The NANOGrav Nine-year Data Set: Mass and Geometric Measurements of Binary Millisecond Pulsars (1603.00545v2)

Abstract: We analyze 24 binary radio pulsars in the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) nine-year data set. We make fourteen significant measurements of Shapiro delay, including new detections in four pulsar-binary systems (PSRs J0613$-$0200, J2017+0603, J2302+4442, and J2317+1439), and derive estimates of the binary-component masses and orbital inclination for these MSP-binary systems. We find a wide range of binary pulsar masses, with values as low as $m_{\rm p} = 1.18^{+0.10}{-0.09}\text{M}{\odot}$ for PSR J1918$-$0642 and as high as $m_{\rm p} = 1.928^{+0.017}{-0.017}\text{M}{\odot}$ for PSR J1614$-$2230 (both 68.3\% credibility). We make an improved measurement of the Shapiro timing delay in the PSR J1918$-$0642 and J2043+1711 systems, measuring the pulsar mass in the latter system to be $m_{\rm p} = 1.41^{+0.21}{-0.18}\text{M}{\odot}$ (68.3\% credibility) for the first time. We measure secular variations of one or more orbital elements in many systems, and use these measurements to further constrain our estimates of the pulsar and companion masses whenever possible. In particular, we used the observed Shapiro delay and periastron advance due to relativistic gravity in the PSR J1903+0327 system to derive a pulsar mass of $m_{\rm p} = 1.65^{+0.02}{-0.02}\text{M}{\odot}$ (68.3\% credibility). We discuss the implications that our mass measurements have on the overall neutron-star mass distribution, and on the "mass/orbital-period" correlation due to extended mass transfer.

• The paper presents 14 significant Shapiro delay measurements that precisely determine neutron star masses and orbital orientations in binary MSPs.
• It employs high-precision timing data and innovative reparametrizations to reduce statistical uncertainties in mass and geometry estimates.
• The study reveals a wide mass range—from 1.18 to 1.93 M☉—highlighting diverse formation histories and improving models of stellar evolution.

The NANOGrav Nine-year Data Set: Mass and Geometric Measurements of Binary Millisecond Pulsars

The paper presents an analysis of 24 binary radio pulsars observed as part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) nine-year data set. This analysis is notable for its focus on measuring Shapiro delay, a relativistic effect that can yield precise determinations of the masses and orbital inclinations of binary millisecond pulsars (MSPs). The paper provides new insights into the mass distribution of neutron stars as well as the geometric configurations of binary pulsar systems.

The researchers report 14 significant measurements of Shapiro delay, with new detections in binary systems including PSRs J0613$-$0200, J2017+0603, J2302+4442, and J2317+1439. In particular, impressive masses as low as $m_{\rm p} = 1.18^{+0.10}_{-0.09}\text{ M}_{\odot}$ for PSR J1918$-$0642 and as high as $$m_{\rm p} = 1.928^{+0.017}_{-0.017}\text{ M}_{\odot}$$ for PSR J1614$-$2230 were calculated. These results contribute valuable data regarding the range of mass values in MSP systems. Moreover, the paper includes refined determinations of Shapiro timing delay for several pulsars, which allow improved precision in the mass estimates and orbital parameters.

The methodology involved leveraging high precision timing data from the NANOGrav observations, including both regular monitoring and targeted observational campaigns designed to maximize the sensitivity to Shapiro delay. Both traditional parametrizations of the delay and redefined orthometric parametrizations were applied in order to mitigate numerical instability arising from collinear parameters. Such reparametrizations do not change the physical implications of the measurements but offer a statistical advantage by reducing parameter degeneracy, especially in systems with low orbital inclinations.

Importantly, the paper also explores secular variations in the orbital parameters of several MSP systems. Many of these are attributed either to gravitational wave damping (notably in PSR J1614$-$2230) or kinematic effects from the system's relative motion. By accounting for these variations, the analysis could infer stronger constraints on the mass and geometric parameters in select systems, particularly those expected to exhibit relativistic corrections like periastron advance.

The implications of this paper are significant both observationally and for theoretical modeling of neutron star systems. The diverse masses measured imply a variety of formation histories and physical conditions at neutron star birth and during mass transfer phases. Such insights are pivotal for constraining models of stellar evolution and the end-stages of massive stellar lifecycles.

Furthermore, this comprehensive paper underscores the utility of pulsar timing arrays not only for direct gravitational wave detection but also as laboratories for testing fundamental physics in extreme regimes. As NANOGrav and similar collaborations extend their data collection and observational strategies, such binary MSP analyses offer promising prospects for refining our understanding of gravity and compact objects. Future work may involve reconciling the pulsar mass distribution with theoretical expectations from neutron star formation and evolution models, as well as further refining geometric orientation estimates for these precise celestial laboratories.