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 galaxies1. Previous candidate flares2,3,4,5,6 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 jet7,8,9,10. Here we report 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 decay of the light curve follow 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 two 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.
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We thank H. Tananbaum for approving our Chandra Director’s Discretionary Time request. We are grateful to G. Lodato for providing the tidal disruption event models in tabular form, and to S. Moran for running software to calculate the host-galaxy K-corrections. We thank R. E. Williams for discussions on the line emission in the spectra. S.G. was supported by NASA through a Hubble Fellowship grant awarded by the Space Telescope Science Institute, which is operated by AURA Inc. for NASA. Partial support for this work was provided by the National Science Foundation. The PS1 survey has been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Inc. and the National Central University of Taiwan, and by NASA under a grant issued through the Planetary Science Division of the NASA Science Mission Directorate. We acknowledge NASA’s support for construction, operation, and science analysis of the GALEX mission, which was developed in cooperation with Centre National d’Etudes Spatiales of France and the Korean Ministry of Science and Technology. Some of the observations reported here were obtained at the MMT Observatory, which is a joint facility of the Smithsonian Institution and the University of Arizona, and at the Liverpool Telescope, which is operated with financial support from the UK Science and Technology Facilities Council. The computations in this paper were run on the Odyssey cluster supported by the FAS Science Division Research Computing Group at Harvard University. R.J.F. is a Clay Fellow.
This file contains Supplementary Text and Data 1-7, Supplementary Figures 1-5, Supplementary Tables 1-3 and additional references.