Super-Massive Neutron Stars

TL;DR

This paper reports the measurement of a very massive neutron star in a binary system, providing evidence against soft equations of state for dense neutron matter and suggesting a possible bimodal distribution of millisecond pulsar masses.

Contribution

It presents the measurement of one of the largest neutron star masses to date, constraining the equation of state of dense matter and analyzing pulsar mass distribution.

Findings

Measured neutron star mass of approximately 1.94 solar masses.

Excluded soft equations of state for neutron star matter.

Evidence suggests a bimodal distribution of millisecond pulsar masses.

Abstract

We present here the results of Arecibo timing of PSR B1516+02B, a 7.95-ms pulsar in a binary system with a ~0.17 solar mass companion and an orbital period of 6.85 days located in the globular cluster M5. The eccentricity of the orbit (e = 0.14) has allowed a measurement of the rate of advance of periastron: (0.0136 +/- 0.0007) degrees per year. It is very likely that the periastron advance is due to the effects of general relativity; the total mass of the binary system is (2.14 +/-0.16) solar masses. The small measured mass function implies, in a statistical sense, that a very large fraction of this total mass is contained in the pulsar: (1.94+0.17 -0.19) solar masses (1-sigma); there is a 5% probability that the mass of this object is below 1.59 solar masses. With the possible exception of PSR J1748-2021B, this is the largest neutron star mass measured to date. When combined with similar measurements made previously for Terzan 5 I and J, we can exclude, in a statistical sense, the ``soft'' equations of state for dense neutron matter, implying that matter at the center of a neutron star is highly incompressible. There is also some evidence for a bimodal distribution of MSP masses, the reasons for that are not clear.