Constraining electromagnetic couplings of ultralight scalars from compact stars

TL;DR

This paper investigates how ultralight scalar fields interacting with electromagnetic fields of compact stars can be constrained through astrophysical observations, providing new bounds on scalar-photon couplings and potential signals in electromagnetic and gravitational phenomena.

Contribution

It introduces a method to constrain ultralight scalar-photon couplings using observations of compact stars and predicts how future measurements could improve these bounds.

Findings

Current bounds from Crab pulsar, SGR 1806-20, and GRB 080905A.

Projected bounds from binary star systems like PSR J0737-3039.

Potential for future observations to tighten coupling constraints.

Abstract

If an ultralight scalar interacts with the electromagnetic fields of a compact rotating star, then a long-range scalar field is developed outside the star. The Coulomb-like profile of the scalar field to the leading order is equivalent to an effective scalar charge on the star. In a binary star system, the scalar-induced charge would result in a long-range force between the stars, with the scalar field acting as the mediator. The scalar-photon interactions would modify Maxwell's equations for electromagnetic fields in vacuum, resulting in a modified dispersion relation. This could be observed as an apparent redshift for photons emitted by such sources. The scalar field would also induce additional electric and magnetic fields and hence affect the electromagnetic energy radiated from such compact objects. A scalar field sourced by time-varying electromagnetic fields can also carry away energy from a compact star in the form of radiation, and hence contribute to its spin-down luminosity. We constrain the scalar-photon coupling from the measurements of the electromagnetic radiation of a compact star and from its spin-down luminosity, using the Crab pulsar, the soft gamma repeater SGR 1806-20, and the gamma ray burst GRB 080905A. We also project the prospective bounds on the coupling from future measurements of the long-range force between two compact stars in a binary such as PSR J0737-3039, and from the apparent redshifts of compact stars. Future advances in precision-clock sensitivity and targeted observations of stars with strong surface magnetic fields, large radii, and low-frequency emission can substantially tighten these coupling limits.