Persistent X-ray emission from ASASSN-15lh: massive ejecta and pre-SLSN dense wind?

TL;DR

This paper explains the persistent X-ray emission from the luminous supernova ASASSN-15lh as shock-accelerated electrons scattering UV photons, revealing a dense stellar wind and massive ejecta consistent with a pre-superluminous supernova environment.

Contribution

It provides a novel interpretation of the X-ray emission as inverse-Compton scattering and constrains the properties of the circumstellar medium and ejecta mass for ASASSN-15lh.

Findings

X-ray emission explained by inverse-Compton scattering of UV photons.

Dense stellar wind with high mass-loss rate prior to explosion.

Massive ejecta mass exceeding 60 solar masses.

Abstract

The persistent soft X-ray emission from the location of the so-far most luminous supernova (SN), ASASSN-15lh (or SN 2015L), with $L \sim 10^{42} \rm erg~ s^{-1}$, is puzzling. We show that it can be explained by radiation from the SN-shock accelerated electrons inverse-Compton scattering the intense UV photons. The non-detection in radio requires strong free-free absorption in the dense medium. In these interpretations, the circumstellar medium is derived to be a wind ($n\propto R^{-2}$) with mass-loss rate of $\dot M \gtrsim 3 \times10^{-3}M_\odot(v_{\rm w}/10^3 \rm km\,s^{-1}) yr^{-1}$, and the initial velocity of the bulk SN ejecta is $\lesssim 0.02c$. These constraints imply a massive ejecta mass of $\gtrsim 60(E_0/2\times10^{52} \rm erg)M_\odot$ in \asn, and a strong wind ejected by the progenitor star within $\sim8 (v_{\rm w}/10^3 \rm km\,s^{-1})^{-1}$ yrs before explosion.