[Paper Review] Listening for Dark Photon Radio from the Galactic Centre

Edward Hardy, Ningqiang Song|arXiv (Cornell University)|Dec 19, 2022

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper proposes that dark photon dark matter, via kinetic mixing with standard model photons, can resonantly convert into detectable radio waves in the plasma environments of neutron stars and accreting white dwarfs near the galactic centre. Using detailed plasma and magnetic field models, the authors show future radio telescopes like SKA, ALMA, and GBT could probe kinetic mixing parameters up to orders of magnitude below current limits for dark photon masses between 6×10⁻⁶ eV and 7×10⁻⁴ eV.

ABSTRACT

Dark photon dark matter that has a kinetic mixing with the Standard Model photon can resonantly convert in environments where its mass $m_{A'}$ coincides with the plasma frequency. We show that such conversion in neutron stars or accreting white dwarfs in the galactic centre can lead to detectable radio signals. Depending on the dark matter spatial distribution, future radio telescopes could be sensitive to values of the kinetic mixing parameter that exceed current constraints by orders of magnitude for $m_{A'} \in \left(6 imes 10^{-6},7 imes 10^{-4} ight)$ eV.

Motivation & Objective

• To investigate resonant conversion of dark photon dark matter into radio photons in dense astrophysical environments with plasma frequencies matching the dark photon mass.
• To assess the detectability of such signals using upcoming radio telescopes like SKA, ALMA, and GBT.
• To model the effects of magnetic fields and plasma anisotropy on conversion efficiency and photon propagation.
• To improve sensitivity limits on kinetic mixing parameters by leveraging the galactic centre's high dark matter density.
• To quantify the impact of dephasing and trajectory deviations on conversion probability in realistic astrophysical plasma conditions.

Proposed method

• Derives general equations for dark photon-to-photon conversion in anisotropic, magnetized plasma using relativistic field theory and dispersion relations.
• Applies the resonant conversion condition where the dark photon mass equals the plasma frequency, enabling efficient conversion.
• Models neutron star and white dwarf environments with realistic electron density profiles, magnetic fields, and plasma temperatures.
• Uses ray-tracing and phase integral analysis to estimate dephasing effects from refractive path differences between dark photons and photons.
• Computes conversion probabilities and resulting radio signal fluxes for different telescope sensitivities and observation times.
• Evaluates detector sensitivity by projecting signal-to-noise ratios for SKA1, SKA2, ALMA, and GBT across a range of dark photon masses and mixing parameters.

Experimental results

Research questions

• RQ1 Can resonant conversion of dark photons into radio photons occur in the plasma of neutron stars and white dwarfs near the galactic centre?
• RQ2 How do magnetic fields and plasma anisotropy affect the efficiency and phase coherence of the conversion process?
• RQ3 To what extent can future radio telescopes like SKA and ALMA surpass existing experimental constraints on kinetic mixing for dark photon dark matter?
• RQ4 What is the impact of dephasing due to refractive path differences on the detectable radio signal strength?
• RQ5 How does the dark matter density profile near the galactic centre influence the cumulative signal from multiple neutron stars?

Key findings

• Future radio telescopes such as SKA, ALMA, and GBT could achieve sensitivity to kinetic mixing parameters that exceed current constraints by up to three orders of magnitude for dark photon masses between 6×10⁻⁶ eV and 7×10⁻⁴ eV.
• The cumulative signal from neutron stars within 3 pc of the galactic centre could be detectable after 100 hours of observation with SKA2, assuming a gNFW dark matter profile.
• Accreting white dwarfs located 0.3 pc from the galactic centre can produce detectable radio signals, with sensitivity comparable to or better than current haloscope limits.
• Dephasing effects from refractive path differences are negligible in isotropic plasma environments, with phase shifts φ ≪ 1, indicating minimal reduction in conversion efficiency.
• The inclusion of a dark matter density spike near the galactic centre enhances detectability, making the Breakthrough Listen limit visible only under such conditions.
• The study demonstrates that white dwarfs are promising targets due to their high surface escape velocity and favorable plasma conditions for resonant conversion.