Revisiting the evidences for spectral anomalies in distant blazars: new data on the photon-ALP mixing. (arXiv:2204.08865v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Cenedese_F/0/1/0/all/0/1">Francesco Cenedese</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Franceschini_A/0/1/0/all/0/1">Alberto Franceschini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Galanti_G/0/1/0/all/0/1">Giorgio Galanti</a>
We re-examine possible dependencies on redshift of the spectral parameters of
blazars observed at very-high energies (VHEs) with Imaging Atmospheric
Cherenkov telescopes (IACTs). This is relevant to assess potential effects with
the source distance of the photon to axion-like particle (ALP) mixing, that
would deeply affect the propagation of VHE photons across the Universe. We
focus our spectral analysis on 38 BL Lac objects (32 high-peaked and 6
intermediate-peaked) up to redshift $zsimeq 0.5$, and a small sample of 5 Flat
Spectrum Radio Quasars up to $z=1$ treated independently to increase the
redshift baseline. The 78 independent spectra of these sources are first of all
carefully corrected for the gamma-gamma interaction with photons of the
Extragalactic Background Light, that are responsible for the major
redshift-dependent opacity effect. Then, the corrected spectra are fitted with
simple power-laws to infer the intrinsic spectral indices $Gamma_{rm em}$ at
VHE, to test the assumption that such spectral properties are set by the local
rather than the global cosmological environment. We find some systematic
anti-correlations with redshift of $Gamma_{rm em}$ that might indicate,
although with low-significance, a spectral anomaly potentially requiring a
revision of the photon propagation process. More conclusive tests with higher
statistical significance will require the observational improvements offered by
the forthcoming new generation of Cherenkov arrays (CTA, ASTRI, LHAASO).
We re-examine possible dependencies on redshift of the spectral parameters of
blazars observed at very-high energies (VHEs) with Imaging Atmospheric
Cherenkov telescopes (IACTs). This is relevant to assess potential effects with
the source distance of the photon to axion-like particle (ALP) mixing, that
would deeply affect the propagation of VHE photons across the Universe. We
focus our spectral analysis on 38 BL Lac objects (32 high-peaked and 6
intermediate-peaked) up to redshift $zsimeq 0.5$, and a small sample of 5 Flat
Spectrum Radio Quasars up to $z=1$ treated independently to increase the
redshift baseline. The 78 independent spectra of these sources are first of all
carefully corrected for the gamma-gamma interaction with photons of the
Extragalactic Background Light, that are responsible for the major
redshift-dependent opacity effect. Then, the corrected spectra are fitted with
simple power-laws to infer the intrinsic spectral indices $Gamma_{rm em}$ at
VHE, to test the assumption that such spectral properties are set by the local
rather than the global cosmological environment. We find some systematic
anti-correlations with redshift of $Gamma_{rm em}$ that might indicate,
although with low-significance, a spectral anomaly potentially requiring a
revision of the photon propagation process. More conclusive tests with higher
statistical significance will require the observational improvements offered by
the forthcoming new generation of Cherenkov arrays (CTA, ASTRI, LHAASO).
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