  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 ($B > 10^{14}$ 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 (~3ms), which results in a powerful dynamo action, amplifying the seed
magnetic field to $> ~10^{15}$ G. Part of this rotation energy is then expected
to power the supernova through rapid magnetic braking. It is therefore possible
that magnetars 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 $10^{51}$~erg, suggesting an initial
spin period of > ~5ms.
  We therefore do not find evidence that magnetars are formed from rapidly
rotating proto-neutron stars, allowing for the possibility that they descend
from stellar progenitor with high magnetic field cores, and we discuss the
merits of both formation scenarios.
  In an appendix we describe the analysis of XMM-Newton observations of Kes 73
and N49 used to derive the explosion energies for these remnants.
