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Astron. Astrophys. 359, 242-250 (2000)

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5. Conclusions

Estimates of the GW emission from the total pulsar population in the Galaxy require knowledge of the period and distance distributions. Selection and detection bias are strong and population synthesis methods are necessary in order to model these effects,to recover the "true" population properties from the observed distributions.

Pulsars are probably born with a range of periods and magnetic fields. Here we have assumed ad-hoc initial distributions for these parameters, which were optimized in order to reproduce actual data. Our numerical experiments suggest that the mean initial period at the pulsar birth is equal to 290 ms, confirming the conclusions of other simulations. Nevertheless, we predict about 60-90 single pulsars in the galaxy with periods less than 80 ms, as a consequence of the dispersion in the initial periods.

The pulsar population satisfying the condition P[FORMULA]0.4 s, able to contribute to the GW emission within the pass-band of VIRGO, amounts to about 5100-7800 objects. These numbers are considerably smaller than the estimate adopted by GBG97 (1.4[FORMULA]). Those authors assumed that the population with P[FORMULA] 0.4 corresponds to about 28% of the total population. The adopted fraction is that derived from cataloged pulsars. However, our simulations indicate that this fraction is only about 3.5%, exemplifying the consequences of the detection bias present in all surveys.

VIRGO will be able to detect a gravitational strain amplitude of h[FORMULA] with three years of integration. Under these conditions, our simulations indicate 2 detections if [FORMULA] and up to 12-18 detections if [FORMULA]. If the average ellipticity is smaller than 10-6, no detections are expected, at least with the presently planned antenna sensitivity. The total square amplitude resulting from this population is below the VIRGO threshold and a detectable signal would be produced only if the average ellipticity is at least of order of [FORMULA], a value which seems to be excluded by a recent re-analysis of the magnetic and gravitational torques in some (observed) young pulsars (Palomba 1999).

We emphasize that the present results concern uniquely the radio pulsar population and the contribution of a binary millisecond population to the continuous GW emission remains to be estimated. These simulations are more difficult,since the rotation period evolution has a complex history, including decelerating and accelerating phases, which will be discussed in a future paper.

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© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000