New constraints on light Axion-Like Particles using Chandra Transmission Grating Spectroscopy of the powerful cluster-hosted quasar H1821+643

Abstract: Axion-Like Particles (ALPs) are predicted by several Beyond the Standard Model theories, in particular, string theory. In the presence of an external magnetic field perpendicular to the direction of propagation, ALPs can couple to photons. Therefore, if an X-ray source is viewed through a magnetised plasma, such as a luminous quasar in a galaxy cluster, we may expect spectral distortions that are well described by photon-ALP oscillations. We present a $571 \ \mathrm{ks}$ combined High and Low Energy Transmission Grating (HETG/LETG) Chandra observation of the powerful radio-quiet quasar H1821+643, hosted by a cool-core cluster at redshift $0.3$. The spectrum is well described by a double power-law continuum and broad$+$narrow iron line emission typical of type-1 Active Galactic Nuclei (AGN), with remaining spectral features $< 2.5\%$. Using a cell-based approach to describe the turbulent cluster magnetic field, we compare our spectrum with photon-ALP mixing curves for 500 field realisations assuming that the thermal-to-magnetic pressure ratio remains constant up to the virial radius. At $99.7\%$ credibility and taking $\beta = 100$, we exclude all couplings $g_\mathrm{a\gamma} > 6.3 \times 10^{-13} \ {\mathrm{GeV}}^{-1}$ for most ALP masses $< 10^{-12} \ \mathrm{eV}$. Our results are moderately more sensitive to constraining ALPs than the best previous result from Chandra observations of the Perseus cluster, albeit with a less constrained field model. We reflect on the promising future of ALP studies with bright AGN embedded in rich clusters, especially with the upcoming Athena mission.

New Constraints on Light Axion-Like Particles Using Chandra Observations

This paper presents a detailed study on constraining the properties of Axion-Like Particles (ALPs) using high-resolution X-ray spectroscopy of the luminous quasar H1821+643, located in a cool-core galaxy cluster at redshift 0.3. The research capitalizes on observations from the Chandra X-ray Observatory, employing both the Low-Energy Transmission Grating (LETG) and High-Energy Transmission Grating (HETG) to derive a combined spectral profile over a significant observing time totaling approximately 571 ks.

The underlying motivation for this research stems from the coupling properties of ALPs to photons, which can induce photon-ALP oscillations in the presence of strong magnetic fields, such as that found in galaxy clusters. This effect potentially leads to observable spectral distortions in X-ray emissions from bright cosmic sources like quasars situated within such magnetized environments.

The spectrum of H1821+643 is characterized primarily by a double power-law continuum, in addition to broad and narrow iron line emissions typical of type-1 AGN. These spectral features, accurate to within 2.5%, provide a stringent testbed for examining the potential impact of ALPs on the X-ray spectra.

The study employs a Bayesian framework to derive posterior probability distributions over a grid of ALP masses and coupling constants. The authors ensure a comprehensive treatment of the systematics related to the turbulent magnetic fields in the intracluster medium (ICM), which plays a critical role in shaping the propagation of X-rays and potential ALP interactions. The magnetic field model assumes a thermal-to-magnetic pressure ratio ((\beta)) of 100, covering radial distances from the quasar that avoid resonant conversion regimes which could artificially amplify ALP signals.

At 99.7% confidence, the analysis excludes axion-photon couplings greater than (6.3 \times 10^{-13} \ \mathrm{GeV}^{-1}) for ALP masses below (10^{-12} \ \mathrm{eV}), a noteworthy improvement over prior constraints derived from similar studies using the Perseus cluster. These results extend the sensitivity of current techniques used to probe ALP parameter space, surpassing the projected bounds for future experiments such as IAXO+.

The research underscores the value of cluster-embedded, X-ray bright AGNs as astrophysical laboratories for ALP searches, provided systematics like spectral calibration and disentanglement of AGN and cluster emissions are well-managed. The study also highlights the potential of next-generation X-ray missions like Athena, which are expected to refine these constraints further owing to improved spectral resolution and effective area, subject to realistic calibration considerations.

The findings contribute to a broader understanding of AGN-cluster systems and offer a refined technique for investigating astrophysical signatures of Beyond the Standard Model particles like ALPs, whose theoretical implications extend to explaining phenomena such as dark matter and early-Universe dynamics.