The Case for a Misaligned Relativistic Jet from SN 2001em

Abstract

SN 2001em, identified as a Type Ic supernova, has recently been detected in the radio and X-rays, $\gtrsim$ 2 yr after the explosion. The high luminosities at such late times might arise from a relativistic jet viewed substantially off-axis that becomes visible only when it turns mildly relativistic and its emission is no longer strongly beamed away from us. Alternatively, the emission might originate from the interaction of the SN shell with the circumstellar medium. We find that the latter scenario is hard to reconcile with the observed rapid rise in the radio flux and optically thin spectrum, $F_ν\proptoν^{-0.36\pm 0.16}t^{1.9\pm 0.4}$, while these features arise naturally from a misaligned relativistic jet. The high X-ray luminosity provides an independent and more robust constraint -- it requires $\sim 10^{51}$ erg in mildly relativistic ejecta. The source should therefore currently have a large angular size ($\sim$ 2 mas) which could be resolved in the radio with VLBA. It is also expected to be bipolar and is thus likely to exhibit a large degree of linear polarization ($\sim 10%-20%$). The presence of a relativistic outflow in SN 2001em would have interesting implications. It would suggest that several percent of SNe Ib/c produce mildly relativistic jets, with an initial Lorentz factor $Γ_0\gtrsim 2$, while the fraction that produce GRB jets (with $Γ_0\gtrsim 100$) is $\sim 100$ times smaller. This could considerably increase the expected number of transients similar to orphan GRB afterglows in the radio, and to a lesser extent in the optical and X-rays, if there is a continuous distribution in $Γ_0$. Furthermore, this may give further credence to the idea that core collapse SNe, and in particular SNe Type Ib/c, are triggered by bipolar jets.