Enhancing the Spectral Hardening of Cosmic TeV Photons by Mixing with Axionlike Particles in the Magnetized Cosmic Web

Large-scale extragalactic magnetic fields may induce conversions between very-high-energy photons and axionlike particles (ALPs), thereby shielding the photons from absorption on the extragalactic background light. However, in simplified “cell” models, used so far to represent extragalactic magnetic fields, this mechanism would be strongly suppressed by current astrophysical bounds. Here we consider a recent model of extragalactic magnetic fields obtained from large-scale cosmological simulations. Such simulated magnetic fields would have large enhancement in the filaments of matter. As a result, photon-ALP conversions would produce a significant spectral hardening for cosmic TeV photons. This effect would be probed with the upcoming Cherenkov Telescope Array detector. This possible detection would give a unique chance to perform a tomography of the magnetized cosmic web with ALPs.

Figure 1

Limits on ALP parameter space in the plane ($m_a$, $g_{a\gamma }$). The parameter space where ALPs could explain the low $\gamma$-ray opacity is shown in light blue [17]. The horizontal grey bands represent the sensitivity of future ALPS-II and IAXO experiments. Dashed lines represent the photon transfer function $T_{\gamma }$ for the cell model and simulated model of the extragalactic $B$-field. The two small squares represent cases of low-mass and high-mass ALPs where conversions in simulated extragalactic magnetic fields would affect the TeV photon transparency. (See text for details).

Figure 2

Photon transfer function $T_{\gamma }$ in function of the distance $d$ for a given line of sight (upper panels) and corresponding variations of the extragalactic magnetic field (lower panels). We have taken the photon energy $E=10\mathrm{TeV}$ and ALP parameters $m_a=2\times {10}^{-8}\mathrm{eV}$ and $g_{a\gamma }={10}^{-11}{\mathrm{GeV}}^{-1}$ (HM). Left panels refer to the “cell” model, while right panels represent the more realistic model of our cosmological simulation. In both cases we have chosen as average value of the magnetic field $⟨B_T⟩=1.6\times {10}^{-9}\mathrm{G}$.

Figure 3

Probability distribution functions for the transfer function $T_{\gamma }$ for photons emitted from a source at redshift $z=0.3$, with $E=10\mathrm{TeV}$ and ALPs parameters $m_a=2\times {10}^{-8}\mathrm{eV}$ and $g_{a\gamma }={10}^{-11}{\mathrm{GeV}}^{-1}$ (HM). Left distribution refers to the cell model, while the right one to the more realistic case. The vertical line corresponds to the no ALPs case.

Figure 4

Photon transfer function $T_{\gamma }$ as a function of photon energy for a source at redshift $z=0.3$ for $m_a={10}^{-10}\mathrm{eV}$ and $g_{a\gamma }=4\times {10}^{-12}{\mathrm{GeV}}^{-1}$ (LM, left panels) and $m_a=2\times {10}^{-8}\mathrm{eV}$ and $g_{a\gamma }={10}^{-11}{\mathrm{GeV}}^{-1}$ (HM, right panels). The upper panels refer to more realistic models of an extragalactic magnetic field, while lower panels are for the cell model. The black curve corresponds to the case of only absorption onto EBL. The solid grey curve represents the median $T_{\gamma }$ in the presence of ALPs conversions. The orange curve corresponds to conversions for a particular realization of the extragalactic magnetic field. The shaded band is the envelope of the results on all the possible realizations of the extragalactic magnetic field at 68% (dark blue), 90% (blue), and 99% (light blue) C.L., respectively.