Chandra Study of a Complete Sample of Millisecond Pulsars in 47Tuc and NGC6397

Abstract

We report Chandra observations of the complete sample of millisecond pulsars (MSPs) with precise radio positions in the globular clusters 47Tuc (NGC104) and NGC6397. The 47Tuc MSPs are predominantly soft sources suggestive of thermal emission from small (r_x < 0.6km) polar caps on the neutron star rather than magnetospheric emission and are a relatively homogeneous sample, with most x-ray luminosities in a surprisingly narrow range (Lx ~1-4 X 10^30 erg/s). We use previously derived intrinsic Pdot values and find a new relation between Lx and spindown luminosity, Edot: Lx ~ Edot^beta, with beta ~0.5 vs. ~1.0 for both pulsars 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) for thermal emission from polar cap heating although the cluster MSPs are relatively longer-lived (in thermal x-rays) than either the models or field MSPs. We suggest the cluster MSPs may have altered surface magnetic field topology (e.g.multipole) or their neutron stars are more massive from repeated accretion episodes due to encounters and repeated exchange interactions. MSP binary companions on or just off the main sequence are likely to have been re-exchanged and might show anomalous Pdot and Edot values due to relaxation of misaligned core-crust spins. The radial distribution of 40 soft Chandra sources in 47Tuc is consistent with a 1.4Msun component. The implied total MSP population in 47Tuc with Lx >10^30 erg/s is ~35-90, and can constrain the relative beaming in radio vs. soft x-rays. NGC6397 is relatively deficient in MSPs; its single detected example may have been re-exchanged out of the cluster core. (abridged)