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

TL;DR

We address ultralight scalars with electrophilic couplings to the magnetospheric Goldreich-Julian density and a dilatonic coupling to photons in magnetized neutron stars. By treating general magnetospheric geometry, we identify a quadrupolar time-dependent scalar radiation that modestly contributes to pulsar spin-down, along with a static quadrupolar scalar field hair; photon propagation through the oscillating scalar background yields a time-dependent, scalar-induced photon mass and a modulated redshift, while the scalar-induced EM fields perturb the surface magnetic field. Using Crab, SGR 1806-20, and GRB 080905A data, we derive constraints showing that scalar-photon couplings are currently bounded more stringently than existing astrophysical limits, particularly from redshift and residual time-delay measurements, whereas scalar-electron couplings from spin-down are weaker than laboratory bounds. The results highlight that lower-frequency observations (LOFAR, SKA) and highly magnetized sources such as magnetars and FRBs can greatly improve sensitivity to these ultralight scalars, providing a promising path for future tests of beyond-Standard-Model physics in the strong-field regime.

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.

Figures (1)

• Figure 1: Constraints on the scalar-photon coupling $g_{\varphi\gamma\gamma}$ derived from the frequency-dependent redshift measurement of GRB 080905A (red curve), its magnetic field estimation (purple curve) and from the residual time delay of dedispersed giant pulses from the Crab pulsar (blue curve). These bounds are compared with the existing astrophysical constraint from globular clusters (black dashed curve). The shaded regions denote the parameter space excluded by each probe.