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 (Lorimer & Kramer 2005). One of these, called relativistic Shapiro delay (Shapiro 1964), 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 $\mathbf{2.14^{+0.10}_{-0.09}}$ solar masses (68.3% credibility interval; 95.4% credibility interval is $\mathbf{2.14^{+0.20}_{-0.18}}$ solar masses). It is highly likely to be the most massive neutron star yet observed, and serves as a strong constraint on the neutron star interior EoS.

• The paper provides the first precise mass measurement of MSP J0740+6620 using relativistic Shapiro delay, establishing a mass of 2.14 solar masses.
• It employs data from NANOGrav and targeted Green Bank Telescope observations to enhance measurement accuracy and constrain the neutron star equation of state.
• The finding challenges conventional nuclear physics models by suggesting a stiffer equation of state, thereby refining our understanding of supranuclear matter.

Relativistic Shapiro Delay Measurements of an Extremely Massive Millisecond Pulsar

The study conducted by Cromartie et al. offers a substantial contribution to the understanding of neutron stars' internal structure, specifically through the mass measurement of a massive millisecond pulsar (MSP) designated as J0740+6620. This research exploits the relativistic Shapiro delay to measure the pulsar mass, which is significant in constraining the equation of state (EoS) of supranuclear matter within neutron stars.

Key Contributions

The primary focus of this paper is the precise calculation of the mass of MSP J0740+6620 using data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and tailored observations from the Green Bank Telescope. The study provides a measured pulsar mass of 2.14 solar masses with a credibility interval at both 68.3% and 95.4%, suggesting J0740+6620 as possibly the most massive neutron star reliably measured to date. This finding challenges existing theoretical models of nuclear-density matter, offering vital empirical data against which these models can be further tested and refined.

Implications for Neutron Star EoS

The measurement holds considerable significance for the field of high-density nuclear physics. The presence of such a massive neutron star poses a test to proposed EoS models, especially those involving non-nucleonic states such as hyperons and quark matter. The observed data suggest a stiffer EoS, which supports the existence of higher maximum neutron star masses, contradicting models that predict softer, more compressible states under similar conditions. This mass threshold impels a reevaluation of the nuclear physic scenarios previously assumed to dominate the core nuclei interactions.

Methodological Insights

The methodology centers on leveraging the unique geometry and timing properties of the MSP binary system to isolate and measure the Shapiro delay signal, particularly focusing on specific orbital phases to enhance the measurement accuracy. The Shapiro delay, characterized by the range and shape parameters, serves as a key mechanism to derive masses of the pulsar and its companion when analyzed in conjunction with the Keplerian mass function.

Contextualization in Astrophysical Studies

The study enriches the scope of neutron star research by linking the mass constraints observed with potential implications for neutron star mergers and the processes underlying MSP binary system evolution. The findings suggest a potential bimodal distribution of MSP masses and postulate scenarios where such heavy stars may naturally emerge via processes other than gradual mass accretion, such as direct progenitor evolution.

Furthermore, this research contributes to the broader objectives of gravitational-wave astronomy, with implications for detecting gravitational waves from asymmetric neutron star mergers, which might be more prevalent than previously anticipated.

Future Prospects

This study holds promise for further refinement of neutron star models through ongoing observations and collaborations. Future work might focus on precise mass measurements using more regular and high-cadence observations, such as those possible with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Neutron Star Interior Composition Explorer (NICER), to corroborate and improve these initial findings. These advancements could significantly fine-tune the mass and radius parameters, thereby expanding our understanding of neutron star interiors.

In summary, Cromartie et al.'s research provides pivotal empirical evidence that reinforces the hypothesis that neutron stars can be more massive than previously validated, thereby advancing the study of neutron star physics and challenging conventional theoretical frameworks.