Chandra on Millisecond Pulsars in Globular Clusters

We summarize the x-ray properties of the complete samples of millisecond pulsars (MSPs) detected in our Chandra observations of the globular clusters 47 Tuc and NGC 6397. The 47 Tuc MSPs are predominantly soft sources suggestive of thermal emission from the neutron star polar cap and have x-ray luminosities in a surprisingly narrow range (Lx ~--4 x 10^{30} erg/s). The single MSP in NGC 6397 is both hard and apparently extended, probably due to shocked hot gas evaporating from its main sequence companion. In contrast to MSPs in the field and the cluster M28, which show correlation between x-ray luminosity and spindown luminosity Lx \propto \Edot^{\beta} with \beta ~1 - 1.4, the 47 Tuc (and NGC 6397) sample display a relatively tight correlation with \beta = 0.5 +/-0.15. The correlations of Lx vs. P/2P-dot and light cylinder magnetic field values are also different. It is possible the magnetic field configuration has been altered (by episodic accretion) for old MSPs in dense cluster cores.