ChandraStudy of a Complete Sample of Millisecond Pulsars in 47 Tucanae and NGC 6397

We report Chandra observations of the complete sample of millisecond pulsars(MSPs) with precise radio positions in the globular clusters 47Tuc (NGC104) andNGC6397. The 47Tuc MSPs are predominantly soft sources suggestive of thermalemission from small (r_x < 0.6km) polar caps on the neutron star rather thanmagnetospheric emission and are a relatively homogeneous sample, with mostx-ray luminosities in a surprisingly narrow range (Lx ~1-4 X 10^30 erg/s). Weuse previously derived intrinsic Pdot values and find a new relation between Lxand spindown luminosity, Edot: Lx ~ Edot^beta, with beta ~0.5 vs. ~1.0 for bothpulsars and MSPs in the field. This Lx-Edot relation and also the Lx/Edot vs.spindown age are each similar to that found by Harding & Muslimov (2002) forthermal emission from polar cap heating although the cluster MSPs arerelatively longer-lived (in thermal x-rays) than either the models or fieldMSPs. We suggest the cluster MSPs may have altered surface magnetic fieldtopology (e.g.multipole) or their neutron stars are more massive from repeatedaccretion episodes due to encounters and repeated exchange interactions. MSPbinary companions on or just off the main sequence are likely to have beenre-exchanged and might show anomalous Pdot and Edot values due to relaxation ofmisaligned core-crust spins. The radial distribution of 40 soft Chandra sourcesin 47Tuc is consistent with a 1.4Msun component. The implied total MSPpopulation in 47Tuc with Lx >10^30 erg/s is ~35-90, and can constrain therelative beaming in radio vs. soft x-rays. NGC6397 is relatively deficient inMSPs; its single detected example may have been re-exchanged out of the clustercore. (abridged)Comment: 16 pages, 11 figures, 5 tables. Accepted for Ap