Late-time observations of the extraordinary Type II supernova iPTF14hls

TL;DR

This paper presents extensive late-time optical and space-based observations of the unusual long-lived Type II supernova iPTF14hls, providing critical data to evaluate various theoretical models of its extraordinary behavior.

Contribution

It offers the most comprehensive late-time observational dataset for iPTF14hls, including photometry, spectroscopy, and high-resolution imaging, to inform and challenge existing supernova models.

Findings

The supernova's luminosity remained nearly constant for hundreds of days before a steep decline.

Spectra transitioned to nebular phase, revealing signatures from the supernova's interior.

Late-time observations show a steep decline, weak X-ray emission, and emission lines from dense core material.

Abstract

We study iPTF14hls, a luminous and extraordinary long-lived Type II supernova, which lately has attracted much attention and disparate interpretation. We present new optical photometry that extends the light curves until more than 3 yr past discovery. We also obtained optical spectroscopy over this period, and furthermore present additional space-based observations using Swift and HST. 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 the very last optical spectrum likely displays signatures from the deep and dense interior of the explosion. The high-resolution HST image highlights the complex environment of the explosion in this low-luminosity galaxy. 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, the lack of very strong X-ray emission, and the emergence of intermediate-width emission lines including of [S II] that likely originate from dense, processed material in the core of the supernova ejecta, are all key observational tests for existing and future models.