Astron. Astrophys. 331, 535-540 (1998)

1. Introduction

Old isolated neutron stars (NS) and black holes (BH) form a large populations of galactic objects (about - objects in the Galaxy), but most of them are unobserved today. Less than young NS appear as radio pulsars, and no isolated BH has been observed (probably, some of them are detected, for example, in the ROSAT survey, but no one is identified). This article will be concerned only about isolated compact objects, that will simply designated as NS or BH.

During the last years, the spatial distribution and other properties of NS became of great interest, because NS can be observed by the ROSAT satellite in soft X-rays due to accretion from the interstellar medium (ISM) (see, for example, Treves & Colpi 1991). Several sources of this type have been observed (Walter et al. 1996). BH also can appear as similar X-ray sources (Heckler & Kolb 1996) with some differences in spectrum and temporal behaviour (absence of pulsations, for example). That is why we try here to obtain a picture of the distribution of the accretion luminosity of these sources.

Fast rotation and/or a strong magnetic field can prevent accretion onto the surface of the NS. In this case the X-ray luminosity will be very low (except for transient sources due to the formation of an envelope around the NS: see Popov 1994 and Lipunov & Popov 1995). Here we consider only accreting NS. Most NS are in the stage of accretion, because their magneto-rotational evolution usually finishes at this stage approximately years after their birth. The NS properties (periods etc.) in the stage of accretion depend upon the magnetic field decay (see Konenkov & Popov 1997). BH, of course, can only be seen as accretors.

In the articles of Gurevich et al. (1993)and of Prokhorov & Postnov (1993, 1994) it was shown that the population of NS forms a ring (or toroidal) structure in the Galaxy. The distribution of the ISM (see, for example, Bochkarev 1992) also has a ring structure. The maxima of both distributions roughly coincide.

Therefore, most of the NS (and probably BH) are located in the dense regions of the ISM. Thus the accretion luminosity in these regions should be high. The results of computer simulations of this situation are presented in this paper.

The trajectories of NS and BH were computed directly for a specified initial velocity distribution, the Galaxy gravitational potential and the distribution of the ISM density. Preliminary results of such computations for NS for - function and maxwellian velocity distributions were presented in Popov & Prokhorov (1998, paper I).

In Sect. 2 we briefly describe our model. In Sect. 3 the results and a short discussion are presented. The last section contains the conclusions.

© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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