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# 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 <108M, 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 >108M12,13, 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.

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## Acknowledgements

We acknowledge support from the European Union FP7 programme through the following European Research Council grants: 320360 (M.F., H.C.), 647208 (P.G.J.), 291222 (S.J.S.), 615929 (M.S.). We also acknowledge: Einstein Postdoctoral Fellowship PF5-160145 (N.C.S.), Hubble Postdoctoral Fellowship HST-HF2-51350 (S.v.V.), STFC grants ST/I001123/1 ST/L000709/1 (S.J.S.) and ST/L000679/1 (M.S.), Australian Research Council Future Fellowship FT140101082 (J.C.A.M.-J.), a Royal Society University Research Fellowship (J.R.M.), a Sofja Kovalevskaja Award to P. Schady (T.Kr., T.-W.C.), a Ramón y Cajal fellowship and the Spanish research project AYA 2014-58381 (A.de U.P.), CONICYT-Chile FONDECYT grants 3130488 (S.K.), 3140534 (S.S.), 3140563 (H.K.), 3150238 (C.R.-C.), a PRIN-INAF 2014 project (N.E.-R.), support from IDA (D.M.), an Ernest Rutherford Fellowship (K.M.), CAASTRO project number CE110001020 (B.E.T.), National Science Foundation grant AST 11-09881 and NASA grant HST-AR-13726.02 (J.C.W.). This work used observations from the Las Cumbres Observatory Global Telescope Network (LCOGT) and was based upon work supported by National Science Foundation grant 1313484. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by Commonwealth Scientific and Industrial Research Organisation. This work was based partially on observations collected as part of the Public European Southern Observatory Spectroscopic Survey for Transient Objects Survey (PESSTO) under European Southern Observatory (ESO) programmes 188.D-3003 and 191.D-0935, and on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 095.D-0633. We thank M. Della Valle for comments.

## Author information

Authors

### Contributions

G.L. coordinated the PESSTO observations, was principal investigator (PI) of the FORS2 program, analysed the data and wrote the paper. M.F. provided the astrometric localization and reduced the PESSTO spectra. N.C.S. calculated the relationship between the black hole spin and the Hills mass and edited the manuscript. S.v.V. performed the Swift photometry. P.G.J. analysed the X-Ray Multi-Mirror Mission data and helped coordinate the project. I.A. is the PESSTO PI for TDEs and provided LCOGT data. C.F. made the LCOGT photometry. J.R.M. reduced the FORS2 spectra. S.J.S. is the PI of PESSTO and helped coordinate the project. T.Kr. provided the spectral energy distribution fit of the host galaxy. J.C.A.M.-J. provided the radio observations. P.M.V. helped with the analysis of the spectra. G.L., M.F., N.C.S., S.v.V., P.G.J., I.A., S.J.S., J.C.A.M.-J., A.G.-Y. and P.A.M. contributed to the discussions. A.d.U.P. and A.De C. worked on the UV spectrum. D.A.H. is the PI of the LCOGT observations. C.I. and O.Y. are PESSTO builders and helped with the analysis. F.P., D.M., J.S. and J.C.W. provided FORS2 data and analysis. M.C. and B.E.T. provided the WIFES spectra. S.S. and S.K. provided the Magellan spectrum. G.H., C.V.McC. and S.V. obtained and reduced LCOGT data. E.K., K.M., K.W.S., M.S., and D.R.Y. are PESSTO builders and C.A., J.H., S.M., T.-W.C., T.Ka., S.P., C.R.-C., H.K., M.N., J.L., N.E.-R., H.C., I.B., J.J., M.R.M. contributed PESSTO observations or data reductions. Many authors provided comments on the manuscript.

### Corresponding author

Correspondence to G. Leloudas.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

Supplementary Figures 1–8, Supplementary Tables 1–2 (PDF 1050 kb)

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Leloudas, G., Fraser, M., Stone, N. et al. The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole. Nat Astron 1, 0002 (2017). https://doi.org/10.1038/s41550-016-0002

• Accepted:

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• DOI: https://doi.org/10.1038/s41550-016-0002

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