A New Bound on Axion-Like Particles (1703.07354v2)

Abstract: Axion-like particles (ALPs) and photons can quantum mechanically interconvert when propagating through magnetic fields, and ALP-photon conversion may induce oscillatory features in the spectra of astrophysical sources. We use deep (370 ks), short frame time Chandra observations of the bright nucleus at the centre of the radio galaxy M87 in the Virgo cluster to search for signatures of light ALPs. The absence of substantial irregularities in the X-ray power-law spectrum leads to a new upper limit on the photon-ALP coupling, $g_{a\gamma}$: using a conservative model of the cluster magnetic field consistent with Faraday rotation measurements from M87 and M84, we find $g_{a \gamma} < 2.6\times10^{-12}$ GeV$^{-1}$ at 95% confidence level for ALP masses $m_a \leq 10^{-13}$ eV. Other consistent magnetic field models lead to stronger limits of $g_{a \gamma} \lesssim 1.1$--$1.5 \times 10^{-12}$ GeV$^{-1}$. These bounds are all stronger than the limit inferred from the absence of a gamma-ray burst from SN1987A, and rule out a substantial fraction of the parameter space accessible to future experiments such as ALPS-II and IAXO.

An Upper Bound on Axion-Like Particles from M87 Observations

The paper "A New Bound on Axion-Like Particles" presents a detailed paper aimed at constraining the properties of axion-like particles (ALPs) through their conversions with photons in the astronomical setting of a galaxy cluster, specifically the Virgo cluster centered around the radio galaxy M87. The authors leverage X-ray observations made with the Chandra X-ray Observatory to examine potential spectral irregularities indicative of ALP-photon conversions, thus placing new, stringent limits on the ALP parameter space.

Axion-like particles are theorized remnants of potential broken symmetries in grand unified theories, akin to the well-studied QCD axions. These particles, if they exist, interact weakly with photons via a coupling constant $g_{a\gamma}$. This interaction becomes particularly noteworthy in the presence of strong magnetic fields, such as those found in galaxy clusters where the ALP-photon interconversion can lead to observable effects on the energy spectra of X-ray sources.

The paper scrutinizes the dataset from Chandra observations of M87—a central AGN within the Virgo cluster. This setting is advantageous due to its proximity and allows for robust studies of both the electron density distribution, which affects photon propagation, and the magnetic field through observations of Faraday rotation measures. The authors use data reduction and background subtraction processes refined through simulations and detailed X-ray analysis ensuring high accuracy of the extracted nuclear spectra.

Within their framework, the authors apply models of magnetic fields and electron densities, consistent with previous studies, and simulate ALP-photon conversion probabilities using path-ordered transfer matrices. Their approach acknowledges spatial and energy separations in effects induced by ALPs, allowing them to distinguish putative ALP influences from observational noise. The observed nuclear spectrum of M87 is expressed well via an absorbed power law, absent of substantial modulations that could suggest ALP interactions.

Their findings reveal no significant spectral distortions attributable to ALPs, leading to a new upper limit on the ALP-photon coupling, $g_{a\gamma}$, of $1.49 \times 10^{-12} \, {\rm GeV}^{-1}$ at a 95% confidence level for ALP masses $m_a \leq 10^{-13} \, {\rm eV}$. This limit surpasses previous constraints derived from other astrophysical sources, including supernova SN1987A, thereby excluding larger regions of the ALP parameter space than previously accessible.

The paper concludes that M87 observations provide significant constraints on ALPs, further tightening bounds on physics beyond the Standard Model. Moreover, these constraints inform future experimental endeavors like ALPS-II and IAXO, which seek to identify the existence of ALPs across a broader range of masses. The methodologies and analytical rigor presented in the paper highlight the collaborative potential between astrophysical observations and particle physics to advance our understanding of fundamental physics.

In summary, this work demonstrates the power of using high-quality astronomical data, combined with sophisticated modeling techniques, to explore and constrain the parameter space of axion-like particles. The achievements herein paves the way for further astrophysical-based investigations into the existence of ALPs and emphasizes the critical role played by observations of X-ray spectra, especially from environments rich in magnetic fields such as galaxy clusters.