An ultraviolet-optical flare from the tidal disruption of a helium-rich stellar core

TL;DR

This paper reports the discovery of a helium-rich stellar core disruption causing a luminous ultraviolet-optical flare, enabling precise timing of the event and insights into the star's nature and black hole characteristics.

Contribution

It provides the first well-sampled light curve of a tidal disruption event with detailed modeling, revealing the disruption of a helium-rich stellar core.

Findings

The flare's light curve matches predicted mass accretion rates.

The disruption occurred approximately two days before peak brightness.

The black hole mass is estimated at about 2 million solar masses.

Abstract

The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies. Previous candidate flares have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two `relativistic' candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet. Here we report the discovery of a luminous ultraviolet-optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decline of its light curve follows the predicted mass accretion rate, and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about 2 million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.