Probing $μ$eV ALPs with future LHAASO observation of AGN $γ$-ray spectra

Abstract

Axion-like particles (ALPs) are predicted in some well-motivated theories beyond the Standard Model. The TeV gamma-rays from active galactic nuclei (AGN) suffers attenuation by the pair production interactions with the cosmic infrared background light (EBL/CMB) during its travel to the earth. The attenuation can be circumvented through photon-ALP conversions in the AGN and Galaxy magnetic-field, and a flux enhancement is expected to arise in the observed spectrum. In this work, we study the potential of the AGN gamma-ray spectrum for energy up to above 100 TeV to probe ALP-parameter space at around µeV, where the coupling gaγ is so far relatively weak constrained. We find the nearby and bright sources, Mrk 501, IC 310 and M 87, are suitable for our objective. Assuming an intrinsic spectrum exponential cutoff energy at Ec=100 TeV, we extrapolate the observed spectra of these sources up to above 100 TeV by the models with/without ALPs. For gaγ 2×10 −11 GeV −1 with ma 1 µeV, the flux at around 100 TeV predicted by the ALP model can be enhanced more than an order of magnitude than that from the standard absorption, and could be detected by LHAASO. Our result is subject to the uncertainty from the intrinsic spectrum above tens of TeV, which require further observations on these sources by the forthcoming CTA, LHAASO, SWGO and so on.

Axion-Like Particle Searches with IACTs

“…This enhancement is expected at energies E crit ∼ 100 TeV [29] and could be detected by the Large High-Altitude Air Shower Observatory (LHAASO) [84], CTA, and by the planned Southern Wide-field Gamma-ray Observatory (SWGO) [85]. Long et al [29] analyzed HE and VHE γ-ray data from three promising AGNs and the spectra were extrapolated to the energies E ∼ 100 TeV. Further on, the assumed intrinsic spectra were folded with the P γγ , assuming the photon-ALP conversions in the source magnetic field and the back-conversion in the magnetic field of the Milky Way.…”

“…Probably the most interesting part of yet-unexplored ALPs parameter space is accounting for ALPs which are able to explain and constitute most or all of the DM in the universe. This region is anticipated in the future γ-ray experiments, such as CTA [42] and SWGO [85], or in LHAASO [29], that will be able to explore higher energies of up to about 100 TeV and exclude masses of ALPs of m a ∼ 200 neV [42]. With these and other upcoming laboratory axion experiments, constraints on the ALPs parameter space, or even a possible detection of the ALPs, are increasingly anticipated.…”

“…These spectra were compared to the ones obtained only by including the EBL and CMB attenuation. The results showed that, in the respective energy range (above E ∼ 100 TeV), predicted flux enhancement is above one order of magnitude and higher than the sensitivity of the instrument, which will allow for the constraints to be set on the ALPs parameter space [29]. It is also emphasized that, to set stringent constraints, a better estimation of the intrinsic spectra, magnetic fields and EBL attenuation needs to be established, all of which are expected with the upcoming experiments in HE and VHE γ-ray astronomy.…”

“…As investigated by Long et al [29], new observations of VHE γ-ray sources could lead to the detection of the flux enhancement due to the ALP-photon back-conversion in the Galactic magnetic field. This enhancement is expected at energies E crit ∼ 100 TeV [29] and could be detected by the Large High-Altitude Air Shower Observatory (LHAASO) [84], CTA, and by the planned Southern Wide-field Gamma-ray Observatory (SWGO) [85]. Long et al [29] analyzed HE and VHE γ-ray data from three promising AGNs and the spectra were extrapolated to the energies E ∼ 100 TeV.…”

“…While the first term is governed by the microscopic nature of ALP, there are several magnetic field realizations in the universe. Therefore, one needs to consider all different magnetic field environments in the path, from the source to the detector: photon-ALP mixing can be assumed in the magnetic field at the source, in the local environment around the source, in the intergalactic magnetic field and, finally, in the galactic magnetic field [29]. Depending on the observed source, different combinations of magnetic fields can be considered, and as reported by Sikivie [13], there are clearly several ways this problem can be approached.…”