Late-time observations of the extraordinary Type II supernova iPTF14hls

¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: jesper@astro.su.se
² Department of Physics, University of California, Santa Barbara, CA, 93106-9530 USA
³ Las Cumbres Observatory, 6740 Cortona Dr Ste 102, Goleta, CA, 93117-5575 USA
⁴ The Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978 Israel
⁵ Division of Physics, Math and Astronomy, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA, 91125 USA
⁶ Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA, 91125 USA
⁷ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MA, 20771 USA
⁸ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218 Hawthorn, 3122 VIC, Australia
⁹ Department of Astronomy, University of California, Berkeley, CA, 94720-3411 USA
¹⁰ Miller Institute for Basic Research in Science, University of California, Berkeley, CA, 94720 USA
¹¹ Benoziyo Center for Astrophysics, Weizmann Institute of Science, Rehovot, 76100 Israel
¹² Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Browlow Hill, Liverpool, L3 5RF UK
¹³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL, 60208 USA
¹⁴ Department of Physics, University of California, 1 Shields Avenue, Davis, CA, 95616-5270 USA
¹⁵ Physics Department and Tsinghua Center for Astrophysics (THCA), Tsinghua University, Beijing, 100084 PR China

Received: 21 June 2018
Accepted: 23 October 2018

Abstract

Aims. We study iPTF14hls, a luminous and extraordinary long-lived Type II supernova, which lately has attracted much attention and disparate interpretation.

Methods. We have presented new optical photometry that extends the light curves up to more than three years past discovery. We also obtained optical spectroscopy over this period, and furthermore present additional space-based observations using Swift and HST.

Results. After an almost constant luminosity for hundreds of days, the later light curve of iPTF14hls finally fades and then displays a dramatic drop after about 1000 d, but the supernova is still visible at the latest epochs presented. The spectra have finally turned nebular, and our very last optical spectrum likely displays signatures from the deep and dense interior of the explosion. A high-resolution HST image highlights the complex environment of the explosion in this low-luminosity galaxy.

Conclusions. We provide a large number of additional late-time observations of iPTF14hls, which are (and will continue to be) used to assess the many different interpretations for this intriguing object. In particular, the very late (+1000 d) steep decline of the optical light curve is difficult to reconcile with the proposed central engine models. The lack of very strong X-ray emission, and the emergence of intermediate-width emission lines including [S II] that we propose originate from dense, processed material in the core of the supernova ejecta, are also key observational tests for both existing and future models.