Resonant Axion-Photon Conversion in the Early Inspiral of Neutron Star Binaries

Abstract

We consider the early binary neutron star inspiral phase as a scenario to probe environmental axion--photon resonant conversion. For this we approximately model the merger site electromagnetic fields as the superposition of two rotating dipolar stellar magnetic fields at the thousand--km scale when both magnetospheres are not largely distorted. We capture the time-sliced near-zone magnetospheric geometry relevant for axion--photon mixing. Plasma effects are incorporated through an effective Goldreich--Julian charge density, used to determine the effective plasma frequency and the location of resonant conversion surfaces. Our results show that axion--photon resonant conversion in binary magnetospheres mostly occurs on extended peanut-shaped surfaces whose global geometry evolves as the binary inspiral evolves. As a consequence, the total electromagnetic power emitted through axion--photon conversion exhibits a characteristic dependence on axion mass and a slow temporal modulation correlated with the gravitational wave frequency emission. This feature is potentially detectable for $m_a \in [50,170] \,\rm μeV$ and set $g_{a γ} \lesssim 10^{-11}\rm \,GeV^{-1}$ as it lies within the sensitivity limits of current or planned radio observation missions. In light of our results we discuss the opportunity of binary neutron star inspirals as time-dependent, multimessenger probes of axion physics, and motivate coordinated searches combining gravitational wave observations with radio and millimeter wavelength electromagnetic measurements.

Figures (5)

• Figure 1: Configuration of magnetic field lines in the meridional plane of the binary neutron star system for time-sliced near-zone geometry. The panel shows the superposition of dipolar fields: a) isolated NS dipole b) superposition of both dipoles c) misaligned dipole moment for the right-hand star.
• Figure 2: Validity region of our model in the $d-\omega$ space. The purple area between solid and dashed black lines limits the boundary where our model's accuracy begins to degrade. In the lower pink band region tidal deformation effects start to manifest. Blue, orange and green curves denote orbital angular frequency $\Omega$ considering total BNS mass $M_\mathrm{tot}=$2,4, 6$M_\odot$ respectively, are superimposed on the same horizontal axes for the sake of comparison. The red dot shows our reference case at 1000 km.
• Figure 3: Gravitational wave frequency as a function of orbital separation for a quasi-circular BNS system. The blue curve shows the Keplerian relation, while the red point marks the reference case $d = 1000\,\mathrm{km}$, $f_{\mathrm{GW}} \simeq 6.14\,\mathrm{Hz}$ for two $1.4\,M_{\odot}$ NSs) belonging to the DECIGO/BBO and 3G (ET/CE) band. Shaded regions indicate the approximate sensitivity bands of current and future detectors, with the tidal regime excluded.
• Figure 4: Resonant 2D isocontours for a binary system of two NSs with aligned spins and parallel magnetic fields, sharing the same spin frequency $\omega$ and surface magnetic field strength $B_S=10^{14}$ G. Each isocontour corresponds to a fixed axion mass at resonance. Arc length associated with each resonant contour for selected cases: blue circle $m_a=0.58 \, \rm \mu eV,\,L=7600 \rm \, km$, orange squared $m_a=1.4 \, \rm \mu eV, \,L=5400\, \rm km$, green triangle $m_a=5.5 \, \rm \mu eV, L=2300\, \rm km$), red squared $m_a=23 \, \rm \mu eV,\, L=929\, \rm km$, purple triangle $m_a=92 \, \rm \mu eV,\, L=365\, \rm km$.
• Figure 5: Predicted spectral flux density $F_{\nu,a\gamma}$ as a function of the photon frequency $\nu_\gamma$ and the GW frequency $f_{\mathrm{GW}}$ (from $d=1000\,\mathrm{km}$ to $400\,\mathrm{km}$) for a source distance of $10\,\mathrm{kpc}$. We set $g_{a \gamma}=10^{-12}\rm \,GeV^{-1}$ and $\rho_a\sim 10^{23}\, \rm GeV cm^{-3}$ for this plot. Vertical dashed lines mark the representative observing bands of radio and millimeter facilities, SKA (from 0.05 to 15 GHz) and ALMA (from 35 to 50 GHz). VLA (not shown) covers from 0.01 to 50 GHz. The band-like structures reflect the precursor signal in the inspiral-driven modulation of the resonant axion--photon conversion signal as successive resonant surfaces appear and disappear during the binary evolution.