Hint at an axion-like particle from the redshift dependence of blazar spectra

Author:

Galanti G1,Roncadelli M23,De Angelis A45,Bignami G F6

Affiliation:

1. INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy

2. INFN, Sezione di Pavia, Via A. Bassi 6, I-27100 Pavia, Italy

3. INAF, IASF Milano, Via A. Corti 12, I-20133 Milano, Italy

4. Dipartimento di Fisica e Astronomia G. Galilei, Via F. Marzolo 8, I-35131 Padova, Italy

5. INFN, Sezione di Padova, Via F. Marzolo 8, I-35131 Padova, Italy

6. Accademia dei Lincei, Via della Lungara 10, I-00165 Roma, Italy

Abstract

ABSTRACT We consider the largest observed sample including all intermediate-frequency peaked (IBL) and high-frequency peaked (HBL) flaring blazars above 100 GeV up to redshift z = 0.6. We show that the best-fitting regression line of the emitted spectral indices Γem(z) is a concave parabola decreasing as z increases, thereby implying a statistical correlation between the {Γem(z)} distribution and z. This result contradicts our expectation that such a distribution should be zindependent. We argue that the above correlation does not arise from any selection bias. We show that our expectation naturally emerges provided that axion-like particles (ALPs) are put into the game. Moreover, ALPs can also explain why flat spectrum radio quasars emit up to 400 GeV, in sharp contradiction with conventional physics (CP). So, the combination of the two very different but consistent results – taken at face value – leads to a hint at an ALP with mass $m = {\cal O} (10^{-10} \, {\rm eV})$ and two-photon coupling in the range $2.94 \times 10^{- 12} \, {\rm GeV}^{- 1} \lt g_{a \gamma \gamma } \lt 0.66 \times 10^{- 10} \, {\rm GeV}^{- 1}$. As a bonus, the Universe would become considerably more transparent above energies $E \gtrsim 1 \, {\rm TeV}$ than dictated by CP. Our prediction can be checked not only by the new generation of observatories like CTA, HAWC, GAMMA-400, LHAASO, TAIGA-HiSCORE, and HERD, but also thanks to the planned laboratory experiments ALPS II (upgraded), STAX, IAXO and with other techniques now being developed by Avignone and collaborators. Dedication: We wish to dedicate the present work to the memory of our dear friend Nanni Bignami.

Funder

INFN

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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