Mass measurements of the double neutron star system PSR J0641+0448

Abstract

Pulsar timing of double neutron star (DNS) systems is one of the best methodologies to study the neutron star masses distribution. Here we report the discovery of a double neutron star system PSR J0641+0448 in the Five-hundred-meter Aperture Spherical radio Telescope (FAST) Galactic Plane Pulsar Snapshot (GPPS) survey. This pulsar has a 25.7 ms spin period and moves in a 3.73-days eccentric orbit with an eccentricity of 0.145. Using FAST observations, we obtained its phase-connected timing solution with periastron advance and Shapiro delay detected. Using $χ^2$ analysis based on DDGR model, we constrain the pulsar mass to $1.319^{+0.021}_{-0.035}~M_\odot$, and the companion mass to $1.269^{+0.022}_{-0.016}~M_\odot$ with a 68.3\% confidence level. The low companion mass and mild orbital eccentricity is consistent with the correlation between neutron masses and orbital eccentricities.

Figures (4)

• Figure 1: Polarization profiles of PSR J1856--0039, integrated over 13 hours of observations. Panel (A): Position angles (PAs) of the linear polarization at infinite frequency. Error bars represent $\pm1\sigma$ uncertainties. Panel (B): Total intensity $I$ (black solid line), linear polarization $L$ (blue dashed line), and circular polarization $V$ (red dotted line, with positive values corresponding to left-hand circular polarization). The small square in the bottom left corner indicates the $\pm2\sigma_{\rm I,off}$ noise level in total intensity and a width of one phase bin.
• Figure 2: Timing residuals for PSR J0641+0448. The post-fit timing residuals are plotted versus epoch and orbital phase in subpanels (A) and (B), respectively. No systematic trends are apparent. The weighted root-mean-square (RMS) of the residuals, $\sigma_{\rm res}$, is 5.62 µ s. Subpanel (C) shows the Shapiro delay predicted by the best-fit values of DDGR binary model ($M_{\rm c}=1.269~M_\odot,i=79^\circ.6$).
• Figure 3: Constraints on the component masses and orbital inclination of PSR J0641+0448 derived from $\chi^2$ analysis. The two‐dimensional probability distributions in the $M_{\rm{tot}}$–$\cos i$ and $M_{\rm{p}}$–$M_{\rm{c}}$ planes are shown in subpanels (A) and (D), respectively. The contours correspond to $\Delta\chi^2$ levels of 1, 4, and 9. The best‐fit values, $M_{\rm{tot}} = 2.588~M_\odot$, $i = 79^\circ.6$, $M_{\rm{p}} = 1.319~M_\odot$, and $M_{\rm{c}} = 1.269~M_\odot$ are marked by red crosses. The highest density intervals containing 68.3% probability (enclosed by the blue dashed lines), derived from the one‐dimensional cumulative distribution functions (indicated by the black lines) in panels (B), (C), (E), and (F), are correspond to: $M_{\rm{tot}} = 2.557~\text{to}~2.617~M_\odot$, $i = 75.5~\text{to}~82.3$ deg, $M_{\rm{p}} = 1.284~\text{to}~1.340~M_\odot$, and $M_{\rm{c}} = 1.253 ~\text{to}~1.291~M_\odot$.
• Figure 4: The masses of the second-born neutron star, $M_{\rm NS,2}$ and orbital eccentricities, $e$ of known DNS systems in the Galactic field. PSR J0641+0448 is marked by the red star. The error bars show the $1\sigma$ uncertainties of their masses.