Where May Ultrafast Rotating Neutron Stars Be Hidden?

The existence of ultra-fast rotating neutron stars (spin period P < 1 ms) isexpected on the basis of current models for the secular evolution ofinteracting binaries, though they have not been detected yet. Their formationdepends on the quantity of matter accreted by the neutron star which, in turn,is limited by the mechanism of mass ejection from the binary. An efficient massejection can avoid the formation of ultra-fast pulsars or their accretioninduced collapse to a black hole. We propose that significant reductions of themass-transfer rate may cause the switch-on of a radio pulsar phase, whoseradiation pressure may be capable of ejecting out of the system most of thematter transferred by the companion. This can prevent, for long orbital periodsand if a sufficiently fast spin has been reached, any further accretion, evenif the original transfer rate is restored, thus limiting the minimum spinperiod attainable by the neutron star. We show that close systems (orbitalperiods P_orb \sim 1 hr are the only possible hosts for ultra-fast spinningneutron stars. This could explain why ultra-fast radio pulsars have not beendetected so far, as the detection of pulsars with very short spin periods inclose systems is hampered, in current radio surveys, by strong Dopplermodulation and computational limitations.Comment: 6 pages, including 1 figure. To appear in ApJ