Constraining Axion-like Particles through Multi-epoch Monitoring of Strong Gravitational Lenses

TL;DR

The paper develops and applies a multi-epoch polarimetric approach to constrain ultralight axion-like particles via differential birefringence between lensed images in CLASS B1152+199. By modeling the ALP-induced rotation as a coherent or incoherent oscillation with mass $m_a$ and coupling $g_{a\gamma}$, and by employing RM synthesis to mitigate chromatic Faraday rotation, the authors extract differential birefringence angles across six epochs (spanning 9.5 years when including archival data). A profile-likelihood analysis under coherent oscillation and a weighted-average method under incoherent phases yield upper limits on $g_{a\gamma}$ that surpass CAST in the $m_a \sim 10^{-22}$–$10^{-18}$ eV range, with strongest bounds around $m_a \sim 5\times10^{-22}$ eV. The results demonstrate the power of multi-epoch, strong-lens polarimetry and highlight the potential of future facilities (e.g., SKA-Mid, LOFAR2.0) to further extend the ALP search, while noting caveats such as washout and spatial coherence effects. Overall, the work provides a robust, scalable framework for probing ultralight ALPs using differential birefringence in strong gravitational lenses.

Abstract

We present new constraints on ultralight axion-like particles (ALPs) through multi-epoch measurements of differential birefringence induced due to a coupling ($g_{aγ}$) between the ALP and electromagnetic fields. Broadband polarimetric observations in the 2-8 GHz range of the gravitationally lensed system CLASS B1152+199 were carried out over five epochs spanning three months with a cadence of roughly 20 days, and the differential birefringence angle ($Δ\,θ_{a,{\rm lens}}$) between the lensed images were estimated. We also combined an archival observation that effectively increases the span to 9.5 yr to probe the effect of an oscillating ALP field imprinted as oscillating ${Δ\,θ_{a,{\rm lens}}}$ over time. Here we present a new technique for combining multi-epoch measurements of ${Δ\,θ_{a,{\rm lens}}}$ by considering the coherence of the ALP field, such that, ${Δ\,θ_{a,{\rm lens}}}$ over these observations are related. The time scale of coherence depends on the mass of the ALP field ($m_a$). With these new observations, we constrain $g_{aγ} \leq 9.0\times 10^{-12} \,\left( {ρ_{a,\text{em}}}/{20 \text{ GeV cm}^{-3}} \right)^{-1/2}\;\mathrm{GeV}^{-1}$ to $\leq 3.5\times 10^{-8} \,\left( {ρ_{a,\text{em}}}/{20 \text{ GeV cm}^{-3}} \right)^{-1/2}\;\mathrm{GeV}^{-1}$ for $m_a$ between $1.6\times 10^{-22}\;\mathrm{eV}$ and $3.8\times 10^{-18}\;\mathrm{eV}$, where $ρ_{a,{\rm em}}$ is the density of the ALP field at emission. This improves over the constraint provided by the CERN Axion Solar Telescope by up to an order of magnitude in the $m_a$ range $1.6\times 10^{-22}\;\mathrm{eV}$ to $3\times 10^{-21}$ eV.

Figures (9)

• Figure 1: Hubble Space Telescope image of CLASS B1152+199 observed using the F814W filter is overlaid by the VLA total intensity radio contour at 4.5 GHz. The resolution of the radio image is 0.5 arcsec shown as the white circle, and the blue contours are at 2, 10, 18, 26 and 34 $\rm mJy\,beam^{-1}$. The two lensed images A and B are observed in both radio and optical frequencies whereas the lensing galaxy is only visible in the optical observations.
• Figure 2: Left panel shows the Stokes $I$ spectra averaged over 20 MHz for the lensed images A and B in B1152+199. At all the epochs, the Stokes $I$ flux densities are within the 5% flux scale uncertainty of the VLA. Right panel shows the lens magnification factor, $\mu = S_{\rm A}(\nu)/S_{\rm B}(\nu)$, of the lensed images as a function of frequency for the five epochs. To avoid overlap, each epoch is plotted with an offset shown in the right-hand $y$-axis. The grey lines are the corresponding best-fit $\mu$, again plotted with an offset. The shaded region in both the panels covering 1--2 GHz shows significant fluctuations due to blending of image A and B and the data in that frequency range were not used in our analyses.
• Figure 3: Faraday depth spectra of the two lensed images in B1152+199 for the 5-Epochs obtained from RM synthesis. The two peaks correspond to Image A (dashed lines) and B (solid lines).
• Figure 4: The PDFs of $\Delta \theta_{a,\mathrm{lens}}$ obtained from the 5-Epochs observation of B1152+199, drawn from a sample of 50,000 relisations are well-approximated by the Gaussian distribution shown as solid lines. The shaded green and orange histograms are for 5-Epochs and 6-Epochs combined ${\Delta\,\theta_{a,{\rm lens}}^{(n)}}$ through weighted average discussed in section \ref{['sec:WtAvg']}. For comparison, the distribution of ${\Delta\,\theta_{a,{\rm lens}}^{(n)}}$ obtained for a single epoch in Basu et al. Basu2021 is shown as the shaded grey histograms.
• Figure 5: Estimated differential birefringence angle, $\Delta\,\theta_{a,{\rm lens}}^{(n)}$, from B1152+199 as a function of time at emission, $\Delta\,t_{\rm em}$, where the observations on 08 March 2022 is assumed as the reference, are shown as the data points. The dashed magenta and dash-dotted turquoise lines show the best-fit to eq. (\ref{['Eq:DiffBir_multi']}) obtained using maximum likelihood estimation (MLE) for 5-Epochs and 6-Epochs, respectively. The model captures oscillation of ALP field at emission. For comparison, the horizontal grey dashed line shows the null hypothesis.