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Astron. Astrophys. 321, 696-702 (1997)

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3. Halo neutron stars

Recently it has been reported that radio pulsars are born in the galactic disk with very high velocities (Lyne & Lorimer 1994 ). If neutron stars have kick velocities of [FORMULA] km s-1 as implicated by radio pulsar observations (Hartmann et al. 1995 ), thus most of them have been ejected from the disk and today populates the galactic halo. Assuming a constant supernova rate of [FORMULA] yr-1, one can estimate the presence of [FORMULA] neutron stars in the galactic halo 1. Some of them, which we named HNS, are crossing today dark clusters accreting matter in molecular clouds and thus emitting in the X-ray band.

Moreover, it is also expected that a second population of neutron stars, born in globular clusters, should exist in the galactic halo. Arguments from both the initial stellar mass function and the observations of a relatively high number of radio pulsars in globular clusters lead to an estimation of [FORMULA] neutron stars born in globular clusters with velocities less than 100 km s-1 and thus slower then disk pulsars 2. Since the typical escape velocity of a globular cluster is [FORMULA] km s-1, their retention fraction is less then a few percent so that very few neutron stars remain in globular clusters (Drukier 1996 ). These neutron stars today populate the galactic halo and are expected to be in virial or near virial equilibrium in the galactic gravitational well.

What changes with respect to the neutron stars ejected from the disk is that in the case of neutron stars ejected from globular clusters there is an appreciable chance that some of these objects are captured by the dark clusters (this essentially happens due to the lower relative velocity between dark clusters and neutron stars), making a continuous accretion of the gas in molecular clouds. A neutron star can be captured by a dark cluster only if the relative velocity of these objects is less then the internal dark cluster velocity dispersion ([FORMULA] km s-1). Assuming a maxwellian velocity distribution with [FORMULA] km s-1 (corresponding to a flat rotation velocity [FORMULA] km s-1), we find that at least a few percent of the neutron stars born in globular clusters are today bound in dark clusters. We refer to this class of [FORMULA] neutron stars as DCNS.

Clearly, we expect very different X-ray features for the two classes of HNS and DCNS as a consequence of both their different velocity with respect to the accreting matter and the time of permanence within dark clusters 3.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998