Evolutionary history of the 6.1-ms pulsar

Boriakoff et al.¹ have reported the discovery of a 6.1-ms radio pulsar in a binary with an orbital period of 120 days, a₁ sin i = 0.99 × 10¹² cm, and e ~ 0. This is the fourth binary radio pulsar, and the third to have a nearly circular orbit². The origin of such systems has been discussed frequently^3,4 and I now report that evolutionary history of the new millisecond pulsar is perhaps the simplest of them all. The rapid rotation of the pulsar, small mass function, circular orbit, and long binary period suggest that following the formation of a neutron star there had to be a phase of mass transfer from the low-mass secondary onto the neutron star primary. That mass transfer circularized the orbit and spun up the primary, and it required the secondary to be a red giant at that time. The present binary period of 120 days implies that the secondary has a mass of 0.3 M_solar. The observed mass function of 0.0027M_solar implies that the primary must be less massive than 3.1M_solar, not an interesting mass limit for a neutron star. The secondary is probably a white dwarf at present. However, if the mass transfer terminated < 10⁶ yr ago then the secondary may be still burning hydrogen at the rate of about 300L_solar, and it may be optically detectable.