Astrophysical limits on very light axion-like particles from Chandra grating spectroscopy of NGC 1275

Axions and axion-like particles (ALPs) are a well motivated extension of the Standard Model and are generic in String Theory. The X-ray transparency of the magnetized intracluster medium (ICM) in galaxy clusters is a powerful probe of very light ALPs (masses $0<10^-11, m eV$); as X-ray photons propagate through the magnetic field of the ICM, they may undergo energy-dependent quantum mechanical conversion into ALPs (and vice versa), imprinting distortions on the observed X-ray spectrum. We present new Chandra data for the active galactic nucleus NGC1275 at the center of the Perseus cluster. Employing the High-Energy Transmission Gratings (HETG) with a 490ks exposure, we obtain a high-quality 1-9keV spectrum free from photon pileup and ICM contamination. Apart from iron-band features, the spectrum is accurately 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 $g_agamma$ for an ensemble of ICM magnetic field models, and then use the NGC1275 spectrum to derive constraints on the $(m_a, g_agamma)$-plane. Marginalizing over the magnetic field realizations, the 99.7% credible region limits the ALP-photon coupling to $g_agamma<6-8 imes 10^-13, m GeV^-1$ (depending upon the magnetic field model) for masses $m_a<1 imes 10^-12, m eV$. These are the most stringent limit to date on $g_agamma$ for these very 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.