Log N -- Log S distributions of accreting and cooling isolated neutron stars.pdf

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Log N -- Log S distributions of accreting and cooling isolated neutron stars.pdf

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Log N -- Log S distributions of accreting and cooling isolated neutron stars

a r X i v : a s t r o - p h / 0 0 0 9 2 2 5 v 2 2 0 J u n 2 0 0 3 Draft version February 1, 2008 Preprint typeset using LATEX style emulateapj v. 04/03/99 LOG N – LOG S DISTRIBUTIONS OF ACCRETING AND COOLING ISOLATED NEUTRON STARS S.B. Popov1, M. Colpi2, M.E. Prokhorov1, A. Treves3 and R. Turolla4 Draft version February 1, 2008 ABSTRACT We model populations of isolated neutron stars in the Galaxy following their orbital and magneto-rotational evolution. It is shown that accretors become more abundant than coolers at fluxes below ～ 10?13 erg cm?2 s?1, and one can predict that about one accreting neutron star per square degree should be observed at the Chandra and Newton flux limits of ～ 10?16 erg cm?2 s?1. The soft ROSAT sources associated with isolated neutron stars can be relatively young cooling objects only if the neutron star birth rate in the Solar vicinity during the last ～ 106 yr is higher than that inferred from radiopulsar observations. Subject headings: accretion, accretion disks — stars: kinematics — stars: magnetic fields — stars: neutron — stars: statistics — X–rays: stars 1. INTRODUCTION Despite intensive observational campaigns, no ir- refutable identification of an isolated accreting neu- tron star (NS) has been presented so far. Six soft sources have been found in ROSAT fields which are most probably associated to isolated radioquiet NSs. Present X-ray and optical data however do not allow an unambiguous identification of the physical mech- anism responsible for their emission. These sources can be powered either by accretion of the interstellar gas onto old (≈ 1010 yr) NSs or by the release of inter- nal energy in relatively young (≈ 106 yr) cooling NSs (see Treves et al. 2000 and Motch 2000 for recent re- views). The ROSAT candidates, although relatively bright (up to ≈ 1 ct s?1), are intrinsically dim and their inferred luminosity (L ≈ 1031 erg s?1) is near to that expected from either a close-by cooling NS or from an accreting NS among the