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Abstract
Abstract 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 ); 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 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 for an ensemble of ICM magnetic field models, and then use the NGC 1275 spectrum to constrain the -plane. Marginalizing over magnetic field realizations, the 99.7% credible region limits the ALP-photon coupling to (depending upon magnetic field model) for masses . These are the most stringent limit to date on 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.
Highlights
Astrophysical observations have great potential to uncover new physics beyond the Standard Model (SM)
The X-ray transparency of the intracluster medium (ICM) in galaxy clusters is a powerful probe of light axion-like particles (ALPs); 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, imprinting distortions on the X-ray spectrum
7 keV), we find that the 1–9 keV spectrum of NGC 1275 is accurately described by a power-law continuum form modified by the effects of modest Galactic absorption; deviations from the power-law are at the ±3% level
Summary
Astrophysical observations have great potential to uncover new physics beyond the Standard Model (SM). The ICM emission that forms our background is centrally concentrated around the AGN, so we expect that the spectrum extracted from the background/flanking regions will be normalized too low It is not possible, to estimate the size of this effect from the surface brightness profile — the dispersion of the ICM emission in this slit-less grating system together with the order-sorting algorithm (where the intrinsic energy resolution of the ACIS is used to reject all photons that definitely lie outside of the expected map of dispersed position to energy) makes the background normalization a non-trivial function of the ICM spatial and spectral structure. Having probably reached the level dominated by systematic calibration uncertainties, this is the highest quality 1-9 keV band spectrum of this AGN obtained to date and, as we shall see, permits the most sensitive search yet for light ALPs
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