PSR J1227$-$6208 and its massive white dwarf companion: pulsar emission analysis, timing update and mass measurements

PSR J1227$-$6208 is a 34.53-ms recycled pulsar with a massive companion. This system has long been suspected to belong to the emerging class of massive recycled pulsar-ONeMg white dwarf systems such as PSR J2222$-$0137, PSR J1528$-$3146 and J1439$-$5501. Here we present an updated emission and timing analysis with more than 11 years of combined Parkes and MeerKAT data, including 19 hours of high-frequency data from the newly installed MeerKAT S-band receivers. We measure a scattering timescale of 1.22 ms at 1 GHz with a flat scattering index 3.33<$\beta$<3.62, and a mean flux density of 0.53-0.62 mJy at 1 GHz with a steep spectral index 2.06<$\alpha$<2.35. Around 15% of the emission is linearly and circularly polarised, but the polarisation angle does not follow the rotating vector model. Thanks to the sensitivity of MeerKAT, we successfully measure a rate of periastron advance of 0.0171(11) deg/yr, and a Shapiro delay with an orthometric amplitude of 3.6$\pm$0.5 $\mu$s and an orthometric shape of 0.85$\pm$0.05. The main source of uncertainty in our timing analysis is chromatic correlated dispersion measure noise, which we model as a power law in the Fourier space thanks to the large frequency coverage provided by the Parkes UWL receiver. Assuming general relativity and accounting for the measurements across all the implemented timing noise models, the total mass, companion mass, pulsar mass and inclination angle are constrained at 2.3<Mt/$M_{\odot }$<3.2, 1.21<Mc/$M_{\odot }$<1.47, 1.16<Mp/$M_{\odot }$<1.69 and 77.5<i/deg<80.3. We also constrain the longitude of ascending node to either 266$\pm$78 deg or 86$\pm$78 deg. We argue against a neutron star nature of the companion based on the very low orbital eccentric of the system (e=1.15e-3), and instead classify the companion of PSR J1227-6208 as a rare, massive ONeMg white dwarf close to the Chandrasekhar limit.