The Superluminous Transient ASASSN-15lh as a Tidal Disruption Event from a Kerr Black Hole

TL;DR

This paper presents observations of a luminous transient initially thought to be a supernova, which is more consistent with a tidal disruption event caused by a rapidly spinning supermassive black hole, explaining its extreme brightness.

Contribution

It provides evidence that the superluminous transient ASASSN-15lh is a tidal disruption event from a Kerr black hole, challenging the supernova interpretation.

Findings

Rebrightening in ultraviolet observed over 10 months

Spectroscopic phases without nebular features

Localization in the galaxy nucleus supports TDE hypothesis

Abstract

When a star passes within the tidal radius of a supermassive black hole, it will be torn apart. For a star with the mass of the Sun ($M_\odot$) and a non-spinning black hole with a mass $<10^8 M_\odot$, the tidal radius lies outside the black hole event horizon and the disruption results in a luminous flare. Here we report observations over a period of 10 months of a transient, hitherto interpreted as a superluminous supernova. 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 evolution, the presence of highly ionised CNO gas in the line of sight and our improved localisation of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely. While the supermassive black hole has a mass $> 10^8 M_\odot$, a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly. The rapid spin and high black hole mass can explain the high luminosity of this event.