Astronomy and Astrophysics – Astrophysics
Scientific paper
1998-02-03
Astronomy and Astrophysics
Astrophysics
10 pages Latex, 5 ps figures. To be formatted with the AASTeX package. Accepted for publication in ApJ
Scientific paper
10.1086/305810
Old neutron stars (ONSs) which have radiated away their internal and rotational energy may still shine if accreting the interstellar medium. Despite their large number, only two promising candidates have been detected so far and rather stringent limits on their observability follow from the analysis of ROSAT surveys. This contrasts with optimistic theoretical estimates that predicted a large number of sources in ROSAT fields. We have reconsidered the issue of ONSs observability, accounting for the spin and magnetic field evolution over the neutron star lifetime. In the framework of a spin-induced field decay model, we show that the total number of ONSs which are, at present, in the accretion stage is reduced by a factor ~5 over previous figures if the characteristic timescale for crustal current dissipation is ~ 10^8 - 10^9 yr. This brings theoretical predictions much closer to observational limits. Most ONSs should be at present in the propeller phase and, if subject to episodic flaring, they could be observable.
Colpi Monica
Treves Aldo
Turolla Roberto
Zane Silvia
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