Fast rotating neutron stars with realistic nuclear matter equation of state

Abstract

We construct equilibrium configurations of uniformly rotating neutron stars for selected relativistic mean-field nuclear matter equations of state (EOS). We compute in particular the gravitational mass ($M$), equatorial ($R_{\rm eq}$) and polar ($R_{\rm pol}$) radii, eccentricity, angular momentum ($J$), moment of inertia ($I$) and quadrupole moment ($M_2$) of neutron stars stable against mass-shedding and secular axisymmetric instability. By constructing the constant frequency sequence $f=716$ Hz of the fastest observed pulsar, PSR J1748-2446ad, and constraining it to be within the stability region, we obtain a lower mass bound for the pulsar, $M_{\rm min}=[1.2$-$1.4] M_\odot$, for the EOS employed. Moreover we give a fitting formula relating the baryonic mass ($M_b$) and gravitational mass of non-rotating neutron stars, $M_b/M_\odot=M/M_\odot+(13/200)(M/M_\odot)^2$ [or $M/M_\odot=M_b/M_\odot-(1/20)(M_b/M_\odot)^2$], which is independent on the EOS. We also obtain a fitting formula, although not EOS independent, relating the gravitational mass and the angular momentum of neutron stars along the secular axisymmetric instability line for each EOS. We compute the maximum value of the dimensionless angular momentum, $a/M\equiv c J/(G M^2)$ (or "Kerr parameter"), $(a/M)_{\rm max}\approx 0.7$, found to be also independent on the EOS. We compare and contrast then the quadrupole moment of rotating neutron stars with the one predicted by the Kerr exterior solution for the same values of mass and angular momentum. Finally we show that, although the mass quadrupole moment of realistic neutron stars never reaches the Kerr value, the latter is closely approached from above at the maximum mass value, as physically expected from the no-hair theorem. In particular the stiffer the EOS is, the closer the Kerr solution is approached.