Constraints on axionlike particles from 16.5 years of Fermi-LAT data and prospects for VLAST

TL;DR

We address the search for axionlike particles (ALPs) by modeling photon-ALP oscillations in realistic magnetic fields and applying a detailed likelihood analysis to 16.5 years of Fermi-LAT data from NGC 1275. Using a grid of ALP masses $m_a$ and couplings $g_{a\gamma}$, the study exploits the characteristic spectral modulations produced by the Primakoff effect, incorporating energy dispersion and turbulent magnetic-field realizations with Monte Carlo-derived significance thresholds. The analysis excludes $g_{a\gamma} \gtrsim 3\times 10^{-12}\,\mathrm{GeV^{-1}}$ for $4\times10^{-10}\lesssim m_a \lesssim 5\times10^{-9}\ \mathrm{eV}$, improving previous limits by ~2x and removing a previously allowed region. Five-year VLAST projections indicate substantially enhanced sensitivity, with potential to surpass IAXO in a broad mass range ($2\times10^{-11}\lesssim m_a\lesssim 1\times10^{-7}\ \mathrm{eV}$) and to probe $m_a$ below $5\times10^{-12}\ \mathrm{eV}$, including the TeV transparency regime.

Abstract

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.

Figures (9)

• Figure 1: Survival probability of photons from NGC 1275 under different ALP parameters based on one realization of the Gaussian turbulent magnetic field, the "Dominguez" EBL model and the "Jansson12" GMF model. In the left panel, with the fixed coupling ($g_{a\gamma}=5\times 10^{-11}\,\mathrm{GeV^{-1}}$), the photon survival probability is presented for different ALP masses (red solid line: $m_{a}\ll\omega_{\mathrm{pl}}$; green solid line: $m_{a}=1\times 10^{-9}\,\mathrm{eV}$; blue solid line: $m_{a}=1\times 10^{-6}\,\mathrm{eV}$). When the ALP mass is significantly smaller than the plasma frequency, its effect on the photon survival probability becomes negligible. In the right panel, with the ALP mass fixed ($m_{a}=1\times 10^{-9}\,\mathrm{eV}$), the photon survival probability is presented for different couplings (red solid line: $g_{a\gamma}=1\times 10^{-13}\,\mathrm{GeV^{-1}}$; green solid line: $g_{a\gamma}=5\times 10^{-12}\,\mathrm{GeV^{-1}}$; blue solid line: $g_{a\gamma}=5\times 10^{-11}\,\mathrm{GeV^{-1}}$).
• Figure 2: The SEDs analysis results for NGC 1275. Top panel: The SEDs of NGC 1275. Black data points show the Fermi-LAT observational results. The blue dashdot line represents the best-fit spectrum without an ALP model, while the red solid line shows the best-fit spectrum including an ALP model with $m_{a}=4.72\times 10^{-8}\,\mathrm{eV},g_{a\gamma}=4.89\times 10^{-11}\,\mathrm{GeV^{-1}}$. Bottom panel: $\chi^{2}$ values for each energy bin. Blue solid line with circles indicates $\chi^{2}$ values without the ALP model, and red solid line with triangles correspond to $\chi^{2}$ values including the ALP model with the above specified parameters.
• Figure 3: Results of 2500 Monte Carlo simulations based on the null hypothesis spectral model. The red vertical dashed line indicates the statistical significance threshold $\Delta\chi^{2}_{\mathrm{thr}}=9.28$ corresponding to the 95% confidence level.
• Figure 4: Two-dimensional $\Delta\chi^{2}$ distribution in the parameter space of ALP masses versus photon-ALP couplings for NGC 1275. The solid black line indicates the 95% confidence level threshold ($\Delta\chi^{2}$=9.28).
• Figure 5: Sixty-eight percent energy resolution as a function of incident energy. The blue dashed line represents the energy resolution of Fermi-LAT (data version: P8R3_SOURCE_V3), while the red solid line shows the preliminary simulated energy resolution for VLAST. The curve in the low-energy regime corresponds to Compton scattering events, whereas the high-energy portion reflects pair-production events.