Sensitivity to Axion-like Particle dark matter with very-high-energy gamma-ray observations of Active Galactic Nuclei located behind Galaxy Clusters

Abstract

Axion-Like-Particles (ALPs) are hypothetical pseudo-scalar particles actively searched as light dark matter candidates. The coupling of ALPs to photons can give rise to distinctive spectral features in the observed gamma-ray spectrum of astrophysical sources. We perform a forecast study on the sensitivity to ALP-photon interactions using stacked mock observations of selected active galactic nuclei (AGNs) located behind galaxy clusters (GC). The ALP-photon conversion in the magnetic fields of galaxy clusters give rise to absorption-like features in AGN spectra that are subject to large variance in their prediction for individual sources. We consider here a stacking analysis of multiple AGN-cluster pairs, which yields a more controlled prediction of the expected ALP-induced spectral patterns in the observed gamma-ray spectra. Using realistic mock observations of selected Fermi-LAT AGNs by ongoing Imaging Atmospheric Cherenkov Telescopes such as H.E.S.S., MAGIC and VERITAS, we provide a careful assessment of the expected sensitivity of a combined statistical analysis of many AGN-GC pairs, together with the impact of modelling and instrumental uncertainties. The sensitivity reaches ALP-photon couplings down to 6$\times$10$^{-13}$ GeV$^{-1}$ for an ALP mass of 3$\times$10$^{-8}$ eV, and is currently statistically dominated indicating further improvements from more observations. Such a stacking analysis approach enables exploration of the yet-uncharted ALP dark matter parameter space in the 10$^{-8}$ - 10$^{-7}$ eV mass range.

Figures (7)

• Figure 1: Left panel: Photon survival probability $P_{\gamma\gamma}$and its averages as a function on the photon energy. Photon survival probability when passing through the galaxy cluster for different realizations of the cluster magnetic field properties (blue lines). All realizations have the same radial profile of the magnetic field but vary randomly in their orientation in the photon polarization plane and in the sizes of the magnetic domains where the field remains approximately constant. Green and red lines demonstrate the effect of averaging over 16 and 100 randomly selected realizations, respectively. Black dotted-dashed line shows the analytical approximation to these lines in case of infinite number of objects. The ALP parameters for all curves are (m$_a$, g$_{a\gamma\gamma}$) = (20 neV, 7$\times$10$^{-13}$ GeV$^{-1}$). Blue curves were obtained by numerically solving ALP propagation equations via the ALPro code Matthews:2022gqi. Right panel: 1$\sigma$ scatter value $\delta p_0/p_0$ as a function of the number of stacked objects for various values of the ALP mass $m_a$ and photon-ALP coupling $g_{12}\equiv g_{a\gamma\gamma} \cdot 10^{12}$ GeV. See text for further details.
• Figure 2: Comparison of the absorption coefficient $e^{-\tau(E)}$ as a function of energy $E$ for three different EBL models, Franceschini Franceschini:2008tp (solid line), Dominguez Dominguez:2010bv (dashed line) and Finke Finke:2009xi (dotted line), respectively. The absorption coefficient is computed for the source redshifts $z$ of 0.02, 0.5 and 1, respectively.
• Figure 3: Sensitivity on the $g_{a\gamma\gamma}$ coupling versus ALP mass $m_a$ for combined mock datasets of IACT observations assuming the Francheschini EBL model. The sensitivity is expressed as the 95% C. L. mean expected upper limit. The sensitivity is given for mock observations of 11 AGNs by H.E.S.S. (H, red line), 15 by VERITAS (V, orange line), 15 AGNs by MAGIC (M, yellow line). Combined mock observations by H.E.S.S. and MAGIC (H+V, 26 datasets, pink line), and by H.E.S.S., MAGIC and VERITAS (H+M+V, 41 datasets, blue line), respectively, are also displayed. The grey-shaded region corresponds to excluded parameter space by present constraints axionlimits. Below the dashed thick line corresponds to the region of the parameter space where ALP can comprise all the DM in the universe Arias:2012az.
• Figure 4: Sensitivity on the $g_{a\gamma\gamma}$ coupling versus ALP mass $m_a$ for combined mock datasets of IACT observations for different EBL models, i.e., the Francheschini (solid line), Dominguez (dashed line), and Finke (dotted line) models, respectively. The sensitivity is expressed as the 95% C. L. mean expected upper limit. The sensitivity is given for mock observations by H.E.S.S. of 11 AGNs (reds line) and combined mock observations by H.E.S.S., MAGIC and VERITAS of 41 AGNs (blue line), respectively. Below the dashed thick line corresponds to the region of the parameter space where ALP can comprise all the DM in the universe Arias:2012az.
• Figure 5: Top left panel: TS map expressed in the ($g_{a\gamma\gamma}$, $m_a$) for mock observations of 11 datasets seen by H.E.S.S. for an exposure of 50h for each. The mock observations are simulated with the Francheschini EBL model, and the computation of the sensitivity to $g_{a\gamma\gamma}$ as a function of $m_a$ assumes the Finke EBL model. Top right panel: TS map for mock observations of 41 datasets with combined observation by H.E.S.S., MAGIC and VERITAS. Bottom panels: TS map for mock observations of the J1144.9+1937 (z=0.02) AGN by H.E.S.S. for an observation time of 100 h on the left and 500 h on the right.