Abstract
We investigate the distribution of neutron star masses in different populations of binaries, employing Bayesian statistical techniques. In particular, we explore the differences in neutron star masses between sources that have experienced distinct evolutionary paths and accretion episodes. We find that the distribution of neutron star masses in non-recycled eclipsing high-mass binaries as well as of slow pulsars, which are all believed to be near their birth masses, has a mean of 1.28 M_solar and a dispersion of 0.24 M_solar. These values are consistent with expectations for neutron star formation in core-collapse supernovae. On the other hand, double neutron stars, which are also believed to be near their birth masses, have a much narrower mass distribution, peaking at 1.33 M_solar but with a dispersion of only 0.05 M_solar. Such a small dispersion cannot easily be understood and perhaps points to a particular and rare formation channel. The mass distribution of neutron stars that have been recycled has a mean of 1.48 M_solar and a dispersion of 0.2 M_solar, consistent with the expectation that they have experienced extended mass accretion episodes. The fact that only a very small fraction of recycled neutron stars in the inferred distribution have masses that exceed ~2 M_solar suggests that only a few of these neutron stars cross the mass threshold to form low mass black holes.
Highlights
The mass distribution of neutron stars contains information about the supernova explosion mechanisms, the equation of state of neutron star matter, and the accretion history of each neutron star since its formation
The neutron star mass measurements that were available a decade ago allowed a statistical inference of the mass distribution of double neutron stars (Finn 1994) or of pulsars in binaries, without distinguishing between subgroups (Thorsett & Chakrabarty 1999)
Schwab et al (2010) argued that the distribution of neutron star masses in double neutron stars is bimodal, with one peak centered at ∼1.25 M and the other at ∼1.35 M, which they attributed to different supernova explosion mechanisms
Summary
The mass distribution of neutron stars contains information about the supernova explosion mechanisms, the equation of state of neutron star matter, and the accretion history of each neutron star since its formation. Kiziltan et al (2010), Valentim et al (2011), and Zhang et al (2011), on the other hand, inferred the mass distribution of different neutron star subgroups based either on the pulsar spin period or the binary companion, both of which were taken to be indicative of the accretion history of the system. Fallback of stellar matter onto the collapsing core during the supernova explosion allows for the remnant to increase This is expected to increase the dispersion of masses by a comparable amount (see Zhang et al 2008), which is inconsistent with the narrowness of the inferred mass distribution of double neutron star masses.
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