Equation of state of dense nuclear matter and an upper bound on neutron star masses

We have discussed, in general, the important physical parameters, like maxi mum mass, radius, and the minimum rotation period of self-bound, causally c onsistent, and pulsationally stable neutron stars (Q-star models) by using a realistic stiff EOS (such that, the speed of sound, v proportional to P-N , or nP=K(E-E-a)(n), where K less than or equal to 1 and n =1/(1-2N); where P and E represent respectively, the pressure and the energy-density, and E is the value of E at the surface (r = a) of the configuration) within the two constraints imposed by: (i) The minimum rotation period, P-rot, for the pulsar known to date corresponds to 1.558 ms, and (ii) The maximum number density anywhere inside the structure for the models described as Q-stars c annot exceed similar to 1 nucleon/fm(3). By using the empirical formula giv en by Koranda, Stergioulas and Friedman (1997) (KSF-formula), and imposing constraint (i), we have obtained an upper bound of M-max congruent to 7.76 M-. radius a congruent to 32.5 km, and the central energy-density around 2. 17 x 10(14) g cm(-3) (for n =1.01). Constraint (ii) provides the minimum ro tation period, P-rot similar to 0.489 ms for the maximum mass M-max similar to 2.4 M-., and the central energy-density around 2.20 x 10(15) g cm(-3) ( for n =1.01). The speed of sound at the centre of these models approaches s imilar to 99% of the speed of light 'c' (in the vacuum) and vanishes at the surface of the configuration together with pressure. If we relax the maxim um Kepler frequency imposed by the fastest rotating pulsar known to date (c onstraint (i)), in view of certain observational effects and theoretical ev idences, and allow the present EOS to produce larger rotation rates than th e 1.558 ms pulsar, the maximum mass of the non-rotating model drops down to a value similar to 7.2 M-. The higher values of masses (greater than or equal to 7 M-.) and radii (sim ilar to 31-32 km) obtained in this study imply that these models may repres ent the massive compact objects like Cyg X-1, Cyg XR-1, LMC X-3, and others which are known as black hole candidates (BHCs). This study also suggest t hat the strongest contender for black hole at present might be recurrent no va V404 Cyg (mass estimate similar to 8 -12 M-.).