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

Context. 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. It has been proposed that this is the result of energy transfer between the ultra-relativistic GRB jet and the SN ejecta. However, this cannot fully explain the discrepancy, as some type Ic-BL SNe without a GRB detection (ordinary type Ic-BL SNe) may also harbour GRB jets. Aims. 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, comprising a total of 875 spectra. 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). This will help us understand whether the presence of the jet affects the evolution of the expansion velocity, possibly allowing us to infer the existence of jets in cases where GRB emission is not detected. Methods. We measured the expansion velocities of the Fe II 5169 Å and Si II 6355 Å features observed in the spectra of type Ic-BL SNe using a spline fitting method. We fit the expansion velocity evolution with single and broken power laws. In each analysis, we compared two populations: ordinary type Ic-BL SNe and GRB-SNe. Results. The expansion velocities of the Fe II and Si II features revealed considerable overlap between the two populations. Although some GRB-SNe expand more rapidly than ordinary type Ic-BL SNe, the difference between the population medians is not statistically significant. Our analysis confirms that type Ic-BL SNe and GRB-SNe generally expand more rapidly than type Ic SNe. The marginalised Fe II and Si II power law indices indicate that GRB-SNe decline at similar rates to ordinary type Ic-BL SNe. Broken power law evolution appears to be more common for the Si II feature, which always follows a shallow-steep decay. In contrast, the broken power law Fe II decays are predominantly steep-shallow. The Si II velocity evolution of PTF12gzk and SN2016coi (engine-driven SNe) are similar to GRB060218-SN2006aj, with both showing broken power law decay. This observation may hint at a two-component ejecta model, such as a GRB jet or a cocoon. Conclusions. 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.