Constraints on axionlike particles from 16.5 years of Fermi-LAT data and prospects for VLAST
Axionlike particles (ALPs), hypothetical particles beyond the Standard Model, are considered as promising dark matter candidates. ALPs can convert into photons and vice versa in a magnetic field via the Primakoff effect, potentially generating detectable oscillation in $γ$-ray spectra. This study analyzes 16.5 years of data from the Fermi Large Area Telescope (Fermi-LAT) on NGC 1275, the brightest galaxy in the Perseus cluster, to constrain the ALP parameter space. Our results improve the previous 95\% exclusion limits of the photon-ALP coupling $g_{aγ}$ by a factor of 2 in the ALP mass range of $4\times 10^{-10}\,\mathrm{eV}\lesssim m_{a}\lesssim 5\times 10^{-9}\,\mathrm{eV}$. Moreover, we investigate the projected sensitivity of the future Very Large Area $γ$-ray Space Telescope (VLAST) on searching for ALPs. We find that (i) the expected sensitivity on the ALP-photon coupling can be stronger than that from the upcoming International Axion Observatory (IAXO) in the ALP mass range of $2\times 10^{-11}\,\mathrm{eV}\lesssim m_{a}\lesssim 1\times 10^{-7}\,\mathrm{eV}$, with the best sensitivity of $g_{aγ}\sim 7\times 10^{-13}\,\mathrm{GeV^{-1}}$ at $m_{a}\sim 2\times 10^{-10}\,\mathrm{eV}$; (ii) VLAST can extend the sensitivity of the ALP masses below $5\times 10^{-12}\,\mathrm{eV}$, where the ALP-photon coupling $g_{aγ}\gtrsim 1.5\times 10^{-11}\,\mathrm{GeV^{-1}}$ will be excluded; (iii) the entire parameter space of ALP accounting for TeV transparency can be fully tested. These results demonstrate that VLAST will offer an excellent opportunity for ALPs searches.