3.5 Going further3 The Galactic Pulsar Population3.3 The Population of Normal

3.4 The Population of Neutron Star Binaries 

Neutron star-neutron star (NS-NS) binaries similar to the classic Hulse-Taylor pulsar B1913+16 are of great interest, since they are expected to coalesce due to the emission of gravitational radiation. Only during the final few seconds of coalescence are gravitational waves strong enough to detect [138]. As noted in § 2.4, NS-NS systems are expected to be rare since the binary system has to survive two supernova explosions. However, by including galaxies out to distances of tex2html_wrap_inline2075 Mpc, it is hoped that merging NS-NS binaries will prove numerous enough to make a detection likely by the present and future generation of gravitational wave detectors [153, 139].


Of key importance here is the rate tex2html_wrap_inline2099 of NS-NS mergers in the Galaxy which can then be extrapolated to more distant galaxies to provide event rate predictions for gravity wave detectors [125Jump To The Next Citation Point In The Article]. Statistical studies of the Galactic population of NS-NS systems are, however, hampered by small number statistics. The present sample summarised in Table 1Popup Equation includes only three systems which have merging times significantly smaller than a Hubble time-scale viz: B1913+16 [78], B1534+12 [166Jump To The Next Citation Point In The Article], which are in the disk of our Galaxy, and B2127+11C which is located in the Globular Cluster M15 [128].

Early estimates indicated a Galactic rate as high as tex2html_wrap_inline2101 yr tex2html_wrap_inline1837 [45]. These rather optimistic estimates had to be reduced by a factor of 30 [117, 125] following revised scale factor calculations based on the surveys that discovered PSRs B1534+12 and B2127+11C. These estimates implied that many new NS-NS systems should be found by the all-sky millisecond pulsar surveys [50Jump To The Next Citation Point In The Article]. This was surprisingly not the case; the only discovery being PSR J1518+4904 -- a mildly relativistic binary system that will merge on a time-scale of tex2html_wrap_inline2105 yr [120Jump To The Next Citation Point In The Article]. The most recent estimates of the NS-NS population, which take into account the lack of detections by the all-sky surveys, place a lower limit on the Galactic NS-NS population of tex2html_wrap_inline2107 and a merging rate of tex2html_wrap_inline2109 yr tex2html_wrap_inline1837 [50, 160].

It is important to stress that these estimates are insensitive to systems fainter than some defined luminosity limit and therefore only provide a lower limit to tex2html_wrap_inline2099 . Bailes [22] proposed a means of estimating an upper bound on tex2html_wrap_inline2099 . Bailes postulates that the birth rates of normal pulsars in NS-NS systems must be equal to those of recycled pulsars in NS-NS systems. We know of only one NS-NS system in which the normal pulsar is visible: B2303+46 (see Table 1Popup Equation). This system will, however, not merge within a Hubble time-scale. In this case, tex2html_wrap_inline2099 cannot be larger than tex2html_wrap_inline2119, where tex2html_wrap_inline1997 is the number of observed normal pulsars and tex2html_wrap_inline2057 is their birth rate. Inserting the most recent numbers into this equation (tex2html_wrap_inline2125 ; tex2html_wrap_inline2127 yr tex2html_wrap_inline1837) yields an upper limit of the formation rate of merging NS-NS binaries of tex2html_wrap_inline2131 yr tex2html_wrap_inline1837 .

An independent method of estimating tex2html_wrap_inline2099 can be made via population syntheses of binary stars [89, 55, 132, 158]. The essence of this approach is to follow the orbital and stellar evolution of a large number (tex2html_wrap_inline2137) of binary star systems of varying mass and orbital separation. Based on a number of plausible physical arguments it is possible to predict the relative fractions of the various types of binary systems containing neutron stars. The most recent estimates using this method [92, 127] find tex2html_wrap_inline2099 tex2html_wrap_inline2141 yr tex2html_wrap_inline1837 . Given the uncertainties involved, we conclude that both methods yield consistent results i.e. :


Extrapolations including Galaxies out to tex2html_wrap_inline2075 Mpc suggest that the rate of NS-NS mergers will be high enough to yield detection rates of tex2html_wrap_inline1855 several sources per year. Ultimately, the detection statistics from the gravity wave detectors should provide far tighter constraints on the NS-NS merging rate.

3.5 Going further3 The Galactic Pulsar Population3.3 The Population of Normal

image Binary and Millisecond Pulsars
D. R. Lorimer (dunc@mpifr-bonn.mpg.de)
© Max-Planck-Gesellschaft. ISSN 1433-8351
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