2003ApJ...598.1207C

The observed spectra and X-ray luminosities of millisecond pulsars in 47 Tuc can be interpreted in the context of theoretical models based on strong, small-scale, multipole fields on the neutron star surface. For multipole fields that are relatively strong compared to the large-scale dipole field, the emitted X-rays are thermal and likely result from polar cap heating associated with the return current from the polar gap. On the other hand, for weak multipole fields, the emission is nonthermal and results from synchrotron radiation of e^± pairs created by curvature radiation. The X-ray luminosity L_Xis related to the spin-down power L_sd, expressed in the form L_X∝L^β_sdwith β∼0.5 and ∼1 for strong and weak multipole fields, respectively. If the polar cap size is of the order of the length scale of the multipole field s, the polar cap temperature is ∼3x10⁶K(L_sd/10³⁴ergs/s)^1/8(s/3x10⁴cm)^–1/2. A comparison of the X-ray properties of millisecond pulsars in globular clusters and in the Galactic field suggests that the emergence of relatively strong small-scale multipole fields from the neutron star interior may be correlated with the age and evolutionary history of the underlying neutron star.