Log N – Log S Distributions of Accreting and Cooling Isolated Neutron Stars

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 ∼ 10 6 yr is higher than that inferred from radiopulsar observations. Despite intensive observational campaigns, no irrefutable identification of an isolated accreting neutron 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 mechanism responsible for their emission. These sources can be powered either by accretion of the interstellar gas onto old (≈ 10 10 yr) NSs or by the release of internal energy in relatively young (≈ 10 6 yr) cooling NSs (see Treves et al. 2000 and Motch 2000 for recent reviews). The ROSAT candidates, although relatively bright (up to ≈ 1 ct s −1), are intrinsically dim and their inferred luminosity (L ≈ 10 31 erg s −1) is near to that expected from either a close-by cooling NS or from an accreting NS among the most luminous. Their X-ray spectrum is soft and thermal, again as predicted for both accretors and coolers (Zane, Tur-olla & Treves 2000; Treves et al. 2000). Up to now only two optical counterparts have been identified (RXJ 1856, Walter & Matthews 1997, for which a distance estimate of ∼ 60 pc has been very recently obtained, Walter 2000, and RXJ 0720, Kulkarni & Van Kerkwick 1998). In both cases an optical excess over the low-frequency tail of the black body X-ray spectrum has been reported. While detailed multi-wavelength observations with next-generation instruments may indeed be the key for assessing the true nature of these sources, other, indirect, approaches may be used to discriminate in favor of one of the two scenarios proposed so far. In this Letter we compute and compare the log N – log S distribution of both accreting …