Neutron Star Mass Distribution in Binaries

Abstract

Abstract. Massive neutron stars with ∼ 2M have been observed in neutron star-white dwarf binaries. On the other hand, well-measured neutron star masses in double-neutron-star binaries are still consistent with the limit of 1.5M . These observations raised questions on the neutron star equations of state and the neutron star binary evolution processes. In this presentation, a hypothesis of super-Eddington accretion and its implications are discussed. We argue that a 2M neutron star is an outcome of the super-Eddington accretion during the evolution of neutron star-white dwarf binary progenitors. We also suggest the possibility of the existence of new type of neutron star binary which consists of a typical neutron star and a massive compact companion (high-mass neutron star or black hole) with M ≥ 2M . Neutron Star Mass ObservationsNeutron stars provide opportunities to investigate physics of low-temperature dense Quantum chromodynamics (QCD) matter and are sources of gravitational waves that can be detected by ground-based observatories such as LIGO[1], Virgo [2,3] and KAGRA [4].The central densities of neutron stars are expected to reach several times the normal nuclear matter saturation density (2.04 × 10 14 g cm −3 ). Many theoretical investigations provide massradius relations of neutron stars based on the predicted neutron star equations of state. However, due to the nature of strong interaction which governs the QCD matter, the inner structure of neutron stars are still unknown and the maximum mass of neutron star is still an open question [5]. Until 2010, no firm evidence of a high-mass (≥ 2M ) neutron star has been found. Highmass neutron stars were listed in the X-ray binary catalogue, but they were not taken as the evidences of the existence of high-mass neutron stars due to the large uncertainties. The main uncertainty in mass estimates of X-ray binaries lies in finding the actual gravitational center of companion stars. The situation has been changed due to discoveries of two ∼ 2M neutron stars in neutron star-white dwarf binaries [6,7]. These observations rule out many neutron star equations of state which predict the maximum mass of a neutron star to be less than 2M . Recently, we investigated many equations of state that predict a larger neutron star maximum mass than 2M [8,9,10]. Double-neutron-star binaries are major sources for the neutron star mass measurements and gravitational-wave detections with current ground-based gravitational-wave observatories [11,12]. All well-measured neutron star masses in double-neutron-star binaries are still less than 1.5M , contrary to the recent 2M observation in neutron star-white dwarf binaries. These observations indicate that a neutron star's mass may depend on the evolutionary processes. In this presentation, we discuss a hypothesis of super-Eddington accretion during the evolution of binary progenitors.