Diagnosing the ejecta properties of engine-driven supernovae from observables in their initial phase

Engine-driven explosions with continuous energy input from the central system have been suggested for supernovae (SNe) associated a Gamma-Ray Burst (GRB), super-luminous SNe (SLSNe), and at least fraction of broad-lined Ic (SNe Ic-BL) even without an GRB. In present work, we investigate observational consequences in this scenario, focusing on case where injection is sufficiently brief, which has GRB-SNe. We construct simplified, spherical ejecta model sequence taking into account major effects engine; composition mixing, density structure, outermost velocity. Unlike most previous works GRB-SNe, solve formation photosphere self-consistently, can predict photometric spectroscopic observables. find that these properties strongly affect their appearance initial phase (~ week since explosion), highlighted by blended lines suffering higher-velocity absorptions flatter distribution and/or higher ejeca This behaviour also affects multi-band light curves non-monotonic way. Prompt follow-up observations starting immediately after explosion thus provides key diagnostics to unveil nature engine behind GRB-SNe Ic-BL. For SN 2017iuk GRB 171205A diagnosing data are available, show expected structure engine-driven explosion, i.e., flat power-law extending up >~ 100,000 km/s, explain observed spectral evolution reasonably well.