Black Widow Pulsars: the Price of Promiscuity

Abstract

The incidence of evaporating 'black widow' pulsars (BWPs) among all millisecond pulsars (MSPs) is far higher in globular clusters than in the field. This implies a special formation mechanism for them in clusters. Cluster MSPs in wide binaries with WD companions exchange them for turnoff-mass stars. These new companions eventually overflow their Roche lobes because of encounters and tides. The millisecond pulsars eject the overflowing gas from the binary, giving mass loss on the binary evolution timescale. The systems are only observable as BWPs at epochs where this evolution is slow, making the mass loss transparent and the lifetime long. This explains why observed BWPs have low-mass companions. We suggest that at least some field BWPs were ejected from globular clusters or entered the field population when the cluster itself was disrupted.