Axions/axion-like particles (ALPs) are a well motivated extension of the Standard Model and are generic within String Theory. The X-ray transparency of the intracluster medium (ICM) in galaxy clusters is a powerful probe of light ALPs (with mass $<10^{ -11}\,{ \rm eV}$); as X-ray photons from an embedded or background source propagate through the magnetized ICM, they may undergo energy-dependent quantum mechanical conversion into ALPs (and vice versa), imprinting distortions on the X-ray spectrum. We present Chandra data for the active galactic nucleus NGC1275 at the center of the Perseus cluster. Employing a 490ks High-Energy Transmission Gratings (HETG) exposure, we obtain a high-quality 1-9keV spectrum free from photon pileup and ICM contamination. Apart from iron-band features, the spectrum is described by a power-law continuum, with any spectral distortions at the $<3\%$ level. We compute photon survival probabilities as a function of ALP mass $m_a$ and ALP-photon coupling constant $g_{ a\gamma}$ for an ensemble of ICM magnetic field models, and then use the NGC1275 spectrum to constraint the $(m_a, g_{ a\gamma})$-plane. Marginalizing over magnetic field realizations, the 99.7% credible region limits the ALP-photon coupling to $g_{ a\gamma}<6-8\times 10^{ -13}\, { \rm GeV}^{ -1}$ (depending upon magnetic field model) for masses $m_a<1\times 10^{ -12}\,{ \rm eV}$. These are the most stringent limit to date on $g_{ a\gamma}$ for these light ALPs, and have already reached the sensitivity limits of next-generation helioscopes and light-shining-through-wall experiments. We highlight the potential of these studies with the next-generation X-ray observatories Athena and Lynx, but note the critical importance of advances in relative calibration of these future X-ray spectrometers.
Categories: hep-ph astro-ph.CO astro-ph.HE