Are there any isolated old neutron stars in the ROSAT Wide Field Camera survey?

• Published in: Monthly notices of the Royal Astronomical Society Vol. 278; no. 2; pp. 577 - 585
• Main Authors: Manning, R. A., Jeffries, R. D., Willmore, A. P.
• Format: Journal Article
• Language: English
• Published: Blackwell Science Ltd 11.01.1996
• Subjects: methods: numerical; stars: magnetic fields; stars: neutron; stars: statistics; ultraviolet: stars; ultraviolet: stars; methods: numerical; stars: magnetic fields; stars: statistics; stars: neutron
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/278.2.577

We present the results of Monte Carlo simulations to predict the number of isolated old neutron stars (IONs) that are observed in the ROSAT Wide Field Camera (WFC) survey as a result of accretion-powered extreme ultraviolet (EUV) emission. Magnetic field strengths of 109 to 1012 G are considered along with different models for the local interstellar medium. The most recent calibrations of the instrumental response and spatial sensitivity variations are used. Our conclusions are that as many as 10–20 IONs may have been observed if initial magnetic fields decay quickly to values ≤ 1010 G, but higher fields result in far fewer detections. On the basis of the latest source lists and optical identification information, which essentially rule out the presence of more than one or two IONs in the WFC survey, we conclude that one or all of the following is possible, (i) The number of IONs in the Galaxy is less than 109 or the number of accreting IONs may be depleted by accretion-prevention mechanisms. The latter is considered unlikely because the low-speed IONs that are most likely to have overcome the accretion barriers form the majority of the detected EUV sources, (ii) ION magnetic fields have not decayed to values ≤ 1010 G. (iii) The number of low-speed IONs may be fewer than expected, because of either dynamical heating or uncertainties in the birth distribution of pulsar speeds, (iv) The ION emission spectrum may be significantly non-Planckian, with a reduced flux in the WFC band.