Discovery of a millisecond pulsar with a CO white dwarf companion
We report the discovery and characterization of PSR J1810-0623, a fully recycled millisecond pulsar with a spin period of 4.55 ms, discovered with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and followed up with FAST and the Green Bank Telescope (GBT). A phase-connected timing solution spanning over 6.5 years reveals a 15.4-day binary orbit with extremely low eccentricity (about 1.5E-5). Assuming a neutron star mass of 1.4 Msun, the inferred companion median mass (about 0.64 Msun) is consistent with a carbon-oxygen white dwarf, indicating an evolutionary origin in an intermediate mass Xray binary. The system's properties closely resemble those of other massive white dwarf binaries thought to form via Case A Roche lobe overflow, suggesting a prolonged accretion phase during which the neutron star was efficiently recycled. Polarimetric analysis of FAST data yields a moderate degree of linear polarization and a rotation measure of 86.6 pm 0.6 rad/m^2, offering constraints on the Galactic magnetic field. The inferred characteristic age (about 32 Gyr) and low surface magnetic field (about 1E8 G) indicate a highly recycled pulsar. Proper-motion measurements imply a modest transverse velocity, consistent with those of recycled millisecond pulsars in the Galactic field. Although the proximity of the globular cluster Pal 7 raises the possibility of a dynamical origin, discrepancies in dispersion measure and proper motion argue against a physical association. PSR J1810-0623 adds to the rare class of long-orbital period MSP-COWD systems and provides a valuable laboratory for studying pulsar recycling, binary evolution, and Galactic structure.