2026ApJ..1000..166T

Rapid uniformly rotating neutron stars are expected to be formed, for instance, in the collapse of some massive stars, the accretion of compact object binaries, and double neutron star mergers. The huge amount of rotational energy has been widely believed to be the source of some cosmic gamma-ray bursts and superluminous supernovae. Benefiting from the constraints on the equation of state of the neutron star matter set by the latest multimessenger data, the chiral effective field theory, and perturbative quantum chromodynamics, here we present the maximum gravitational mass as well as the rotational energy for a neutron star at a given spin period. Our nonparametric equation-of-state analysis reveals that the critical Keplerian configurations (${{\rm{\Omega }}}_{{\rm{kep}}}^{{\rm{crit}}}=1.0{2}_{-0.07}^{+0.06}\times 1{0}^{4}\,{{\rm{rad}}\,{\rm{s}}}^{-1}$) can sustain maximum gravitational masses of ${M}_{\mathrm{kep}}^{\mathrm{crit}}=2.73\pm 0.09\,{M}_{\odot }$ with the corresponding rotational energy reaching ${E}_{{\rm{rot}},{\rm{kep}}}^{{\rm{crit}}}=2.3{6}_{-0.22}^{+0.24}\times 1{0}^{53}$ erg. However, the maximum rotational energy that can be feasibly extracted from a neutron star is limited to $1.4{0}_{-0.13}^{+0.14}\times 1{0}^{53}$ erg, which holds for a baryon mass of $2.6{6}_{-0.09}^{+0.10}\,{M}_{\odot }$. All these parameters, obtained via the nonparametric reconstruction of the equation of state, are at the 68.3% confidence level, and the adoption of a quarkonic model yields rather similar results. These findings are found to have already set some intriguing constraints on the millisecond magnetar interpretation of some exciting data.