Implications of a half-millisecond pulsar

New models of rotating neutron stars show that only a surprisingly narrow range of possible equations of state (EOS’s) can simultaneously allow a rotating neutron star with frequency as large as 1968 Hz and a spherical (nonrotating) neutron star with mass as large as 1.44${\mathrm{M}}_{\mathrm{\odot }}$. The mass and baryon mass for the 1968-Hz models exceed 1.5${\mathrm{M}}_{\mathrm{\odot }}$ and 1.7${\mathrm{M}}_{\mathrm{\odot }}$, implying a progenitor mass >1.7${\mathrm{M}}_{\mathrm{\odot }}$. Those EOS’s that allowed 1968-Hz models have, for spherical stars, a stringent upper mass limit <1.7${\mathrm{M}}_{\mathrm{\odot }}$. Each model at 1968 Hz has mass above the spherical upper mass limit for its EOS, implying collapse upon spin down.