The GRB–SLSN connection: misaligned magnetars, weak jet emergence, and observational signatures

Abstract

Multiple observational lines of evidence support a connection between hydrogen-poor superluminous supernovae (SLSNe) and long duration gamma-ray bursts (GRBs). Both events require a powerful central energy source, usually attributed to a millisecond magnetar or an accreting black hole. The GRB-SLSN link raises several theoretical questions: What distinguishes the engines responsible for these different phenomena? Can a single engine power both a GRB and a luminous SN in the same event? We propose a new unifying model for magnetar thermalization and jet formation: misalignment between the rotation (${\bf \Omega}$) and magnetic dipole (${\bf \mu}$) axes thermalizes a fraction of the spindown power by reconnection in the striped equatorial wind, providing a guaranteed source of "thermal" emission to power the supernova. The remaining un-thermalized power energizes a relativistic jet. In this picture, the GRB-SLSN dichotomy is directly linked to ${\bf \Omega \cdot \mu}$. We extend earlier work to show that even weak relativistic jets of luminosity $\sim10^{46}$ erg s$^{-1}$ can escape the expanding SN ejecta hours after the explosion, implying that escaping relativistic jets may accompany many SLSNe. We calculate the observational signature of these jets. We show that they may produce transient UV cocoon emission lasting a few hours when the jet breaks out of the ejecta surface. A longer-lived optical/UV signal may originate from a mildly-relativistic wind driven from the interface between the jet and the ejecta walls. This provides a new explanation for the secondary early-time maximum observed in some SLSNe light curves, such as LSQ14bdq. This scenario also predicts a population of GRB from on-axis jets with extremely long durations, potentially similar to the population of "jetted tidal disruption events", in coincidence with a small subset of SLSNe.