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Measuring the Non-Axially-Symmetric Surface Temperature Distribution of the Central Compact Object in Puppis A

The surface temperature distributions of central compact objects (CCOs) are powerful probes of their crustal magnetic field strengths and geometries. Here we model the surface temperature distribution of RX J0822$-$4300, the CCO in the Puppis A supernova remnant (SNR), using $471$ ks of XMM-Newton data. We compute the energy-dependent pulse profiles in sixteen energy bands, fully including the general relativistic effects of gravitational redshift and light bending, to accurately model the two heated surface regions of different temperatures and areas, in addition to constraining the viewing geometry. This results in precise measurements of the two temperatures: $kT_{\rm warm} = (1+z) \times 0.222_{-0.019}^{+0.018}$ keV and $kT_{\rm hot} = (1+z) \times 0.411\pm0.011$ keV. For the first time, we are able to measure a deviation from a pure antipodal hot-spot geometry, with a minimum value of $1.\!^{\circ}1 \pm 0.\!^{\circ}2$, and an expectation value of $9.\!^{\circ}35 \pm 0.\!^{\circ}17$ among the most probable geometries. The discovery of this asymmetry, along with the factor of $\approx2$ temperature difference between the two emitting regions, may indicate that RX J0822$-$4300 was born with a strong, tangled crustal magnetic field.