Constraining electromagnetic couplings of ultralight scalars from compact stars

TL;DR

The paper investigates how an ultralight scalar that couples to photons, via $g_{\phi\gamma\gamma}$, can source a long-range scalar field around magnetized compact stars. It derives the external scalar profile with an effective charge $Q_{\phi}^{\mathrm{eff}}$, computes scalar-induced EM fields, and analyzes modifications to photon dispersion and potential scalar radiation. By applying observations from the Crab pulsar, SGR 1806-20, GRB 080905A, and prospects with double pulsars and precision clocks, it places bounds on $g_{\phi\gamma\gamma}$ across the scalar mass $m_{\phi}$, finding the strongest astrophysical bound from GRB 080905A spin-down, and the strongest magnetic-field bound from GRB 080905A’s EM effects. The results are complementary to laboratory fifth-force and equivalence-principle tests and suggest that future precision clocks and low-frequency observations could tighten constraints further.

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.

Figures (1)

• Figure 1: Bounds on $g_{\phi\gamma\gamma}$ derived from the measurements of the electromagnetic radiation by a scalar-induced magnetic field (blue shaded region), and pulsar spin-down caused by scalar radiation (red shaded region). The prospective bound from possible future constraints on a new long-range force from a pulsar binary pair with large surface magnetic fields $B_0$ is shown as a magenta dashed line, while the prospective bound from the measurement of the photon wavenumber using a future precision atomic clock ($\Delta k/k \sim 10^{-18}, \omega = 2.1$ GHz) is shown as a purple dashed line. The sensitivity obtainable with $\Delta k/k \sim 10^{-24}$ at $\omega\sim 10$ MHz has also been shown with a green dashed line. The existing constraints are shown as gray-shaded areas.