Modeling the Surface X-ray Emission and Viewing Geometry of PSR J0821-4300 in Puppis A

We present a model for the unusual X-ray pulse profiles of PSR J0821-4300, the compact central object in supernova remnant Puppis A. We show that a pair of thermal, antipodal hot spots on the neutron star (NS) surface is able to fully account for the pulsar's double blackbody spectrum and energy-dependent pulse profile, including the observed 180° phase reversal at ≈1.2 keV. By comparing the recorded pulse modulation and phase to the model predictions, we strongly constrain the hot-spot pole (ξ) and the line-of-sight (ψ) angles with respect to the spin axis. For a nominal radius of R = 12 km and distance D = 2.2 kpc, we find (ξ, ψ) = (86°, 6°), with 1σ error ellipse of (2°, 1°) this solution is degenerate in the two angles. The best-fit spectral model for this geometry requires that the temperatures of the two emission spots differ by a factor of 2 and their areas by a factor of ~20. Including a cosine-beamed pattern for the emitted intensity modifies the result, decreasing the angles to (84°, 3°) however this model is not statistically distinguishable from the isotropic emission case. We also present a new upper limit on the period derivative of \dot{P} < 3.5 × 10^{-16} (2σ), which limits the global dipole magnetic field to B_s < 2.0 × 10¹¹ G, confirming PSR J0821-4300 as an "anti-magnetar." We discuss the results in the context of observations and theories of nonuniform surface temperature on isolated NSs of both weak and strong magnetic field. To explain the nonuniform temperature of PSR J0821-4300 may require a crustal field that is much stronger than the external, global dipole field.