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SUMMARY:Ensemble-Averaged Axion–Photon Conversion in Turbulent Galaxy Cl
 usters
DTSTART;VALUE=DATE-TIME:20260923T123000Z
DTEND;VALUE=DATE-TIME:20260923T125000Z
DTSTAMP;VALUE=DATE-TIME:20260914T090302Z
UID:indico-contribution-544@indico.bitp.kiev.ua
DESCRIPTION:Speakers: Oleksandr Khasai (Bogolyubov Institute for Theoretic
 al Physics of the National Academy of Sciences of Ukraine)\nAxion-like par
 ticles (ALPs) coupled to photons can modify electromagnetic spectra throug
 h conversion in astrophysical magnetic fields. In galaxy clusters\, turbul
 ence makes this effect stochastic and strongly dependent on the magnetic-f
 ield realisation for individual sightlines. Recent work showed that averag
 ing over ensembles of background active galactic nuclei transforms these f
 luctuations into a smooth\, universal absorption-like feature\, enhancing 
 the sensitivity of cluster ALP searches [1\,2]. We develop an analytic des
 cription of this ensemble-averaged signal and establish the connection bet
 ween physical turbulence spectra and the domain models commonly used in nu
 merical simulations.\n\nTurbulent fields are often represented by uniform 
 domains with sizes distributed as $p(\\Delta)\\propto\\Delta^{\\beta-1}$\,
  while a physical turbulent field is characterised by $\\langle|B_k|^2\\ra
 ngle\\propto k^{-a-2}$\, corresponding to an energy spectrum $\\propto k^{
 -a}$ and $a=5/3$ for Kolmogorov turbulence. In the perturbative regime\, t
 he ensemble-averaged conversion probability is determined by the one-dimen
 sional power spectrum of the transverse field along the line of sight [3].
  Matching the exact domain-model correlator to the turbulent structure fun
 ction gives $\\beta = a - 2$ for $1  0$ universally produces an effective 
 $k^{-2}$ spectrum because of the discontinuities at domain boundaries\, wh
 ile spectra with $a > 2$ cannot be reproduced by this class of models.\n\n
 Using this correspondence\, we derive the ensemble-averaged conversion pro
 bability in closed form for arbitrary $\\Delta_{\\min}/\\Delta_{\\max}$ an
 d obtain a simple two-power-law approximation. The resulting feature start
 s at $\\omega_c\\sim m_a^2\\Delta_{\\max}/2$\, rises approximately as $\\o
 mega^a$\, and saturates at high energies with an amplitude controlled by t
 he magnetic-field correlation length. Thus\, the inertial-range turbulence
  fixes the spectral shape\, while the high-energy plateau retains an $\\ma
 thcal{O}(1)$ sensitivity to the modelling of the largest field scales.\n\n
 Our results provide an analytic dictionary between turbulent spectra and d
 omain-size prescriptions\, removing an ad hoc parameter from cluster ALP s
 imulations. They also provide an analytic alternative to Monte-Carlo avera
 ging over magnetic-field realisations\, offering a practical framework for
  interpreting existing ensemble searches and constructing efficient templa
 tes for future observations [1\,2].\n\nReferences:\n[1]D. Malyshev\, L. Za
 dorozhna\, Yu. Bidasyuk\, A. Santangelo\, and O. Ruchayskiy. Constraints o
 n axion-like particles from active galactic nuclei seen through galaxy clu
 sters. Nature Astronomy\, 9:1387–1395\, September 2025. doi:10.1038/s415
 50-025-02621-8\n[2]D. Malyshev\, L. Zadorozhna\, Yu. Bidasyuk\, A. Santang
 elo\, and O. Ruchayskiy. AGN spectral variability across activity states a
 nd searches for axion-like particles. arXiv e-prints\, art. arXiv:2509.223
 44\, September 2025. doi:10.48550/arXiv.2509.22344.\n[3]M. C. D. Marsh\, J
 . H. Matthews\, C. Reynolds\, and P. Carenza. Fourier formalism for relati
 vistic axion-photon conversion with astrophysical applications. Phys. Rev.
  D\, 105(1):016013\, 2022. doi:10.1103/PhysRevD.105.016013\n[4]J. P. Conlo
 n\, F. Day\, N. Jennings\, S. Krippendorf\, and M. Rummel. Constraints on 
 Axion-Like Particles from Non-Observation of Spectral Modulations for X-ra
 y Point Sources. JCAP\, 07:005\, 2017. doi:10.1088/1475-7516/2017/07/005.\
 n\nhttps://indico.bitp.kiev.ua/event/18/contributions/544/
LOCATION:Bogolyubov Institute for Theoretical Physics Conference Hall
URL:https://indico.bitp.kiev.ua/event/18/contributions/544/
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