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Astron. Astrophys. 327, 428-431 (1997)

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1. Introduction

Among possible sources of gravitational radiation (GR), neutron stars (NS), both single and entering binary systems, are considered as most promising (see Thorne 1987; Abramovici et al. 1992; Schutz 1997 for full review). NS are the end product of evolution of massive ([FORMULA] -10 [FORMULA]) stars, so their number in the Galaxy should amount to [FORMULA] [FORMULA] over the galactic lifetime of [FORMULA] [FORMULA] years. The number of binary NS is more controversial. Simple estimate based upon the binary pulsar statistics (Phinney 1991; Curran & Lorimer 1995; van den Heuvel & Lorimer 1996) yield the coalescence rate of binary NS in the Galaxy of order 1 per [FORMULA] years, so to have an acceptable detection rate of these sources one must have a detector sensitivity of at least [FORMULA] - [FORMULA] at the frequency 100 Hz, which the initial laser inetrferometers currently under construction are aimed at. Theoretical estimates of the binary NS coalescence rate are typically an order of magnitude higher (1 per [FORMULA] years), and the possible solution of the discrepancy may be connected with the NS not being observed as a radiopulsar in a binary NS+NS system (see Lipunov, Postnov & Prokhorov (1997) for more detail).

The situation with single NS, however, differs from binary NS in that the latter are the most relaible sources of GR, which is confirmed by observations of the binary pulsar PSR 1913+16 orbit decay (Taylor 1992), whereas to be a noticable source of GR, the form of an isolating rotating NS must deviate from axisymmetry. This deviation is usually described in terms of the relative difference of moment inertia along the different axis of the non-spherical body of the star, [FORMULA], where [FORMULA] is the semimajor axis of the equatorial section, and [FORMULA] the semiminor axis.

In the last years, different mechanisms of symmetry bracking have been proposed for young NS (see, e.g., Lai & Shapiro 1995; Bonazzola, Frieben & Gourgoulhon 1996). It has also been suggested (Zimmermann 1978; Gal'tsov, Tsvetkov & Tsirulev 1984; Bonazzola & Gourgoulhon 1996) that an internal strong magnetic field ([FORMULA] - [FORMULA] G) may cause the asymmetrical shape of the NS. It has been shown by Blair (1996) that the asymmetry of young NS may lead to appearance of a stochastic GR background at frequencies 1-300 Hz, and provided that the supernova explosions in the entire Universe are frequent enough, it can be marginally detected with the advanced LIGO/VIRGO interferometer. Recently, Giazotto, Bonazzola & Gourgoulhon (1997) studied the possibility of the detection of the GR background generated by all old NS in the Galaxy with only one GW interferometer using a quadratic detection technique. We are highly ignorant about old NS distribution in the Galaxy. However, old NS may populate an extensive halo around the galaxy ([FORMULA] 100-300 kpc even without significant kick velocity; see e.g. calculations by Gurevich et al. (1993); Prokhorov & Postnov (1994)), so the modulation of the signal by Earth rotation considered by Giazotto et al. (1997) could be smeared out, making quadratic detection by one GW interferometer questionable.

In this paper, we consider the stochastic GW background produced by the old NS population in the Galaxy and beyond taking into account spin-down evolution of old NS. We show that in the limiting case of angular mometum loss only due to GR, the upper limit on the GR background formed by old NS is determined by NS production rate only. We also briefly discuss the possibility of its detection in future GR experiments.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998