configuration, with the minimum number of free parameters, and allows for self-consistent calculation of the surface temperature distribution and the resulting light curve. One can invoke more complex magnetic field topology, but the general conclusion of our result, that the NS in Kes 79 contains a significant hidden magnetic field (of order a few$\times 10^{14}$G or larger) in the bulk region of its crust, is likely to be robust. We also note that while the “strong toroidal field + weak dipole field” configuration adopted in our model has generally been thought to be unstable in fluid stars (e.g., Braithwaite & Nordlund 2006; Reisenegger 2009), this may not, in fact, be the case, as Braithwaite (2009) demonstrated that the effect of gravitational stratification is such that even a relatively weak poloidal field can be enough to stabilize the configuration. In NSs beyond the fluid stage of evolution, the crust also has a stabilizing effect. If we include a poloidal field of similar strength as the toroidal field, our general conclusion will be unchanged.

While it is beyond the scope of this paper to speculate how the magnetic field configuration adopted in our model may come about, we can estimate how long 