Relativistic Shapiro delay measurements of an extremely massive millisecond pulsar

Abstract

Despite its importance to our understanding of physics at supranuclear densities, the equation of state (EoS) of matter deep within neutron stars remains poorly understood. Millisecond pulsars (MSPs) are among the most useful astrophysical objects in the Universe for testing fundamental physics, and place some of the most stringent constraints on this high-density EoS. Pulsar timing -the process of accounting for every rotation of a pulsar over long time periods -can precisely measure a wide variety of physical phenomena, including those that allow the measurement of the masses of the components of a pulsar binary system [1]. One of these, called relativistic Shapiro delay [2], can yield precise masses for both an MSP and its companion; however, it is only easily observed in a small subset of high-precision, highly inclined (nearly edge-on) binary pulsar systems. By combining data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 12.5-year data set with recent orbital-phase-specific observations using the Green Bank Telescope, we have measured the mass of the MSP J0740+6620 to be 2.14 +0.10 −0.09 solar masses (68.3% credibility interval; 95.4% credibility interval is 2.14 +0.20 −0.18 solar masses). It is highly 1 arXiv:1904.06759v2 [astro-ph.HE] 13 Sep 2019 likely to be the most massive neutron star yet observed, and serves as a strong constraint on the neutron star interior EoS. Relativistic Shapiro delay, which is observable when a pulsar passes behind its stellar companion during orbital conjunction, manifests as a small delay in pulse arrival times induced by the curvature of spacetime in the vicinity of the companion star. For a highly inclined MSP-white dwarf binary, the full delay is of order ∼10 µs. The relativistic effect is characterized by two parameters, "shape" and "range." In general relativity, shape (s) is the sine of the angle of inclination of the binary orbit (i), while range (r) is proportional to the mass of the companion, m c . When combined with the Keplerian mass function, measurements of r and s also constrain the pulsar mass (m p ; [3] provides a detailed overview and an alternate parameterization).Precise neutron star mass measurements are an effective way to constrain the EoS of the ultradense matter in neutron star interiors. Although radio pulsar timing cannot directly determine neutron star radii, the existence of pulsars with masses exceeding the maximum mass allowed by a given model can straightforwardly rule out that EoS.In 2010, Demorest et al. reported the discovery of a 2-solar-mass MSP, J1614−2230 [4] (though the originally reported mass was 1.97 ± 0.04 M , continued timing has led to a more precise mass measurement of 1.928±0.017 M ; Fonseca et al. 2016 [5]). This Shapiro-delay-enabled measurement disproved the plausibility of some hyperon, boson, and free quark models in nuclear-density environments. In 2013, Antoniadis et al. used optical techniques in combination with pulsar timing to yield a mass measurement of 2.01±0.04 M for the pulsar J0...

GW170817 constraints analyzed with Gogny forces and momentum-dependent interactions

“…A very recent value for the observed largest NS mass of 2.14 +0.20 −0.18 M ⊙ at 95.4% credible level and of 2.14 +0.10 −0.09 M ⊙ at 68.3% credible level is proposed in Ref. [75] from the MSP J0740+6620 pulsar. D1M*, D2 and the MDI models with x ≤ −0.2 would be compatible with the lower limit of this mass at the 95.4% level and only D2 in the case of the 68.3% level.…”

Strangeness in nuclei and neutron stars

“…The mass of the Hulse-Taylor pulsar and the observed PSR J1614-2230 [353,354] and PSR J0348+0432 [355] are shown with horitzontal lines. Also, the new mass measurement of PSR J0740+6620 is shown [356]. Figures taken from [370].…”

Constraints on the Muon Fraction and Density Profile in Neutron Stars

“…Within the model framework discussed above, we have recently studied how the three high-density EOS parameters are constrained by NS observations (Zhang et al 2018;Zhang & Li 2019a,b,c;Xie & Li 2019). These include the radii R 1.4 of canonical NSs, e.g., 10.62 < R 1.4 < 12.83 km from analyzing quiescent low-mass X-ray binaries (Lattimer & Steiner 2014), the dimensionless tidal deformability 70 ≤ Λ 1.4 ≤ 580 from the refined analysis of GW170817 data (Abbott et al 2018), and the latest NS maximum mass M = 2.14 +0.10 −0.09 M ⊙ from the observations of PSR J0740+6620 (Cromartie et al 2019). Of course, the causality and dynamical stability conditions have to be satisfied through out the entire NSs.…”

“…While the nuclear symmetry energy especially at supra-saturation densities remains the most uncertain part of the Equation of State (EOS) of dense neutron-rich nucleonic matter, significant constraints on the E sym (ρ) have been obtained recently from both terrestrial experiments and astrophysical observations. In particular, analyses of NS radii from earlier Chandra (Steiner et al 2010) X-ray data as well as the tidal deformability of NSs from GW170817 (Abbott et al 2018) together with the latest NS maximum mass M = 2.14 +0.10 −0.09 M ⊙ from observations of PSR J0740+6620 (Cromartie et al 2019) have facilitated the recent extraction of tighter constraints on the EOS of dense neutron-rich nucleonic matter, especially its symmetry energy term (see, e.g., Zhang et al 2018;Zhang & Li 2019a,b,c;Nakazato & Suzuki 2019;Baillot et al 2019;.…”