Impact of the PSR J0740+6620 radius constraint on the properties of high-density matter
X-ray pulse profile modeling of PSR J0740+6620, the most massive known pulsar, with data from the NICER and XMM-Newton observatories recently led to a measurement of its radius. We investigate this measurement's implications for the neutron star equation of state (EoS), employing a nonparametric EoS model based on Gaussian processes and combining information from other x-ray, radio and gravitational-wave observations of neutron stars. Our analysis mildly disfavors EoSs that support a disconnected hybrid star branch in the mass-radius relation, a proxy for strong phase transitions, with a Bayes factor of 6.9. For such EoSs, the transition mass from the hadronic to the hybrid branch is constrained to lie outside (1,2) M_⊙. We also find that the conformal sound-speed bound is violated inside neutron star cores, which implies that the core matter is strongly interacting. The squared sound speed reaches a maximum of 0.75^(+0.25)_(−0.24) c² at 3.60^(+2.25)_(−1.89) times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR J0740+6620's central density is only 3.57^(+1.3)_(−1.3) times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in β-equilibrium.
© 2021 American Physical Society. (Received 10 June 2021; revised 27 July 2021; accepted 28 July 2021; published 2 September 2021) Research at Perimeter Institute is supported in part by the Government of Canada through the Department of Innovation, Science and Economic Development Canada and by the Province of Ontario through the Ministry of Colleges and Universities. S. H. is supported by the National Science Foundation, Grant No. PHY-1630782, and the Heising-Simons Foundation, Grant No. 2017-228. P. L. is supported by National Science Foundation Grant No. PHY-1836734 and by a gift from the Dan Black Family Foundation to the Nicholas & Lee Begovich Center for Gravitational-Wave Physics & Astronomy. This research has made use of data, software and/or web tools obtained from the Gravitational Wave Open Science Center (https://www.gw-openscience.org), a service of LIGO Laboratory, the LIGO Scientific Collaboration and the Virgo Collaboration. Virgo is funded by the French Centre National de Recherche Scientifique (CNRS), the Italian Istituto Nazionale della Fisica Nucleare (INFN) and the Dutch Nikhef, with contributions by Polish and Hungarian institutes. This material is based upon work supported by NSF's LIGO Laboratory which is a major facility fully funded by the National Science Foundation. The authors are grateful for computational resources provided by the LIGO Laboratory and supported by National Science Foundation Grants No. PHY-0757058 and No. PHY-0823459.
Published - PhysRevD.104.063003.pdf
Accepted Version - 2106.05313.pdf