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Register a new userNew constraints on light Axion-Like Particles using Chandra Transmission Grating Spectroscopy of the powerful cluster-hosted quasar H1821+643
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Added by
J\\'ulia Sisk Reyn\\'es
Publication date
2021
fields
Physics
and research's language is
English
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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, we exclude all couplings $g_mathrm{agamma} > 6.3 times 10^{-13} {mathrm{GeV}}^{-1}$ 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 provide the best constraints on light ALPs, exceeding the projected sensitivity of next generation axion helioscopes. We reflect on the promising future of ALP studies with bright AGN embedded in rich clusters, especially with the upcoming Athena mission.
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The high-energy Universe is potentially a great laboratory for searching new light bosons such as axion-like particles (ALPs). Cosmic sources are indeed the scene of violent phenomena that involve strong magnetic field and/or very long baselines, where the effects of the mixing of photons with ALPs could lead to observable effects. Two examples are archetypal of this fact, that are the Universe opacity to gamma-rays and the imprints of astrophysical magnetic turbulence in the energy spectra of high-energy sources. In the first case, hints for the existence of ALPs can be proposed whereas the second one is used to put constraints on the ALP mass and coupling to photons.
Axion-like particles with masses in the keV-GeV range have a profound impact on the cosmological evolution of our Universe, in particular on the abundance of light elements produced during Big Bang Nucleosynthesis. The resulting limits are complementary to searches in the laboratory and provide valuable additional information regarding the validity of a given point in parameter space. A potential drawback is that altering the cosmological history may potentially weaken or even fully invalidate these bounds. The main objective of this article is therefore to evaluate the robustness of cosmological constraints on axion-like particles in the keV-GeV region, allowing for various additional effects which may weaken the bounds of the standard scenario. Employing the latest determinations of the primordial abundances as well as information from the cosmic microwave background we find that while bounds can indeed be weakened, very relevant robust constraints remain.
We present a deep Suzaku observation of H1821+643, an extremely rare example of a powerful quasar hosted by the central massive galaxy of a rich cooling-core cluster of galaxies. Informed by previous Chandra studies of the cluster, we achieve a spectral separation of emission from the active galactic nucleus (AGN) and the intracluster medium (ICM). With a high degree of confidence, we identify the signatures of X-ray reflection/reprocessing by cold and slowly moving material in the AGNs immediate environment. The iron abundance of this matter is found to be significantly sub-solar (Z~0.4Zsun), an unusual finding for powerful AGN but in line with the idea that this quasar is feeding from the ICM via a Compton-induced cooling flow. We also find a subtle soft excess that can be described phenomenologically (with an additional black body component) or as ionized X-ray reflection from the inner regions of a high inclination (i=57 degrees) accretion disk around a spinning (a>0.4) black hole. We describe how the ionization state of the accretion disk can be used to constrain the Eddington fraction of the source. Applying these arguments to our spectrum implies an Eddington fraction of 0.25-0.5, with an associated black hole mass of 3-6x10^9Msun.
We study the propagation of light in the presence of a parity-violating coupling between photons and axion-like particles (ALPs). Naively, this interaction could lead to a split of light rays into two separate beams of different polarization chirality and with different refraction angles. However, by using the eikonal method we explicitly show that this is not the case and that ALP clumps do not produce any spatial birefringence. This happens due to non-trivial variations of the photons frequency and wavevector, which absorb time-derivatives and gradients of the ALP field. We argue that these variations represent a new way to probe the ALP-photon couping with precision frequency measurements.
Axion-like particles (ALPs) provide a feasible explanation for the observed low TeV opacity of the Universe. If the low TeV opacity is caused by ALP, then the $>{rm TeV}$ fluxes of unresolved extragalactic point sources will be correspondingly enhanced, resulting in an enhancement of the observed EGB spectrum at high energies. In this work, we for the first time investigate the ALP effect on the EGB spectrum. Our results show that the existence of ALPs can cause the EGB spectrum to deviate from a pure EBL absorption case. The deviation occurs at about $sim$1 TeV and current EGB measurements by Fermi-LAT cannot identify such an effect. The observation from forthcoming VHE instruments like LHAASO and CTA may be useful for studying this effect. We find that although most of the sensitive ALP parameters have been ruled out by existing ALP results, some unrestricted parameters could be probed with the EGB observation around 10 TeV.
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