Relevance of axionlike particles for very-high-energy astrophysics

Several extensions of the standard model and, in particular, superstring theories suggest the existence of axionlike particles (ALPs), which are very light spin-zero bosons with a two-photon coupling. As a consequence, photon-ALP oscillations occur in the presence of an external magnetic field, and ALPs can lead to observable effects on the measured photon spectrum of astrophysical sources. An intriguing situation arises when blazars are observed in the very-high-energy (VHE) band—namely, above 100 GeV—as it is the case with the presently operating Imaging Atmospheric Cherenkov Telescopes H.E.S.S, Major Atmospheric Gamma Imaging Cherenkov telescope, Collaboration of Australia and Nippon for a Gamma Ray Observatory in the Outback III, and VERITAS. The extragalactic background light produced by galaxies during cosmic evolution gives rise to a source dimming which becomes important in the VHE band and increases with energy, since hard photons from a blazar scatter off soft extragalactic background light photons thereby disappearing into $e^{+}{e}^{-}$ pairs. This dimming can be considerably reduced by photon-ALP oscillations, and since they are energy independent the resulting blazar spectra become harder than expected. We consider throughout a scenario first proposed by De Angelis, Roncadelli, and Mansutti in which the above strategy is implemented with photon-ALP oscillations triggered by large-scale magnetic fields, and we systematically investigate its implications for VHE blazars. We find that for ALPs lighter than $5·{10}^{-10}\mathrm{eV}$ the photon survival probability is larger than predicted by conventional physics above a few hundred GeV. Specifically, a boost factor of 10 can easily occur for sources at large distance and large energy, e.g. at 8 TeV for the blazar 1ES 0347-121 at redshift $z=0.188$. This is a clear-cut prediction which can be tested with the planned Cherenkov Telescope Array and the High Altitude Water Cherenkov Experiment (HAWC) water Cherenkov $\gamma$-ray observatory and possibly with the currently operating Imaging Atmospheric Cherenkov Telescopes as well as with detectors like Astrophysical Radiation with Ground-based Observatory at YangBaJing and Multiple Institution Los Alamos Gamma Ray Observatory. Moreover, we show that the De Angelis, Roncadelli, and Mansutti scenario offers a new interpretation of the VHE blazars detected so far, according to which the large spread in the values of the observed spectral index is mainly due to the wide spread in the source distances rather than to large variations of their internal physical properties. Finally, we stress that ALPs with the right properties to produce the above effects can be discovered by the Gamma-meV experiment at FERMILAB and more likely with the planned photon regeneration experiment ALPS at Deutsches Elektronen Synchrotron.

Figure 1

The pair-production mean free path ${\lambda }_{\gamma }$ of a VHE photon is plotted versus its energy $E$ within the EBL model of FRV. Only conventional physics is assumed and, in particular, the possibility of photon-ALP oscillations is ignored.

Figure 2

The observed values of the observed spectral index ${\Gamma }_{\mathrm{obs}}$ versus the source redshift for all blazars detected so far in the VHE band are represented by dots and corresponding error bars.

Figure 3

Behavior of $P_{\gamma \to \gamma }^{\mathrm{DARMA}}$ versus the observed energy $E_0$ for: $z=0.031$ (top row), $z=0.188$ (second row), $z=0.444$ (third row), $z=0.536$ (bottom row). The solid black line corresponds to $\xi =5.0$, the dotted-dashed line to $\xi =1.0$, the dashed line to $\xi =0.5$, the dotted line to $\xi =0.1$, and the solid grey line to conventional physics. We have taken $L_{\mathrm{dom}}=4\mathrm{Mpc}$ (left column) and $L_{\mathrm{dom}}=10\mathrm{Mpc}$ (right column).

Figure 4

Behavior of $P_{\gamma \to \gamma }^{\mathrm{DARMA}}$ versus the observed energy $E_0$ for: $z=0.031$ (top row left), $z=0.188$ (top row right), $z=0.444$ (bottom row left), $z=0.536$ (bottom row right) for $L_{\mathrm{dom}}=0.05\mathrm{Mpc}$. The solid black line corresponds to $\xi =5.0$, the dotted-dashed line to $\xi =1.0$, the dashed line to $\xi =0.5$, the dotted line to $\xi =0.1$, and the solid grey line to conventional physics.

Figure 5

Behavior of ${\Gamma }_{\mathrm{obs}}^{\mathrm{DARMA}}$ for the blazars 3C 66B, Mrk 421, Mrk 501 (with the two measurements of ${\Gamma }_{\mathrm{obs}}$ in the literature), 1ES $2344+514$ and Mrk 180. The solid black line corresponds to ${\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}=2.51$ and the grey strip represents the range $2.31<{\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}<2.71$.

Figure 6

Behavior of ${\Gamma }_{\mathrm{obs}}^{\mathrm{DARMA}}$ for the blazars 1ES $1959+650$, BL Lacertae, PKS 0548-322, PKS 2005-489, RGB J0152+017, and W Comae. The solid black line corresponds to ${\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}=2.51$ and the grey strip represents the range $2.31<{\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}<2.71$.

Figure 7

Behavior of ${\Gamma }_{\mathrm{obs}}^{\mathrm{DARMA}}$ for the blazars PKS 2155-304, H $1426+428$, 1ES $0806+524$, 1ES $0229+200$, H 2356-309, and 1ES $1218+304$. The solid black line corresponds to ${\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}=2.51$ and the grey strip represents the range $2.31<{\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}<2.71$.

Figure 8

Behavior of ${\Gamma }_{\mathrm{obs}}^{\mathrm{DARMA}}$ for the blazars 1ES 1101-232, 1ES 0347-121, 1ES $1011+496$, S5 $0716+714$, PG $1553+113$, and PKS $1222+21$. The solid black line corresponds to ${\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}=2.51$ and the grey strip represents the range $2.31<{\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}<2.71$.

Figure 9

Behavior of ${\Gamma }_{\mathrm{obs}}^{\mathrm{DARMA}}$ for the blazars 3C 66A, PKS $1424+240$, and 3C 279. The solid black line corresponds to ${\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}=2.51$ and the grey strip represents the range $2.31<{\Gamma }_{\mathrm{em}}^{\mathrm{DARMA}}<2.71$.

Figure 10

The spectral photon number density in the present Universe $n_{\gamma }(·,0)$ is plotted versus the energy ${ϵ}_0$ in the energy range 0.25–2.5 eV. The solid line represents the result of the FRV model. The dotted and dashed lines correspond to the lower ($\alpha =0.9$) and upper ($\alpha =3.6$) limit, respectively, of our power-law approximation defined in Eq. (b4).

Figure 11

Plot of the behavior of $I(z)$.

Figure 12

The approximate pair-production mean free path ${\lambda }_{\gamma }^{\mathrm{app}}$ of a VHE photon is plotted versus its energy $E$, and it is represented by the shadowed area as the parameter $\alpha$ varies in the range 0.9–3.6. The dotted lines and dashed lines correspond to $\alpha =0.9$ and $\alpha =3.6$, respectively. Superimposed is the exact result obtained within the FRV model and shown in Fig. 1.