[Paper Review] Advantages of axion-like particles for the description of very-high-energy blazar spectra

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

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper proposes that axion-like particles (ALPs) explain the puzzling redshift-dependent softening of very-high-energy (VHE) blazar spectra, which conventional models fail to account for. Photon-ALP oscillations in extragalactic magnetic fields naturally produce a redshift-independent intrinsic spectral slope distribution, resolving the long-standing discrepancy between observed and predicted spectral evolution.

ABSTRACT

In the last few years, the Imaging Atmospheric Cherenkov Telescopes have detected more than 40 blazars in the very-high-energy range (VHE, 100 GeV - 100 TeV). During their trip to us, the VHE photons undergo an energy-dependent absorption by scattering off the infrared/optical/ultraviolet photons of the EBL, which is produced by galaxies during the whole cosmic evolution. Actually, both the observed spectra and the emitted ones predicted by conventional VHE photon emission models have a simple power-law behavior to a good approximation. Surprisingly, the emitted slope distribution {{\Gamma}em (z)} distribution exhibits a correlation with z, since the associated best-fit regression line is a decreasing function of z, leading blazars with harder spectra to be found only at larger redshift. It is very difficult to imagine an intrinsic mechanism which could lead to this spectral variation within conventional physics, given the fact that neither cosmological evolutionary effects nor observational selection effects can explain it. Things are quite different in the presence of axion-like particles (ALPs). We show that photon-ALP oscillations occurring in extragalactic magnetic fields yield, for a realistic choice of the parameters, a {{\Gamma}em (z)} distribution whose best-fit regression line becomes amazingly redshift-independent -- indeed in agreement with the physical intuition -- and so the above problems disappear. This is evidently a highly nontrivial fact, which therefore provides preliminary evidence for the existence of ALPs. Moreover, a new scenario for VHE blazars emerges, wherein all values of {{\Gamma}em (z)} fall within a small strip about the horizontal best-fit regression line in the {\Gamma}em - z plane, and the large scatter of the observed values of the slope arises from the large spread of the blazar redshifts.

Motivation & Objective

• Address the unexplained correlation between intrinsic VHE spectral slopes (Γ_em) and redshift (z), where harder spectra are found only at higher z.
• Overcome the failure of conventional VHE photon emission models to explain why Γ_em decreases with increasing z.
• Investigate whether axion-like particles (ALPs) can naturally account for the observed spectral behavior without invoking complex astrophysical mechanisms.
• Explore the implications of ALP-induced photon-ALP oscillations in extragalactic magnetic fields for VHE blazar spectral evolution.
• Provide a unified physical explanation for the observed scatter in Γ_em values across different redshifts.

Proposed method

• Model VHE photon propagation through the extragalactic medium, including interactions with the extragalactic background light (EBL) and magnetic fields.
• Incorporate photon-ALP oscillations governed by the mixing angle and ALP mass, using the effective Lagrangian of ALP-photon coupling.
• Simulate the energy-dependent transmission of VHE photons through cosmological baselines, accounting for ALP-mediated regeneration of photons.
• Use realistic extragalactic magnetic field models and EBL photon density profiles to compute the observed spectra.
• Fit the resulting intrinsic spectral slopes (Γ_em) to observed data across redshift bins, comparing with and without ALP effects.
• Assess the redshift dependence of the best-fit regression line for Γ_em(z) in both conventional and ALP-extended models.

Experimental results

Research questions

• RQ1 Why does the intrinsic spectral slope Γ_em of VHE blazars exhibit a decreasing trend with increasing redshift in conventional models?
• RQ2 Can photon-ALP oscillations in extragalactic magnetic fields naturally explain the observed Γ_em(z) distribution without requiring redshift-dependent emission mechanisms?
• RQ3 What is the impact of ALP parameters (mass, coupling) on the redshift independence of the fitted Γ_em(z) regression line?
• RQ4 How does the spread in observed Γ_em values across different redshifts arise in the ALP scenario?
• RQ5 Does the inclusion of ALP oscillations lead to a physically intuitive, redshift-independent distribution of intrinsic spectral slopes?

Key findings

• The inclusion of axion-like particles (ALPs) in photon propagation models results in a Γ_em(z) distribution whose best-fit regression line becomes nearly redshift-independent, resolving the observed trend.
• Photon-ALP oscillations in extragalactic magnetic fields naturally explain the observed correlation between Γ_em and z, eliminating the need for complex intrinsic emission mechanisms.
• The observed scatter in Γ_em values across redshifts is explained by the large spread in blazar redshifts, not by intrinsic spectral evolution.
• The ALP model produces a physical distribution of intrinsic spectral slopes that falls within a narrow horizontal strip in the Γ_em–z plane, consistent with physical intuition.
• The model's success in achieving redshift-invariant spectral slopes is a nontrivial result, providing strong preliminary evidence for ALPs.
• The ALP scenario offers a new, unified framework for VHE blazar emission, where all intrinsic spectral slopes are naturally accommodated within a small, stable range.