Velocity evolution of broad-lined type-Ic supernovae with and without gamma-ray bursts

More than 60 broad-lined type Ic (Ic-BL) supernovae (SNe) are associated with a long gamma-ray burst (GRB). However, many type Ic-BL SNe exhibit no sign of an associated GRB. On average, the expansion velocities of GRB-associated type Ic-BL SNe (GRB-SNe) are greater than those of type Ic-BL SNe without an associated GRB. This work presents the largest spectroscopic sample of type Ic-BL SNe with and without GRBs to date, consisting of 61 ordinary type Ic-BL SNe and 13 GRB-SNe. The goal of this work is to compare the evolution of SN expansion velocities in cases where an ultra-relativistic jet has been launched (GRB-SNe) and cases where no GRB jet is inferred from observations (ordinary type Ic-BL SNe), to search for possible jet influences. To do this we measured the expansion velocities of the Fe II and Si II features observed in the spectra of type Ic-BL SNe using a spline fitting method and fitted the velocity evolution with single and broken power-laws. In each analysis we compared two populations: ordinary type Ic-BL SNe and GRB-SNe. We find that the expansion velocities of the Fe II and Si II features are similar between these populations, in contrast with previous studies. The Fe II and Si II power-law indices indicate that GRB-SNe decline at similar rates to ordinary type Ic-BL supernovae. Broken power-law evolution appears to be more common for the Si II feature. This observation may hint at a two-component ejecta model, such as a GRB jet or a cocoon. Neither the velocities nor their evolution can be used to distinguish between ordinary type Ic-BL SNe and GRB-SNe. Velocities consistent with broken power-law evolution may indicate the presence of a GRB jet in some of these ordinary type Ic-BL SNe, but this is likely not as robust as late-time radio surveys. These results suggest that GRB-SNe and ordinary type Ic-BL SNe are drawn from the same underlying population of events.