Ultralight time-oscillating scalars from magnetized compact stars: electrophilic radiation and photon propagation effects

TL;DR

This paper explores how ultralight scalar fields interacting with magnetized compact stars can produce observable electromagnetic effects, leading to new constraints on scalar-photon couplings through astrophysical measurements.

Contribution

It introduces novel mechanisms for scalar radiation and photon propagation effects around magnetized stars, providing the strongest astrophysical bounds on scalar-photon couplings to date.

Findings

Constraints on scalar-electron couplings from pulsar spin-down are weaker than existing limits.

Electromagnetic observations set the strongest bounds on scalar-photon couplings.

Future observations could improve bounds by several orders of magnitude.

Abstract

Ultralight scalars with electrophilic couplings to the time-dependent Goldreich-Julian charge density of magnetized compact stars can be radiated from their magnetospheres, contributing to pulsar spin-down. Coupling to the time-independent component of the charge density instead generates a quadrupolar scalar field profile, which may influence the orbital dynamics of binary systems. Such scalars can also interact with the time-varying electromagnetic fields of magnetized stars, modifying photon propagation and inducing observable effects in the redshift and residual time-delay measurements, as well as corrections to the background electromagnetic fields. We investigate these phenomena for the Crab pulsar, SGR 1806-20, and GRB 080805A. Using spectral and timing observations, we derive constraints on the scalar-electron and scalar-photon couplings. While the bounds obtained on the scalar-electron coupling from pulsar spin-down are weaker than existing limits, electromagnetic radiation measurements yield the strongest astrophysical constraints to date on the scalar-photon coupling. Compact stars with stronger surface magnetic fields and observations at lower photon frequencies can improve these bounds by several orders of magnitude.