ASASSN-15lh: A Superluminous Ultraviolet Rebrightening Observed by Swift and Hubble

TL;DR

This paper reports on the ultraviolet rebrightening of the superluminous supernova ASASSN-15lh, highlighting its extreme luminosity, mild asymmetry, and potential for high-redshift detection, with implications for early universe studies.

Contribution

It provides detailed ultraviolet and optical observations of ASASSN-15lh, revealing its unique properties and demonstrating its detectability at very high redshifts, expanding understanding of superluminous supernovae.

Findings

ASASSN-15lh is the most luminous supernova observed to date.

It exhibits a late rebrightening with UV-dominated spectra.

The supernova can be detected beyond redshift 4, possibly up to 20.

Abstract

We present and discuss ultraviolet and optical photometry from the Ultraviolet/Optical Telescope and X-ray limits from the X-Ray Telescope on Swift and imaging polarimetry and ultraviolet/optical spectroscopy with the Hubble Space Telescope of ASASSN-15lh. It has been classified as a hydrogen-poor superluminous supernova (SLSN I) more luminous than any other supernova observed. ASASSN-15lh is not detected in the X-rays in individual or coadded observations. From the polarimetry we determine that the explosion was only mildly asymmetric. We find the flux of ASASSN-15lh to increase strongly into the ultraviolet, with a ultraviolet luminosity a hundred times greater than the hydrogen-rich, ultraviolet-bright SLSN II SN 2008es. We find objects as bright as ASASSN-15lh are easily detectable beyond redshifts of ~4 with the single-visit depths planned for the Large Synoptic Survey Telescope. Deep near-infrared surveys could detect such objects past a redshift of ~20 enabling a probe of the earliest star formation. A late rebrightening -- most prominent at shorter wavelengths -- is seen about two months after the peak brightness, which is itself as bright as a superluminous supernova. The ultraviolet spectra during the rebrightening are dominated by the continuum without the broad absorption or emission lines seen in SLSNe or tidal disruption events and the early optical spectra of ASASSN-15lh. Our spectra show no strong hydrogen emission, showing only LyA absorption near the redshift previously found by optical absorption lines of the presumed host. The properties of ASASSN-15lh are extreme when compared to either SLSNe or tidal disruption events.