TY - JOUR
T1 - Surface of rapidly-rotating neutron stars
T2 - Implications to neutron star parameter estimation
AU - Silva, Hector O.
AU - Pappas, George
AU - Yunes, Nicolás
AU - Yagi, Kent
N1 - Funding Information:
It is a pleasure to thank Fred Lamb, Cole Miller, Stuart Shapiro, Hajime Sotani, Nikolaos Stergioulas, and Helvi Witek for various discussions. We are particularly indebted to Sharon Morsink for helping us implement our code, sharing with us numerical data used to validate it and numerous discussions on the subject. We also thank the anonymous referee for carefully reading our work and the suggestions to improve it. H. O. S. and G. P. thank Hajime Sotani and the Division of Theoretical Astronomy of the National Astronomical Observatory of Japan (NAOJ) through the NAOJ Visiting Joint Research supported by the Research Coordination Committee, NAOJ, National Institutes of Natural Sciences (NINS) for the hospitality while part of this work was done. N. Y. thanks the hospitality of the Kavli Institute for Theoretical Physics (KITP) where part of this work was carried out. The work of H. O. S. and N. Y. was supported by the NSF Grant No. PHY-1607130 and NASA Grants No. NNX16AB98G and No. 80NSSC17M0041. G. P. acknowledges financial support provided under the European Union’s H2020 ERC, Starting Grant Agreement No. DarkGRA-757480. K. Y. acknowledges support from NSF Grant No. PHY-1806776, NASA Grant No. 80NSSC20K0523, a Sloan Foundation Research Fellowship and the Owens Family Foundation. K. Y. would like to also acknowledge support by the COST Action GWverse CA16104 and JSPS KAKENHI Grants No. JP17H06358.
Publisher Copyright:
© 2021 American Physical Society.
PY - 2021/3/25
Y1 - 2021/3/25
N2 - The Neutron star Interior Composition Explorer (NICER) is currently observing the x-ray pulse profiles emitted by hot spots on the surface of rotating neutron stars allowing for an inference of their radii with unprecedented precision. A critical ingredient in the pulse profile model is an analytical formula for the oblate shape of the star. These formulas require a fitting over a large ensemble of neutron star solutions, which cover a wide set of equations of state, stellar compactnesses and rotational frequencies. However, this procedure introduces a source of systematic error, as (i) the fits do not describe perfectly the surface of all stars in the ensemble and (ii) neutron stars are described by a single equation of state, whose influence on the surface shape is averaged out during the fitting procedure. Here we perform a first study of this systematic error, finding evidence that it is subdominant relative to the statistical error in the radius inference by NICER. We also find evidence that the formula currently used by NICER can be used in the inference of the radii of rapidly rotating stars, outside of the formula's domain of validity. Moreover, we employ an accurate enthalpy-based method to locate the surface of numerical solutions of rapidly rotating neutron stars and a new highly accurate formula to describe their surfaces. These results can be used in applications that require an accurate description of oblate surfaces of rapidly rotating neutron stars.
AB - The Neutron star Interior Composition Explorer (NICER) is currently observing the x-ray pulse profiles emitted by hot spots on the surface of rotating neutron stars allowing for an inference of their radii with unprecedented precision. A critical ingredient in the pulse profile model is an analytical formula for the oblate shape of the star. These formulas require a fitting over a large ensemble of neutron star solutions, which cover a wide set of equations of state, stellar compactnesses and rotational frequencies. However, this procedure introduces a source of systematic error, as (i) the fits do not describe perfectly the surface of all stars in the ensemble and (ii) neutron stars are described by a single equation of state, whose influence on the surface shape is averaged out during the fitting procedure. Here we perform a first study of this systematic error, finding evidence that it is subdominant relative to the statistical error in the radius inference by NICER. We also find evidence that the formula currently used by NICER can be used in the inference of the radii of rapidly rotating stars, outside of the formula's domain of validity. Moreover, we employ an accurate enthalpy-based method to locate the surface of numerical solutions of rapidly rotating neutron stars and a new highly accurate formula to describe their surfaces. These results can be used in applications that require an accurate description of oblate surfaces of rapidly rotating neutron stars.
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U2 - 10.1103/PhysRevD.103.063038
DO - 10.1103/PhysRevD.103.063038
M3 - Article
AN - SCOPUS:85104248763
VL - 103
JO - Physical Review D
JF - Physical Review D
SN - 2470-0010
IS - 6
M1 - 063038
ER -