Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001ap%26ss.275..299n&link_type=abstract
Astrophysics and Space Science, v. 275, Issue 3, p. 299-318 (2001).
Astronomy and Astrophysics
Astrophysics
3
Scientific paper
We have discussed, in general, the important physical parameters, like maximum mass, radius, and the minimum rotation period of self-bound, causally consistent, and pulsationally stable neutron stars (Q-star models) by using a realistic stiff EOS (such that, the speed of sound, v~ P, or nP=K(E-E), where K<= 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, 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 cannot exceed ~ 1 nucleon/fm^3. By using the empirical formula given by Koranda, Stergioulas and Friedman (1997) (KSF-formula), and imposing constraint (i), we have obtained an upper bound of M ≅ 7.76 M radius a≅ 32.5 km, and the central energy-density around 2.17 × 10^14 g cm^-3 (for n =1.01). Constraint (ii) provides the minimum rotation period, P ~ 0.489 ms for the maximum mass M ~ 2.4 M_solar, and the central energy-density around 2.20 × 10^15 g cm^-3 (for n =1.01). The speed of sound at the centre of these models approaches ~ 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 maximum Kepler frequency imposed by the fastest rotating pulsar known to date (constraint (i)), in view of certain observational effects and theoretical evidences, and allow the present EOS to produce larger rotation rates than the 1.558 ms pulsar, the maximum mass of the non-rotating model drops down to a value ~ 7.2 M_solar. The higher values of masses (>= 7 M_solar) and radii (~ 31-32 km) obtained in this study imply that these models may represent 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 that the strongest contender for black hole at present might be recurrent nova V404 Cyg (mass estimate ~ 8 -12 M_solar).
Durgapal M. C.
Negi P. S.
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