PSR J0108–1431 is an old pulsar where the X-ray emission is expected to have a thermal component from the polar cap and a non-thermal component from the magnetosphere .
Although the phase-integrated spectra are fit best with a single non-thermal component modeled with a power-law ( PL ) of photon index \Gamma = 2.9 , the X-ray pulse profiles do show the presence of phase-separated thermal and non-thermal components .
The spectrum extracted from half the rotational phase away from the X-ray peak fits well with either a single blackbody ( BB ) or a neutron star atmosphere ( NA ) model , whereas , the spectrum from the rest of the phase range is dominated by a PL .
From Bayesian analysis , the estimated BB area is smaller than the expected polar cap area for a dipolar magnetic field with a probability of 86 % whereas the area estimate from the NA model is larger with a probability of 80 % .
Due to the ambiguity in the thermal emission model , the polar cap area can not be reliably estimated and hence can not be used to understand the nature of the surface magnetic field .
Instead , we can infer the presence of multipolar magnetic field from the misalignment between the pulsar ’ s thermal X-ray peak and the radio emission peak .
For J0108–1431 , we estimated a phase-offset \Delta \phi > 0.1 between the thermal polar cap emission peak and the radio emission peak and argue that this is best explained by the presence of a multipolar surface magnetic field .