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

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1. Introduction

The detection of gravitational microlensing (Paczyski 1986 ) of stars in the Large Magellanic Cloud (LMC) by the EROS (Aubourg et al. 1993 ) and the MACHO (Alcock et al. 1993 ) collaborations shows that MACHOs (Massive Astrophysical Compact Halo Objects) provide a substantial fraction of the dark matter present in the galactic halo (see e.g. De Rújula, Jetzer & Massó 1992 ). Based on the 6 to 8 microlensing events found so far, the MACHO team (1996 ) announced that MACHOs contribute a fraction as high as 50% to the halo dark matter.

Quite recently we have proposed a scenario in which the formation of dark clusters composed by MACHOs of mass [FORMULA] and cold molecular clouds naturally arises in the outer part (beyond [FORMULA] kpc) of the galactic halo (De Paolis et al. 1995a, 1995b, 1996a ), while in the inner part of the galaxy stellar globular clusters arise.

Besides dark clusters, the galactic halo could also contain two populations of neutron stars: [FORMULA] neutron stars originating in the disk and ejected in the halo (due to their high kick velocities) and a second population of [FORMULA] neutron stars originating with lower velocities in globular clusters (via Type II supernovae and/or accretion induced collapse of white dwarfs) and also ejected in the halo (due to the low escape velocity from globular clusters). Here we suggest that when these halo neutron stars cross dark clusters passing through molecular clouds, accrete material and emit in the X-ray band.

That old neutron stars can be revealed in the X-ray band due to the accretion of interstellar matter and subsequent X-ray emission has been suggested long ago by Ostriker, Rees & Silk (1970 ). The problem of the detectability of disk neutron stars has been faced by several authors (Treves & Colpi 1991 , Blaes & Madau 1993 and Madau & Blaes 1994 ) who also recognized that the most favourable sites for the detection of accreting neutron stars are giant molecular clouds in the galactic disk (Colpi, Campana & Treves 1993 ). However, despite the increasing efforts, only a few X-ray sources have been identified until now as neutron stars accreting the interstellar medium (Walter, Wolk & Neuhäuser 1996 ).

The existence of a new population of yet unrecognized X-ray sources has been recently suggested by an analysis of very deep ROSAT observations which revealed a considerable excess of faint X-ray sources over that expected by Active Galactic Nuclei (AGN) evolution models (Hasinger et al. 1993 ). The possibility that this new population of X-ray sources is galactic in origin has been considered by Maoz & Grindlay (1995 ), which derived the characteristic properties that the unknown population should have to contribute significantly to the diffuse unresolved X-ray background (XRB). The contribution to the XRB of old neutron stars confined in the galactic plane has been also calculated, assuming a given model for this population and taking into account interstellar X-ray absorption (Zane et al. 1995 ).

ROSAT observations have also revealed a new and distinct class of supersoft X-ray sources with luminosities up to [FORMULA] erg s-1 (Kahabka, Pietsch & Hasinger 1994 ). Some of these sources have been optically identified with close binary systems involving mass transfer from a main-sequence star onto a white dwarf burning the accreted matter.

Our main point is that to these populations of X-ray sources can contribute substantially two distinct classes of neutron stars, spherically distributed in the galactic halo and accreting matter from molecular clouds (with mean density between [FORMULA] cm-3) inside dark clusters. The first class consists of halo neutron stars (HNS) passing through dark clusters with high ([FORMULA] 400 km s-1) velocity and producing an X-ray luminosity of [FORMULA] erg s-1. The second class consists of neutron stars bound in dark clusters (DCNS) which continuously emit in the X-ray band and that, due to the low ([FORMULA] 15 km s-1) velocity with respect to the accreting matter, have higher luminosity of [FORMULA] erg s-1. We find that HNS may contribute significantly to the diffuse XRB and that some of the supersoft X-ray sources which cannot be identified with any class of known objects can be DCNS.

The presence of accreting neutron stars in the galactic halo could be also tested with observations in the infrared band, because it is well possible that X-rays are (may be in part) absorbed by the surrounding matter and re-emitted in the infrared band. Correlated observations in the radio and X-ray bands could make sure the identification of halo neutron stars still active as pulsars. It goes without saying that the detection of this kind of X-ray halo sources will also provide a further test of the previously proposed model for the galactic dark matter.

In the following Sect. 2 we briefly review our scenario for the dark cluster formation and in Sect. 3 we assess the arguments for the presence of neutron stars in the galactic halo. The expected number of HNS and DCNS and the corresponding X-ray luminosities are discussed in Sect. 4, while the detectability of these sources is faced in Sect. 5. Relevant ROSAT X-ray observations are analyzed in Sect. 6 and, finally, our conclusions are offered in Sect. 7.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998