Shapiro delay measurement of a two solar mass neutron star

Abstract: Neutron stars are composed of the densest form of matter known to exist in our universe, and thus provide a unique laboratory for exploring the properties of cold matter at super-nuclear density. Measurements of the masses or radii of these objects can strongly constrain the neutron-star matter equation of state, and consequently the interior composition of neutron stars. Neutron stars that are visible as millisecond radio pulsars are especially useful in this respect, as timing observations of the radio pulses provide an extremely precise probe of both the pulsar's motion and the surrounding space-time metric. In particular, for a pulsar in a binary system, detection of the general relativistic Shapiro delay allows us to infer the masses of both the neutron star and its binary companion to high precision. Here we present radio timing observations of the binary millisecond pulsar PSR J1614-2230, which show a strong Shapiro delay signature. The implied pulsar mass of 1.97 +/- 0.04 M_sun is by far the highest yet measured with such certainty, and effectively rules out the presence of hyperons, bosons, or free quarks at densities comparable to the nuclear saturation density.

• The paper demonstrates that high-precision Shapiro delay measurements yield a neutron star mass of 1.97 ± 0.04 M☉.
• It employs advanced pulsar timing techniques and MCMC statistical methods to constrain orbital parameters in an edge-on binary system.
• The findings impose strong constraints on the neutron star equation of state, challenging models that predict exotic matter in dense interiors.

Shapiro Delay Measurement of a 2 Solar Mass Neutron Star

The paper at hand presents a detailed investigation into the precise measurement of the mass of the neutron star PSR J1614-2230 using observations of the Shapiro delay. This study, conducted by Demorest et al., provides significant insights into the constraints of the neutron star equation of state (EOS) and implications for the presence of exotic matter in neutron stars.

Neutron stars, known for their extreme density, serve as vital laboratories for examining matter under conditions of supranuclear density. Accurately measuring their masses imposes critical constraints on their internal compositions and the EOS. The authors achieved such precise measurements by observing the Shapiro delay, a relativistic effect evident in binary systems. The Shapiro delay arises when light from a neutron star is delayed while traversing the space-time curvature created by a companion star. In particular, systems with an edge-on orbit maximally exhibit this effect, allowing for precise determination of the component masses.

Key findings of this paper demonstrate the robustness of this method, with the observation of PSR J1614$-$2230 yielding a neutron star mass of 1.97 ± 0.04 M$_\odot$. This measurement effectively challenges some prevailing models predicting the emergence of non-nucleonic matter such as hyperons, bosons, or quark matter within neutron stars. Such matter would typically soften the equation of state, leading to reduced maximum possible masses. The high mass derived here lends strong empirical constraints against such theoretical models.

The observational strategy involved using the Green Bank Telescope and an advanced pulsar timing instrument, GUPPI, to obtain high-precision timing data over the orbital period of PSR J1614$-$2230. Complementary long-term timing data ensured that model parameters were effectively constrained across different scales. The high inclination of 89.17° for the binary orbit facilitated a significant Shapiro delay effect, enabling the detection and precise characterization of the neutron star and companion masses.

Methodologically, the authors addressed potential sources of bias in data fitting using sophisticated statistical techniques like Markov chain Monte Carlo (MCMC), ensuring robustness in the derived results. The paper provides a detailed discussion on the derived posterior probability distributions for relevant parameters, underscoring the high significance of the Shapiro delay observed.

Practical implications of this study reach beyond neutron star astrophysics into various astrophysical phenomena contingent on the mass constraints. The results stimulate further examination of evolutive models for binary pulsar systems, hinting that massive neutron stars may originate from similar systems with notably massive initial companions or through distinct accretion scenarios.

In conclusion, this investigation represents a substantive advancement in our understanding of neutron star interiors and the fundamental physics governing them. Future observational campaigns could employ similar methodologies to expand upon these findings, providing greater clarity on the distribution of neutron star masses and the nature of ultra-dense matter. Further theoretical exploration concerning the composition of neutron star interiors, possibly involving quark condensates or other exotic states, could pivot upon insights gleaned from such empirical work, driving dialogue within the astrophysical research community.