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

The persistent soft X-ray emission from the location of the most luminous supernova (SN) so far, ASASSN-15lh (or SN 2015L), with L∼ {10}⁴² {erg} {{{s}}}^-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}^-3{{M}}_⊙ ({{v}}_{{w}}/{10}³ {{k}}{{m}} {{{s}}}^-1) {{{y}}{{r}}}^-1, and the initial velocity of the bulk SN ejecta is ≲ 0.02c. These constraints imply a massive ejecta mass of ≳ 60({E}₀/2× {10}⁵² {erg}){M}_⊙ in ASASSN-15lh, and a strong wind ejected by the progenitor star within ∼ 8{({v}_{{w}}/{10}³{km}{{{s}}}^-1)}^-1 yr before explosion.