[Paper Review] Axion-like particles explain the unphysical redshift-dependence of AGN gamma-ray spectra

Giorgio Galanti, M. Roncadelli+2 more|arXiv (Cornell University)|Mar 15, 2015

Astrophysics and Cosmic Phenomena4 citations

TL;DR

The paper proposes that axion-like particles (ALPs) explain the unphysical redshift-dependence observed in very-high-energy (VHE) gamma-ray spectra of blazars. Through photon-ALP oscillations in extragalactic magnetic fields, the observed spectral index becomes redshift-independent, resolving a long-standing inconsistency and providing indirect evidence for ALPs and their role in cold dark matter.

ABSTRACT

Blazars are a class of AGN known to be powerful very-high-energy (VHE, 100 GeV - 100 TeV) celestial gamma-ray emitters. At the time of writing, 41 blazars, spread all over the sky and with known redshift in the range $0.0215 \leq z \leq 0.635$ have been observed in the VHE band by the Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC and VERITAS. Thus, they represent an isotropic and relatively local extragalactic sample, unaffected by significant cosmological evolution. The blazar emitted spectra are well fitted by a power law with index $\Gamma_{ m em}$. We show that the $\Gamma_{ m em}$ distribution exhibits an unexpected and previously unnoticed unphysical redshift-dependence. We demonstrate that this result is not due to any selection effect. It is difficult to imagine an intrinsic mechanism which could lead to such a spectral variation, and so this result seriously challenges the conventional view. We propose that such a behaviour is explained by oscillations between the VHE gamma-rays and Axion-Like Particles (ALPs), taking place in extragalactic magnetic fields. We recall that ALPs are predicted by several extensions of the Standard Model and especially by those based on superstring theories. Moreover, they are attracting growing interest being also good candidates for cold dark matter. As a consequence of the photon-ALP oscillation mechanism, the $\Gamma_{ m em}$ distribution becomes redshift-independent, indeed in agreement with the physical expectation. This is a highly nontrivial fact, which therefore provides a preliminary evidence for the existence of ALPs. Thus, besides physics laboratory data, astrophysical VHE data from e.g. the upcoming CTA can settle this issue. Our Universe may in this way be offering us a compelling reason to push physics beyond the Standard Model along a very specific direction and can shed light on the nature of cold dark matter.

Motivation & Objective

• To address the unphysical redshift-dependence of VHE gamma-ray spectral indices in blazars, which contradicts physical expectations.
• To investigate whether this anomaly could be explained by physics beyond the Standard Model, particularly axion-like particles (ALPs).
• To test whether ALP-induced photon-ALP oscillations in extragalactic magnetic fields can reconcile the observed spectral evolution with cosmological expectations.
• To provide a physical mechanism that explains the apparent spectral softening with redshift as an artifact of vacuum polarization and mixing, rather than intrinsic source evolution.
• To position VHE gamma-ray observations as a probe for ALPs and cold dark matter candidates.

Proposed method

• Analyzing VHE gamma-ray spectra from 41 blazars with redshifts between 0.0215 and 0.635 observed by H.E.S.S., MAGIC, and VERITAS.
• Fitting the observed spectra with a power-law model to extract the intrinsic spectral index Γ_m_em.
• Identifying a systematic, unphysical redshift dependence in Γ_m_em that cannot be attributed to selection effects.
• Modeling photon-ALP oscillations in extragalactic magnetic fields using the mixing angle and ALP mass parameters.
• Simulating the energy-dependent transition probability of photons into ALPs and back, modifying the observed spectrum.
• Comparing the observed redshift evolution of Γ_m_em with predictions from the ALP oscillation model to test consistency.

Experimental results

Research questions

• RQ1 Why does the observed VHE gamma-ray spectral index of blazars show a systematic dependence on redshift, contrary to physical expectations?
• RQ2 Could this redshift dependence be an artifact of vacuum polarization or photon-ALP mixing rather than intrinsic source evolution?
• RQ3 To what extent can axion-like particle oscillations in extragalactic magnetic fields explain the observed spectral softening with redshift?
• RQ4 Does the redshift-invariant behavior of the effective spectral index under the ALP model align with observational data?
• RQ5 Can VHE gamma-ray observations of blazars serve as a probe for the existence of ALPs and their role in cold dark matter?

Key findings

• The observed distribution of VHE gamma-ray spectral indices (Γ_m_em) exhibits a significant, unphysical redshift dependence across 41 blazars with 0.0215 ≤ z ≤ 0.635.
• This redshift dependence is not due to observational selection effects, ruling out instrumental or sample bias as the cause.
• The inclusion of photon-ALP oscillations in extragalactic magnetic fields transforms the redshift-dependent Γ_m_em into a redshift-independent distribution, consistent with physical expectations.
• The model provides a natural explanation for the spectral softening with redshift as a result of photon-ALP mixing, not intrinsic source evolution.
• The agreement between the ALP model and observations constitutes a preliminary, nontrivial evidence for the existence of axion-like particles.
• The results suggest that VHE gamma-ray data from future observatories like CTA could definitively test the ALP hypothesis.