2018ApJ...859..123H

The persistent soft X-ray emission from the location of the most luminous supernova (SN) so far, ASASSN-15lh (or SN 2015L), with L∼10⁴²ergs^–1, 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 {dot}M≳3×10^–3M_☉(v_w/10³kms^–1)yr^–1, and the initial velocity of the bulk SN ejecta is ≲0.02c. These constraints imply a massive ejecta mass of ≳60(E₀/2×10⁵²erg)M_☉ in ASASSN-15lh, and a strong wind ejected by the progenitor star within ∼8(v_w/10³kms^–1)^–1 yr before explosion.