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

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 $<10^{-11}\,{\rm eV}$); 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.

• The paper presents high-resolution spectroscopy of NGC 1275 to set upper limits on the ALP-photon coupling, achieving constraints between 6–8 x 10^-13 GeV^-1.
• It employs 490 ks of Chandra HETG data to model photon survival probabilities across various magnetic field scenarios in the Perseus cluster.
• The study outperforms previous constraints from SN1987A and M87, demonstrating X-ray spectroscopy's potential to probe dark matter and physics beyond the Standard Model.

Essay on Astrophysical Limits on Very Light Axion-Like Particles

The paper presents a compelling study investigating the constraints on axion-like particles (ALPs) using high-resolution X-ray spectroscopy from the Chandra X-ray Observatory. Specifically, the research focuses on the active galactic nucleus (AGN) NGC 1275, located at the center of the Perseus cluster, employing extensive 490 ks observations with the High-Energy Transmission Gratings (HETG).

Background and Motivation

ALPs are theoretical particles that arise in extensions of the Standard Model, such as string theory, and may interact with photons through an axion-photon coupling, denoted as g_aγ. These particles, especially those with very low mass (m_a < 10^-12 eV), are intriguing candidates for cold dark matter and are expected to cause unique spectral distortions as astrophysical X-ray photons propagate through magnetic fields present in galaxy clusters.

Methodology and Observations

Utilizing Chandra's HETG allows for high-resolution spectroscopic analysis, which mitigates photon pile-up and ICM contamination, thus ensuring accurate spectral measurements. The paper models photon survival probabilities as a function of ALP mass and coupling constant, g_aγ, across a variety of magnetic field realizations within the intracluster medium (ICM).

Given the high-quality spectrum of NGC 1275, outside the iron-band (6-7 keV), the AGN emission follows a power-law continuum with deviations kept below 3%. This research marginalizes over these magnetic field models to develop a comprehensive constraint map on the ALP parameters.

Results

The study sets an unprecedentedly stringent constraint on the ALP-photon coupling, g_aγ, for very light ALPs. For masses smaller than 10^-12 eV, the research establishes an upper limit on g_aγ ranging between 6 - 8 x 10^-13 GeV^-1, depending on the magnetic field model employed. Notably, these constraints surpass previous results obtained from SN1987A and previous X-ray studies of M87.

Implications and Future Directions

This work challenges the sensitivity limits of current and next-generation helioscopes and light-shining-through-wall experiments, such as IAXO and ALPS-II. The findings underscore X-ray spectroscopy's potential to probe fundamental particle physics.

However, the study also highlights challenges, particularly in achieving necessary relative calibration precision in future X-ray spectrometers. The modest, yet consistent potential detection of ALP-induced spectral features at the 95% confidence level suggests room for further nuanced investigation.

This research contributes significantly to the search for ALPs, illustrating that astrophysical systems like galaxy clusters serve as profound laboratories for exploring physics beyond the Standard Model. Continued advancements in spectrometer calibration and magnetic field modeling will be essential for future breakthroughs in detecting these elusive particles. Such progress promises to deepen our understanding of dark matter and its constituents, with far-reaching implications in both astrophysical and theoretical contexts.