Neutron Star Masses
In recent years, the number of pulsars with secure mass measurements has increased to a level that allows us to probe the underlying neutron star mass distribution in detail. We critically review radio pulsar mass measurements and present a detailed examination through which we are able to put stringent constraints on the underlying neutron star mass distribution. For the first time, we are able to analyze a sizable population of neutron star-white dwarf systems in addition to double neutron star systems with a technique that accounts for systematically different measurement errors. We find that neutron stars that have evolved through different evolutionary paths reflect distinctive signatures through dissimilar distribution peak and mass cutoff values. Neutron stars in double neutron star and neutron star-white dwarf systems show consistent respective peaks at 1.35 Msun and 1.50 Msun which suggest significant mass accretion (dm~ 0.15 Msun) has occurred during the spin up phase. The width of the mass distribution implied by double neutron star systems is indicative of a tight initial mass function while the inferred mass range is significantly wider for neutron stars that have gone through recycling. We find a mass cutoff at 2 Msun for neutron stars with white dwarf companions which establishes a firm lower bound for the maximum neutron star mass. This rules out the majority of strange quark and soft equation of state models as viable configurations for neutron star matter. The lack of truncation close to the maximum mass cutoff suggests that the 2 Msun limit is set by evolutionary constraints rather than nuclear physics or general relativity, and the existence of rare super-massive neutron stars is possible. ..
Posterior predictive density estimates for the neutron star mass distribution. DNS and NS-WD systems have respective peaks at 1.35 MSun and 1.50 Msun. The solid part of the curves show the 95% probability range. The filled circles mark the 68% central region of the distribution which correspond to 1.35+/-0.13 Msun and $1.50+/-0.25 Msun for the DNS and NS-WD system, respectively.
Measured masses of radio pulsars. All error bars indicate the central 68% confidence limits. Vertical solid lines are the peak values of the underlying mass distribution for DNS (m=1.35 Msun) and NS-WD (m=1.50 MSun) systems. The dashed and dotted vertical lines show the central 68% and 95% predictive probability intervals of the underlying mass distribution shown above.
Likelihood surfaces (a) and posterior densities (b) of model parameters mu and sigma for the NS mass distribution. In each panel, the tight contours on the left and wider contours on the right correspond to the data from DNS and NS-WD systems, respectively.
