  Magnetars are regarded as the most magnetized neutron stars in the Universe.
Aiming to unveil what kinds of stars and supernovae can create magnetars, we
have performed a state-of-the-art spatially resolved spectroscopic X-ray study
of the supernova remnants (SNRs) Kes 73, RCW 103, and N49, which host magnetars
1E 1841-045, 1E 161348-5055, and SGR 0526-66, respectively. The three SNRs are
O- and Ne-enhanced and are evolving in the interstellar medium with densities
of >1--2 cm$^{-3}$. The metal composition and dense environment indicate that
the progenitor stars are not very massive. The progenitor masses of the three
magnetars are constrained to be < 20 Msun (11--15 Msun for Kes 73, < 13 Msun
for RCW 103, and ~13 --17 Msun for N49). Our study suggests that magnetars are
not necessarily made from very massive stars, but originate from stars that
span a large mass range. The explosion energies of the three SNRs range from
$10^{50}$ erg to ~2$\times 10^{51}$ erg, further refuting that the SNRs are
energized by rapidly rotating (millisecond) pulsars. We report that RCW 103 is
produced by a weak supernova explosion with significant fallback, as such an
explosion explains the low explosion energy (~$10^{50}$ erg), small observed
metal masses ($M_{\rm O}\sim 4\times 10^{-2}$ Msun and $M_{\rm Ne}\sim 6\times
10^{-3}$ Msun), and sub-solar abundances of heavier elements such as Si and S.
Our study supports the fossil field origin as an important channel to produce
magnetars, given the normal mass range ($M_{\rm ZAMS} < 20$ Msun) of the
progenitor stars, the low-to-normal explosion energy of the SNRs, and the fact
that the fraction of SNRs hosting magnetars is consistent with the magnetic OB
stars with high fields.
