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

Abstract

We report Chandra observations of the complete sample of millisecond pulsars (MSPs)with precise radio positions in the globular clusters 47 Tuc (NGC104) and NGC 6397. The X-ray luminosities and colors are derivedor constrained and compared to X-ray MSPs previously detected inthe field as well as one previously detected in a globular cluster(M28). The 47 Tuc MSPs are predominantly soft sources suggestiveof thermal emission from small (r X <0.6 km) polar capson the neutron star rather than magnetospheric emission and are arelatively homogeneous sample, with most X-ray luminosities in asurprisingly narrow range [L X ~(1-4)�10 30 ergss -1 ]. We use previously derived intrinsic P values andfind a new relation between L X and spin-down luminosity,E:L X ~ E beta , with beta~0.5+/-0.2 versus ~1.0 for bothpulsars and MSPs in the field. Adding the single MSP in NGC 6397constrains betato0.5+/-0.15. This L X -E relation and also theL X /E versus spin-down age are each similar to that foundby Harding & Muslimov for thermal emission from polar cap heating.However, the cluster MSPs are relatively longer lived (in thermalX-rays) than either the models or field MSPs, which may have additionalmagnetospheric (nonthermal) components. We suggest that the cluster MSPs mayhave altered surface magnetic field topology (e.g., multipole), ortheir neutron stars are more massive from repeated accretion episodesdue to encounters and repeated exchange interactions. MSP binarycompanions on or just off the main sequence (e.g., NGC 6397) arelikely to have been reexchanged and might show anomalous P and Evalues due to relaxation of misaligned core-crust spins. The radialdistribution of ~40 soft Chandra sources in 47 Tuc is consistentwith a ~1.4M solar component in a multimass King model andwith the identified MSP sample. The implied total MSP populationin 47 Tuc with L X ~10 30 ergs s -1 is ~35-90 and can constrain the relative beaming in radio versussoft X-rays. NGC 6397 is relatively deficient in MSPs: its singledetected example may have been reexchanged out of the cluster core.