Persistent X-Ray Emission from ASASSN-15lh: Massive Ejecta and Pre-SLSN Dense Wind?

Abstract

The persistent soft X-ray emission from the location of the most luminous supernova (SN) so far, ASASSN-15lh (or SN 2015L), with , is puzzling. We show that it can be explained by radiation from electrons accelerated by the SN shock 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  ∝  R −2 ) with mass-loss rate of , and the initial velocity of the bulk SN ejecta is . These constraints imply a massive ejecta mass of in ASASSN-15lh, and a strong wind ejected by the progenitor star within yr before explosion.