Heated Polar Caps in PSR 0656+14 and PSR 1055−52

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Abstract
We present new ASCA observations covering the 0.5-10 keV X-ray range of the cooling neutron star candidates PSR 0656+14 and PSR 1055-52. Previous ROSAT observations had shown that two-component models, either two blackbodies or a blackbody plus a power-law, provided the best spectral fits to their X-ray emission. The combined ASCA and ROSAT spectrum of PSR 0656+14 reveals two blackbody components with T ≈ 8 × 105 K and T ≈ 1.5 × 106 K and shows evidence that a power-law component is needed to account for higher energy photons. This three-component fit gives a reduced χ2 that is half the value of a more conventional two component fit (1.3 as compared to 2.4). The fit to the combined spectrum for PSR 1055-52 yields a two-blackbody fit with T ≈ 8 × 105 K and T ≈ 3.7 × 106 K. Our results favor the existence of a hot polar cap in each of these pulsars with the ratio of the polar cap area to the neutron star surface area being 7 × 10-3 and 3 × 10-5 for PSR 0656+14 and PSR 1055-52, respectively. The results are compared to models that make predictions of polar cap heating processes.