Map of ALP parameter CLs values. See Figure 3 below for more details.

Reference: C.B. Adams et al. (The VERITAS Collaboration), accepted for publication in Phys. Rev. D (2025)

Full text version

ArXiv: ArXiV: 2510.19010

Contacts: Qi Feng

Background: Axion-like particles (ALPs) are hypothetical particles that emerge in numerous theoretical extensions to the Standard Model. Their coupling to the 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 γ-ray sources.

Data: In 2017, the VERITAS γ-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 µG) 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 2×10⁻⁷ eV ≲ m_a ≲ 4×10⁻⁷ eV and at the coupling strength of g_aγ ≳ 3×10⁻¹¹ GeV⁻¹ up to 80% confidence level, which are consistent with previous studies.

Conclusions: We find the CLs to be a trustworthy approach, and advocate for its continued usage in future studies. We note that many of the limitations contributing to the limited sensitivity seen by VERITAS in this study will be improved with next-generation γ-ray instruments, such as the Cherenkov Telescope Array Observatory (CTAO).
 

FITS files: N/A

Figures from paper (click to get full size image):

 

Figure 1:  SED for the January 2nd, 2017 observations of the NGC 1275 flare. The best fit model of the joint likelihood without ALPs (with an ALP of ma = 1.61 × 10−8 eV and gaγ = 3.16 × 10−11 GeV−1 ) is shown as a solid (dashed) black line for one realization of the ICMF.

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Figure 2: Cumulative distribution function (CDF) for the TS values calculated from 500 simulations produced assuming the no-ALP case. The CDF is fitted with a modified Γ 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 confidence level shown in the legend.

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Figure 3: Map of ALP parameter CLs values. Points are excluded with confidence 1 − CLs. The point of highest VERITAS 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 14×9 grid has been upsampled by a factor of three and smoothed with linear interpolation.

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