, hitherto interpreted7 as a superluminous supernova8. Our data show that the transient rebrightened substantially in the ultraviolet and that the spectrum went through three different spectroscopic phases without ever becoming nebular. Our observations are more consistent with a tidal disruption event than a superluminous supernova because of the temperature evolution6, the presence of highly ionized CNO gas in the line of sight9 and our improved localization of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely10,11. While the supermassive black hole has a mass >108 M ⊙ 12,13, a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly14. The rapid spin and high black hole mass can explain the high luminosity of this event.

ASASSN-15lh was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) on 14 June 2015 at a redshift of z = 0.2326. Its light curve peaked at V ≈ 17 mag implying an absolute magnitude of M = −23.5 mag, more than twice as luminous as any known supernova 7 . Our long-term spectroscopic follow-up reveals that ASASSN-15lh went through three different spectroscopic phases (Fig. 1). During the first phase 7 , the spectra were dominated by two broad absorption features. While these features appear similar to those observed in superluminous supernovae (SLSNe; Supplementary Fig. 1), their physical origin is different. The features in SLSNe are due to O II 8,15 , but this would produce an additional strong feature at ∼4,400 Å (Supplementary Fig. 2). The feature at ∼4,100 Å cannot be easily identified in the tidal disruption event (TDE) framework either. Two possibilities are that it could be due to absorption of Mg ii or high-velocity He ii 16 . After the initial broad absorption features disappeared, the spectra of ASASSN-15lh were dominated by two emission features. A possible identification for these features is He ii λλ3,202 and 4,686 Å, which are both consistently blue-shifted by ∼15,000 km s−1 (Supplementary Fig. 3). He ii emission is commonly seen in optically discovered TDEs 4,5 at different blueshifts, albeit typically at lower velocities, but it has not been seen in H-poor SLSNe. These features disappeared after day +75 (measured in rest frame from the peak) and the later spectra were mostly featureless, with the exception of two emission features at ∼4,000 and 5,200 Å. The spectra remained much bluer than those of SLSNe 17 for many months after the peak and never revealed nebular features, even up to day +256.