Constraints on Axion-photon coupling from the Global 21-cm Signal

TL;DR

The paper investigates how an oscillating ultralight axion field coupled to electromagnetism through a Chern-Simons term can resonantly excite radiation, potentially seeding primordial magnetic fields and providing the LW background needed for direct collapse black hole formation. The author develops global and halo-resonance frameworks, characterizes the resulting radiation spectra (energy cascade or thermalized), and derives constraints from global 21 cm observations by evaluating the ratio ${\cal R}$ of resonant to CMB radiation at the 21 cm line. Key findings show that global 21 cm data constrain the cascade radiation strongly, yielding regions in parameter space (spectral index $n$, coupling $g_{\phi\gamma}$, and energy fraction $f$) that are compatible with magnetic-field generation, while halo-resonance with thermalization is largely unconstrained; halo cascade scenarios admit narrow viable regions demanding small $n$, and escape fraction assumptions influence the allowed space. Overall, the study highlights a parameter window where axion-photon resonant processes can simultaneously support early-universe magnetic fields and DCBH seeds without conflicting with 21 cm data, with the tightest bounds arising for cascade spectra and smaller couplings.

Abstract

A radiation field can be excited via parametric resonance when an oscillating axion field couples to the electromagnetic sector through a Chern-Simons interaction. As demonstrated in previous works, this mechanism can generate primordial magnetic fields shortly after recombination and provide sufficient ultraviolet radiation for the formation of direct collapse black holes (DCBHs). In this study, I analyze constraints on the parametric resonance scenario from global 21cm observations. I find that there exist viable regions in the parameter space that satisfy both observational limits and the physical requirements of the magnetic field and DCBH formation scenarios.

Figures (7)

• Figure 1: Constraints on the $n-f$ plane from global 21cm signal at redshift $(z+1)=20$ The shaded regions represent excluded parameter space for coupling constant ${\tilde{g}_{10}}=1,$ (blue) and $10^{-4}$ (orange).
• Figure 2: The shaded regions indicate the allowed parameter space where the generation of resonant magnetic field is consistent with global 21cm constraints. The color scheme is the same as in Fig. \ref{['fig1']}.
• Figure 3: The ratio ${\cal R}$ as a function of observed reshift $z$. In this figure, we set ${\tilde{g}_{10}}=1$ and $f=10^{-4}$.
• Figure 4: Constraints on the parametric resonance in halos from the global 21cm absorption feature at $(z+1)=20$. The blue and orange shaded regions represent the excluded parameter space for ${\tilde{g}_{10}}=1$ and $10^{-2}$, respectively. The dashed and dotted lines indicate the lower bounds on the radiation energy fraction required to generate sufficient LW radiation for the formation of DCBHs JH-DCBH, assuming escape fraction $\epsilon=1$, and $0.1$, respectively.
• Figure 5: The shaded regions indicate the allowed parameter space where the formation of DCBHs in the scenario presented in JH-DCBH is consistent with global 21cm constraints, under the assumption of an energy cascade spectrum. The color scheme and line styles are the same as in Fig. \ref{['fig4']}. Note that the range of the vertical axis is 3 orders of magnitude smaller than that in the previous figures.