1. Black Widow Pulsars(BWP): the Price of Promiscuity A. R. King, M. B. Davies and M. E. Beer, 2003,MNRAS,345,678 Fu Lei 2010.04.12

2. Outline Introduction Widowhood Companion exchange Binary bereavement BWP in the field Conclusions

3. Introduction Formation of millisecond pulsars(MSP) Accretion spin up Accretion ceases and pulse appears First BWP PSR1957+20 discovered in 1988 by Fruchter et al. (Fruchter, A.S., Stinebring, D.R. and Taylor, J.H. 1988, Nature 333, 237.)

4. MSP in globular clusterMSP in the field

5. Why the BWPs favour in globular cluster? Author suggest here that is because the ability of partner exchange in GC. Consequences Leave the most massive star within binaries. Subsequent evolution depends on the mass of the donor and the separation of the two star when donor fills its Roche lobe(RL). IMXB or MSP

6. Widowhood and Companion exchange BWP ingredients Recycling by accretion torques and spun up to ms periods Accretion must stop to attains a near-vacuum magnetosphere  Search for pulsed radio emission from quiescent soft X- ray transients yield no results even  Companion detaches from its RL Companion which be ablated

7. MSP exchange it's unpromising WD partner with a potential BWP victim Encounter make wide binaries to be left with high e then circularize with final separation The outcome for all binaries which undergo exchanges is thus a close binary in a circular orbit contains MSP and a MS star close to the cluster turn-off mass

8. Binary bereavement Roche lobe radius evolution Stellar radius evolution For nuclear or thermal expansion For radius change results from mass loss

9. Mass loss time scale The evolution of the system depend on whether the mass loss timescale longer or shorter than angular momentum loss timescale or a nuclear or thermal timescale If we see that the companion fill the RL on timscale or

10. Expect the matter which flowing through the Lagrange point L1 can be blow away Comparing the ram pressure of the matter and the pressure of the pulsar wind thus companion with mass fill the RL at so with R of this order

11. The process stabilize when or so or BWP loses mass on a stellar expansion or angular momentum loss timescale Constraints on the observability of BWP mass loss may obscure the pulsar i.e. by increasing the DM

12. Free-free optical depth at typical radio frequencies 0.4-1.7 GHz is >=1 for mass loss rates This evidently correspond to cases of very low mass loss. Since mass loss timescale is also the binary evolution timescale, so the lifetime of such system is so long that increasing their discovery probability.

13. BWPs in the field&Conclusions Occam's razor suggests that we should not look for a second formation mechanism, but a variation on the first. BWPs made in GC ejected by some dynamical event (or the GC was disrupted) Captures may occur in the field (very unlikely because the low space density of stars in the field) Two BWPs in the field namely 1957+20 and 2051-0827. Tangential velocity and galactic latitude is 190km/s, -5.2° and 14km/s, -30.4° respectively. It's more probable that these two systems have either been ejected from a cluster or that the clusters themselves have been broken up, though it's not impossible that both objects formed from encounters in the field.

14. The difficulty of the BWP formation Attempts to use the same companion which spun up the pulsar to ms periods as the target which be ablates run into difficulty. The fact that the incidence of BWPs in GC is much higher than in the field make the companion exchange a reasonable explanation.

15. ms Pulsar with WD companion Ms Pulsar with a turn-off mass star Encounters and tides bring the system into tight orbits Companion fill the RL, mass loss on binary evolution timescale. In the meantime material ejected by ms Pulsar. Exchange encounter The systems are observable only the mass loss rate is low.

16. Detached evaporating Pulsar's total energy emission is The amount of energy which driven the wind from the companion is thus Orbit period ~5hr

18. Thanks !