The Neutron Star Census

Abstract

The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity 0 ≤ V ≤ 550 km s −1 . The spin-down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky Survey an upper limit of ∼ 10 accreting neutron stars within ∼ 140 pc from the Sun, we infer a lower bound for the mean kick velocity, V ∼ > 200 − 300 km s −1 , corresponding to a velocity dispersion σ V ∼ > 125−190 km s −1 . The same conclusion is reached for both a constant magnetic field (B ∼ 10 12 G) and a magnetic field decaying exponentially with a timescale ∼ 10 9 yr. Such high velocities are consistent with those derived from radio pulsar observations. Present results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to less than 1%.

The Magnificent Seven in the dusty prairie

“…As noted above, there are other more recent theoretical 3D extinction models. However, the analytical H density model by Popov et al (2000) seems to be at least as good as e.g. the underlying model used by Amôres & Lépine (2005).…”

“…The galactic coordinate x varies from 50 to 2500 pc (direction: from the Sun towards the galactic center), y is always 0, z is 25 pc north . For the ISM distribution, we applied the same as used by Popov et al (2000), which is described in more detail below. The effect of less absorption, resulting from the use of the newer routine 'tbabs', is stronger at larger column densities, hence distances.…”

“…It is well suited for statistical studies. The analytical model we apply is described by Popov et al (2000). It is based on the formulae by (Zane et al 1995;Dickey & Lockman 1990;de Boer 1991).…”

“…It is based on the formulae by (Zane et al 1995;Dickey & Lockman 1990;de Boer 1991). Popov et al (2000) included additionally a galactocentric radius dependency for the number density of atomic and molecular hydro- Bochkarev (1992). The Local Bubble was taken into account by Popov et al (2000) as a sphere of 140 pc radius having a constant low density of 0.1 particles cm −3 .…”

“…Popov et al (2000) included additionally a galactocentric radius dependency for the number density of atomic and molecular hydro- Bochkarev (1992). The Local Bubble was taken into account by Popov et al (2000) as a sphere of 140 pc radius having a constant low density of 0.1 particles cm −3 . As noted above, there are other more recent theoretical 3D extinction models.…”