Observational signatures of stellar explosions driven by relativistic jets

Abstract

ABSTRACT The role of relativistic jets in unbinding the stellar envelope during a supernova (SN) associated with a gamma-ray burst (GRB) is unclear. To study that, we explore observational signatures of stellar explosions that are driven by jets. We focus on the final velocity distribution of the outflow in such explosions and compare its observational imprints to SN/GRB data. We find that jet-driven explosions produce an outflow with a flat distribution of energy per logarithmic scale of proper velocity. The flat distribution seems to be universal as it is independent of the jet and the progenitor properties that we explored. The velocity range of the flat distribution for typical GRB parameters is γβ ≈ 0.03–3, where γ is the outflow Lorentz factor and β is its dimensionless velocity. A flat distribution is seen also for collimated choked jets where the highest outflow velocity decreases with the depth at which the jet is choked. Comparison to observations of SN/GRBs strongly disfavors jets as the sole explosion source in these events. Instead, in SN/GRB the collapsing star seems to deposit its energy into two channels – a quasi-spherical (or wide angle) channel and a narrowly collimated one. The former carries most of the energy and is responsible for the SN sub-relativistic ejecta while the latter carries 0.01–0.1 of the total outflow energy and is the source of the GRB. Intriguingly, the same two channels, with a similar energy ratio, were seen in the binary neutron star merger GW170817, suggesting that similar engines are at work in both phenomena.