The diversity of transients from magnetar birth in core collapse supernovae

Strongly magnetized, rapidly rotating neutron stars are contenders for the central engines of both long gamma-ray bursts (LGRBs) and hydrogen-poor superluminous supernovae (SLSNe-I). Models for typical (minute long) LGRBs invoke magnetars with high dipole magnetic fields (B_d ≳ 10¹⁵ G) and short spin-down times, SLSNe-I require neutron stars with weaker fields and longer spin-down times of weeks. Here, we identify a transition region in the space of B_d and birth period for which a magnetar can power both a LGRB and a luminous supernova. In particular, a 2 ms period magnetar with a spin-down time of ~10⁴ s can explain both the ultralong GRB 111209 and its associated luminous SN2011kl. For magnetars with longer spin-down times, we predict even longer duration (~10^5-6 s) GRBs and brighter supernovae, a correlation that extends to Swift J2058+05 (commonly interpreted as a tidal disruption event). We further show that previous estimates of the maximum rotational energy of a protomagnetar were too conservative and energies up to E_max ~ 1-2 × 10⁵³ ergs are possible. A magnetar can therefore comfortably accommodate the extreme energy requirements recently posed by the most luminous supernova ASASSN-15lh. The luminous pulsar wind nebula powering ASASSN-15lh may lead to an 'ionization breakout' X-ray burst over the coming months, accompanied by a change in the optical spectrum.