Study of measured pulsar masses and their possible conclusions

Abstract

We study the statistics of 61 measured masses of neutron stars (NSs) in binary pulsar systems, including 18 double NS (DNS) systems, 26 radio pulsars (10 in our Galaxy) with white dwarf (WD) companions, 3 NSs with main-sequence companions, 13 NSs in X-ray binaries, and one undetermined system. We derive a mean value of M = 1.46 ± 0.30 M . When the 46 NSs with measured spin periods are divided into two groups at 20 milliseconds, i.e., the millisecond pulsar (MSP) group and others, we find that their mass averages are, respectively, M = 1.57 ± 0.35 M and M = 1.37 ± 0.23 M . In the framework of the pulsar recycling hypothesis, this suggests that an accretion of approximately ∼0.2 M is sufficient to spin up a neutron star and place it in the millisecond pulsar group. Based on these estimates, an approximate empirical relation between the accreting mass (ΔM) of recycled pulsar and its spin period is proposed as ΔM = 0.43 (M )(P/1 ms) −2/3 . If we focus only on the DNS, the mass average of all 18 DNSs is 1.32 ± 0.14 M , and the mass averages of the recycled DNSs and the non-recycled NS companions are, respectively, 1.38 ± 0.12 M and 1.25 ± 0.13 M . This is consistent with the hypothesis that the masses of both NSs in DNS system have been affected by accretion. The mass average of MSPs is higher than the Chandrasekhar limit 1.44 M , which may imply that most of binary MSPs form via the standard scenario by accretion recycling. If we were to assume that the mass of a MSP formed by the accretion induced collapse (AIC) of a white dwarf must be less than 1.35 M , then the portion of the binary MSPs involved in the AICs would not be higher than 20%, which imposes a constraint on the AIC origin of MSPs. With accreting mass from the companion, the nuclear matter composition of MSP may experience a transition from the "soft" equation of state (EOS) to a "stiff" EOS or even neutron to quark matter.

Surface emission from neutron stars and implications for the physics of their interiors

“…Such a large mass already argues against significant softening of the equation of state by the emergence of new degrees of freedom at high densities (Özel et al 2010). Mass measurements of a large sample of neutron stars also reveal their intrinsic mass distribution and probe the physics of the supernova explosions and neutron star formation (Thorsett & Chakrabarty 1999;Kiziltan et al 2011;Zhang et al 2011;Özel et al 2012).…”

Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers

“…This one-to-one correspondence between the EoS and the mass-radius relation of NSs allows in principle to determine or at least to constrain the EoS from the simultaneous measurement of the mass and the radius of a NS. While NS masses have been measured very precisely in binary systems [1,[5][6][7], the determination of radii has not yet been achieved with high precision (see e.g. [1,2,8,9] and references therein).…”

On the Mass Distribution and Birth Masses of Neutron Stars

“…More recently, Schwab et al (2010) argued that the distribution of neutron star masses in double neutron stars is actually bimodal, with one peak centered at ∼1.25 M and the other at ∼1.35 M , which they attributed to different supernova explosion mechanisms. Kiziltan et al (2010), Valentim et al (2011), andZhang 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. All groups found that the neutron stars that are thought to have undergone significant accretion are, on average, 0.2-0.3 M heavier than those that have not.…”