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 } { \thinspace 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 NGC 1275 at the center of the Perseus cluster .
Employing a 490 ks High-Energy Transmission Gratings ( HETG ) exposure , we obtain a high-quality 1–9 keV 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 NGC 1275 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 } { GeV } ^ { -1 } ( depending upon magnetic field model ) for masses m _ { a } < 1 \times 10 ^ { -12 } { \thinspace 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 .