Constraints on Axion-Like Particles from VERITAS Observations of a Flaring Radio Galaxy in the Perseus Cluster

TL;DR

This study searches for axion-like particles (ALPs) via ALP–photon mixing in the magnetic field of the Perseus cluster, using VERITAS TeV observations of NGC 1275 during a dramatic 2017 flare. The authors model three propagation environments, compute the ALP survival probability with gammaALPs, and perform a CL_s-based exclusion analysis across a grid in $(m_a, g_{aγ})$, incorporating 100 turbulent magnetic-field realizations per point. They find no evidence for ALPs and report conservative exclusions, with a peak sensitivity around $m_a \sim 2$–$4×10^{-7}$ eV and $g_{aγ} \gtrsim 3×10^{-11}$ GeV$^{-1}$ up to about 80% CL, but no 95% CL exclusions. The work highlights the utility of the CL_s method for high-energy astrophysical searches and discusses limitations and future improvements, particularly with the upcoming CTA, which could tighten ALP constraints through enhanced energy resolution and sensitivity.

Abstract

Background: Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to electromagnetic field implies that ALPs would mix with photons in the presence of external magnetic fields. As ALP phenomenology is governed by the mass and strength of its coupling, there is a subset of this parameter space in which this mixing would be expected to leave an imprint on the spectra of TeV gamma-ray sources. Data: In 2017, the VERITAS gamma-ray observatory recorded the second day of a dramatic flare of the radio galaxy NGC 1275, embedded at the center of the Perseus galaxy cluster. This serendipitous locale provides a spatially-extended magnetic field of strength O(10$μ$G) through which escaping photons traverse, making it an excellent target to study ALPs. Methods: We analyze the VERITAS data of NGC 1275's 2017 flare with the gammapy analysis package. Extensive fitting and modeling are performed to ultimately conduct a likelihood analysis used to search for any evidence of a preference for ALPs and to explore the confidence with which constraints can be set. We adopt the CLs method for this study for its conservative approach to setting limits in regimes where the search has limited sensitivity. Results: No evidence for the existence of ALPs is found, and no combination of mass and coupling strength can be excluded at or above 95% confidence level. We provide a map showing the strength of our exclusions in the mass and coupling parameter space. The strongest exclusions are found in the mass range $2 \times 10^{-7}$eV $\lesssim m_a \lesssim 4 \times 10^{-7}$eV and at the coupling strength of $g_{aγ} \gtrsim 3 \times 10^{-11}$ GeV$^{-1}$ up to 80% confidence level, which are consistent with previous studies. Conclusions: We find the CLs method to be a trustworthy approach, and advocate for its...

Figures (6)

• Figure 1: sed for the January 2nd, 2017 observations of the NGC 1275 flare. The best fit model of the joint likelihood without alp (with an alp of ${m_a\xspace = 1.61\times 10^{-8} \rm~eV\xspace}$ and ${g_{a\gamma}\xspace = 3.16 {\rm~\times~10^{-11}~GeV^{-1}}\xspace}$) is shown as a solid (dashed) black line for one realization of the icmf.
• Figure 2: cdf for the TS values calculated from 500 simulations produced assuming the no-*alp case. The cdf is fitted with a modified $\Gamma$ distribution. Black vertical lines show the 68% and 95% TS thresholds, and the red dotted line shows the TS value of the data, with its associated cl shown in the legend.
• Figure 3: Map of *alp parameter ${\rm CL_s}$ values. Points are excluded with confidence $1-{\rm CL_s}$. The point of highest vts TS is indicated by a yellow triangle and annotated with its value. As a reference, 95% exclusions from *cast are above the white line. For visualization purposes, the sparse $14 \times 9$ grid has been upsampled by a factor of three and smoothed with linear interpolation.
• Figure 4: The distribution of the test statistic ($Q$) values calculated using Equation \ref{['eqn:qstat']} at two test points ${(m_a\xspace = 2\times 10^{-7} \rm~eV\xspace}$, ${g_{a\gamma}\xspace = 1 {\rm~\times~10^{-10}~GeV^{-1}}\xspace)}$ and ${(m_a\xspace = 3\times 10^{-7} \rm~eV\xspace}$, ${g_{a\gamma}\xspace = 1 {\rm~\times~10^{-12}~GeV^{-1}}\xspace)}$, respectively. The blue distributions are for the null hypothesis, and the orange distributions are for the alternative ALP hypothesis. The red vertical line is the test statistic $Q$ calculated from the data. $a$, $\theta$, and $x_0$ are the shape, scale, and location parameters resulting from a fit to a gamma function.
• Figure 5: The map of exclusion confidence levels derived from the $\lambda$ values calculated using Equation \ref{['eqn:lambda']}, covering the same *alp parameter space as Figure \ref{['fig:cls_space']}. The dashed black contours indicate the regions excluded at 68% and 75%. The point of highest vts TS is indicated by a yellow triangle and annotated with its value.