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 so-far most luminous supernova (SN), ASASSN-15lh (or SN 2015L), with $L\sim 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$, 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 $\dot{M}\gtrsim 3\times 10^{-3}{M}_{\odot }(v_{\mathrm{w}}/10^3\mathrm{km}{\mathrm{s}}^{-1})\mathrm{y}{\mathrm{r}}^{-1}$, and the initial velocity of the bulk SN ejecta is $\lesssim 0.02c$. These constraints imply a massive ejecta mass of $\gtrsim 60(E_0/2\times 10^{52}\mathrm{erg}){\mathrm{M}}_{\odot }$ in \asn, and a strong wind ejected by the progenitor star within $\sim 8(v_{\mathrm{w}}/10^3\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$ yrs before explosion.

7 pages, 3 figures and 1 table, Accepted for publication in ApJ

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