Constraints on the environment and energetics of the broad-line Ic SN 2014ad from deep radio and x-ray observations

Broad-line type Ic Supernovae (BL-Ic SNe) are characterized by high ejecta velocity (& 10⁴ km s⁻¹) and are sometimes associated with the relativistic jets typical of long duration (& 2 s) Gamma-Ray Bursts (L-GRBs). The reason why a small fraction of BL-Ic SNe harbor relativistic jets is not known. Here we present deep X-ray and radio observations of the BL-Ic SN 2014ad extending from 13 to 930 days post explosion. SN 2014ad was not detected at either frequency and has no observational evidence of a GRB counterpart. The proximity of SN 2014ad (d ∼ 26 Mpc) enables very deep constraints on the progenitor mass-loss rate Ṁ and on the total energy of the fast ejecta E. We consider two synchrotron emission scenarios for a wind-like circumstellar medium (CSM): (i) uncollimated non-relativistic ejecta, and (ii) off-axis relativistic jet. Within the first scenario our observations are consistent with GRB-less BL-Ic SNe characterized by a modest energy budget of their fast ejecta (E . 10⁴⁵ erg), like SNe 2002ap and 2010ay. For jetted explosions, we cannot rule out a GRB with E . 10⁵¹ erg (beam-corrected) with a narrow opening angle (θ_j ∼ 5^◦) observed moderately off-axis (θ_obs & 30^◦) and expanding in a very low CSM density (Ṁ . 10⁻⁶ M⊙ yr⁻¹). Our study shows that off-axis low-energy jets expanding in a low-density medium cannot be ruled out even in the most nearby BL-Ic SNe with extensive deep observations, and might be a common feature of BL-Ic SNe.