Discovery of a millisecond pulsar with a CO white dwarf companion

Abstract

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.

Figures (5)

• Figure 1: Timing residuals as a function of time (top panel) and orbital phase (bottom panel). The blue points indicate FAST ToAs and the red points are GBT ToAs.
• Figure 2: Integrated polarization profile of the PSR J1810-0623 from FAST data at 1250 MHz. The red line is the linear polarization profile, the blue line is the circular polarization profile, and the black line is the normalized total intensity profile. Black dots in the top panel give the linear position angle (PA) referred to the overall band center of the integrated profiles.
• Figure 3: Binary properties of MSPs in the Galactic field and globular clusters (based on the ATNF Pulsar Catalogue$^{\ref{['ATNF_catalog']}}$). (a) Spin period versus median companion mass. (b) Orbital period versus median companion mass. Different companion types are indicated by distinct symbols: CO white dwarfs (CO-WDs, crosses), He white dwarfs (He-WDs, circles), main-sequence stars (MS, squares), neutron stars (NS, right-pointing triangles), and ultra-light companions (ULs, left-pointing triangles). Systems in the Galactic field are shown in black, while those in globular clusters are shown in light grey. PSR J1810$-$0623 (this work) is highlighted in red, while PSRs J1125$-$6014, J1614$-$2230, and J1933$-$6211 Demorest2010Shamohammadi2023Geyer2023 are highlighted in red and blue, respectively.
• Figure 4: $P$–$\dot{P}$ diagram based on the ATNF pulsar catalogue$^{\ref{['ATNF_catalog']}}$. Dot-dashed and dashed grey lines denote lines of constant surface magnetic field strength (in Gauss) and characteristic age (in years), respectively. PSR J1810$+$0623 is marked by a red star outlined in blue, with an inferred characteristic age of $\sim$32 Gyr. PSRs J1125$-$6014, J1614$-$2230, and J1933$-$6211 Demorest2010Shamohammadi2023Geyer2023 are marked by black stars outlined in blue.
• Figure 5: Predicted maximum amplitude (at superior conjunction) of the Shapiro delay contribution due to the third and higher-order harmonics as a function of the cosine of the inclination, for three different assumed masses for the pulsar. The dashed horizontal line indicates the residual rms of the current timing solution.