Supernova remnant energetics and magnetars: no evidence in favour of millisecond proto-neutron stars

It is generally accepted that anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetars, i.e. neutron stars with extremely high surface magnetic fields (B > 1014 G). The origin of these high magnetic fields is uncertain, but a popular hypothesis is that magnetars are born with an initial spin period not much exceeding the convective overturn time (∼3 ms), which results in a powerful dynamo action, amplifying the seed magnetic field to ≳1015 G. Part of this rotation energy is then expected to power the supernova through rapid magnetic braking. It is therefore possible that magnetar creation is accompanied by supernovae that are an order of magnitude more energetic than normal supernovae, provided their initial spin period is ∼1 ms. However, we list here evidence that the explosion energies of these supernova remnants associated with AXPs and SGRs – Kes 73 (AXP 1E 1841−045), CTB 109 (AXP 1E2259+586) and N49 (SGR 0526−66) – are close to the canonical supernova explosion energy of 1051 erg, suggesting an initial spin period of P0≳ 5 ms.