# Constraints on millicharged dark matter and axion-like particles from timing of radio waves

## TL;DR

This paper uses pulsar timing and fast radio burst data to set new constraints on millicharged dark matter and axion-like particles, improving our understanding of their possible properties and interactions.

## Contribution

It introduces novel astrophysical constraints on millicharged dark matter and axion-like particles using radio wave timing data, expanding the parameter space limits.

## Key findings

- Constraints on millicharged dark matter: ε/mₘᵢₗₗᵢ ≲ 10⁻⁸ eV⁻¹ for mₘᵢₗₗᵢ ≳ 10⁻⁶ eV.
- Constraints on axion-like particles: g/mₐ ≲ 10⁻¹³ GeV⁻¹ / (10⁻²² eV).
- Current pulsar data constrain new regions of parameter space, with future data expected to improve these bounds.

## Abstract

We derive novel constraints on millicharged dark matter and ultralight axion-like particles using pulsar timing and fast radio burst observations. Millicharged dark matter affects the dispersion measure of the time of arrival of radio pulses in a way analogous to free electrons. Light pseudo-scalar dark matter, on the other hand, causes the polarization angle of radio signals to oscillate. We show that current and future data can set strong constraints in both cases. For dark matter particles of charge $\epsilon e$, these constraints are ${\epsilon}/{m_{\rm milli}} \lesssim 10^{-8}{\rm eV}^{-1}$, for masses $m_{\rm milli}\gtrsim 10^{-6}\,$eV. For axion-like particles, the analysis of signals from pulsars yields constraints in the axial coupling of the order of $g/m_a\lesssim 10^{-13} {\rm GeV}^{-1}/(10^{-22}{\rm eV})$. Both bounds scale as $(\rho/\rho_{\rm dm})^{1/2}$ if the energy density $\rho$ of the components is a fraction of the total dark matter energy density $\rho_{\rm dm}$. We do a detailed study of both effects using data from two samples of pulsars in the galaxy and in globular clusters, as well as data from FRB 121102 and PSR J0437$-$4715. We show that in both cases actual pulsar data constrain a new region of the parameter space for these models, and will improve with future pulsar-timing observations.