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  1. Abstract Initially classified as a Type Ib supernova (SN), ∼100 days after the explosion SN 2014C made a transition to a Type II SN, presenting a gradual increase in the H α emission. This has been interpreted as evidence of interaction between the SN shock wave and a massive shell previously ejected from the progenitor star. In this paper we present numerical simulations of the propagation of the SN shock through the progenitor star and its wind, as well as the interaction of the SN ejecta with the massive shell. To determine with high precision the structure and location of the shell, we couple a genetic algorithm to a hydrodynamic and a bremsstrahlung radiation transfer code. We iteratively modify the density stratification and location of the shell by minimizing the variance between X-ray observations and synthetic predictions computed from the numerical model, allowing the shell structure to be completely arbitrary. By assuming spherical symmetry, we found that our best-fit model has a shell mass of 2.6 M ⊙ ; extends from 1.6 × 10 16 cm to 1.87 × 10 17 cm, implying that it was ejected ∼ 60/( v w /100 km s −1 ) yr before the SN explosion; and has a density stratification with an average behavior ∼ r −3 but presenting density fluctuations larger than one order of magnitude. Finally, we predict that if the density stratification follows the same power-law behavior, the SN will break out from the shell by mid-2022, i.e., 8.5 yr after explosion. 
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  2. Abstract We present the results from our 7 yr long broadband X-ray observing campaign of SN 2014C with Chandra and NuSTAR. These coordinated observations represent the first look at the evolution of a young extragalactic SN in the 0.3–80 keV energy range in the years after core collapse. We find that the spectroscopic metamorphosis of SN 2014C from an ordinary type Ib SN into an interacting SN with copious hydrogen emission is accompanied by luminous X-rays reaching L x ≈ 5.6 × 10 40 erg s −1 (0.3–100 keV) at ∼1000 days post-explosion and declining as L x ∝ t −1 afterwards. The broadband X-ray spectrum is of thermal origin and shows clear evidence for cooling after peak, with T ( t ) ≈ 20 keV ( t / t pk ) − 0.5 . Soft X-rays of sub-keV energy suffer from large photoelectric absorption originating from the local SN environment with NH int ( t ) ≈ 3 × 10 22 ( t / 400 days ) − 1.4 cm − 2 . We interpret these findings as the result of the interaction of the SN shock with a dense ( n ≈ 10 5 − 10 6 cm −3 ), H-rich disk-like circumstellar medium (CSM) with inner radius ∼2 × 10 16 cm and extending to ∼10 17 cm. Based on the declining NH int ( t ) and X-ray luminosity evolution, we infer a CSM mass of ∼(1.2 f –2.0 f ) M ⊙ , where f is the volume filling factor. We place SN 2014C in the context of 121 core-collapse SNe with evidence for strong shock interaction with a thick circumstellar medium. Finally, we highlight the challenges that the current mass-loss theories (including wave-driven mass loss, binary interaction, and line-driven winds) face when interpreting the wide dynamic ranges of CSM parameters inferred from observations. 
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