Yagi, Kent and Stein, Leo C. and Yunes, Nicolás
(2016)
*Challenging the presence of scalar charge and dipolar radiation in binary pulsars.*
Physical Review D, 93
(2).
Art. No. 024010.
ISSN 0556-2821.
http://resolver.caltech.edu/CaltechAUTHORS:20160128-135122814

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## Abstract

Corrections to general relativity that introduce long-ranged scalar fields which are nonminimally coupled to curvature typically predict that neutron stars possess a nontrivial scalar field profile anchored to the star. An observer far from a star is most sensitive to the spherically symmetric piece of this profile that decays linearly with the inverse of the distance to the source, the so-called scalar monopole charge, which is related to the emission of dipolar radiation from compact binary systems. The presence of dipolar radiation has the potential to rule out or very strongly constrain extended theories of gravity. These facts may lead people to believe that gravitational theories that introduce long-ranged scalar fields have already been constrained strongly from binary pulsar observations. Here we challenge this “lore” by investigating the decoupling limit of Gauss-Bonnet gravity as an example, in which the scalar field couples linearly to the Gauss-Bonnet density in the action. We prove a theorem that neutron stars in this theory cannot possess a scalar charge, due to the topological nature of the Gauss-Bonnet density. Thus Gauss-Bonnet gravity evades the strong binary pulsar constraints on dipole radiation. We discuss the astrophysical systems which will yield the best constraints on Gauss-Bonnet gravity and related quadratic gravity theories. To achieve this we compute the scalar charge in quadratic gravity theories by performing explicit analytic and numerical matching calculations for slowly rotating neutron stars. In generic quadratic gravity theories, either neutron star–binary or neutron star–black hole systems can be used to constrain the theory, but because of the vanishing charge, Gauss-Bonnet gravity evades the neutron star–binary constraints. However, in contrast to neutron stars, black holes in Gauss-Bonnet gravity do anchor scalar charge, because of the difference in topology. The best constraints on Gauss-Bonnet gravity will thus come from accurate black hole observations, for example through gravitational waves from inspiraling binaries or the timing of pulsar–black hole binaries with radio telescopes. We estimate these constraints to be a factor of 10 better than the current estimated bound, and also include estimated constraints on generic quadratic gravity theories from pulsar timing.

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Additional Information: | © 2016 American Physical Society. Received 7 October 2015; published 8 January 2016. The authors thank Enrico Barausse, Mike Boyle, Gilles Esposito-Farèse, Paulo Freire, Takahiro Tanaka, and Norbert Wex for useful discussions and comments. We also thank Paolo Pani for providing us numerical data on the scalar charge in TEdGB gravity to compare against. L. C. S. and N. Y. would like to thank the Max Planck Institute for Radioastronomy for their hospitality, while this work was started. K. Y. acknowledges support from JSPS Postdoctoral Fellowships for Research Abroad and the NSF Grant No. PHY-1305682. L. C. S. acknowledges that support for this work was provided by NASA through Einstein Postdoctoral Fellowship Award Number PF2-130101 issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060, and further acknowledges support from the NSF Grant No. PHY-1404569. N. Y. acknowledges support from NSF CAREER Grant No. PHY-1250636. Some calculations used the computer algebra system maple, in combination with the GRTensorII package [118]. | ||||||||||||||

Group: | TAPIR, Walter Burke Institute for Theoretical Physics | ||||||||||||||

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Record Number: | CaltechAUTHORS:20160128-135122814 | ||||||||||||||

Persistent URL: | http://resolver.caltech.edu/CaltechAUTHORS:20160128-135122814 | ||||||||||||||

Official Citation: | Challenging the presence of scalar charge and dipolar radiation in binary pulsars Kent Yagi, Leo C. Stein, and Nicolás Yunes Phys. Rev. D 93, 024010 – Published 8 January 2016 | ||||||||||||||

Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. | ||||||||||||||

ID Code: | 64055 | ||||||||||||||

Collection: | CaltechAUTHORS | ||||||||||||||

Deposited By: | Tony Diaz | ||||||||||||||

Deposited On: | 28 Jan 2016 22:38 | ||||||||||||||

Last Modified: | 28 Jan 2016 22:38 |

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