Hints of the existence of axionlike particles from the gamma-ray spectra of cosmological sources

Axionlike particles (ALPs) are predicted to couple with photons in the presence of magnetic fields. This effect may lead to a significant change in the observed spectra of gamma-ray sources such as active galactic nuclei (AGNs). Here we carry out a detailed study that for the first time simultaneously considers in the same framework both the photon/axion mixing that takes place in the gamma-ray source and that one expected to occur in the intergalactic magnetic fields. An efficient photon/axion mixing in the source always means an attenuation in the photon flux, whereas the mixing in the intergalactic medium may result in a decrement and/or enhancement of the photon flux, depending on the distance of the source and the energy considered. Interestingly, we find that decreasing the value of the intergalactic magnetic field strength, which decreases the probability for photon/axion mixing, could result in an increase of the expected photon flux at Earth if the source is far enough. We also find a 30% attenuation in the intensity spectrum of distant sources, which occurs at an energy that only depends on the properties of the ALPs and the intensity of the intergalactic magnetic field, and thus independent of the AGN source being observed. Moreover, we show that this mechanism can easily explain recent puzzles in the spectra of distant gamma-ray sources, like the possible detection of TeV photons from 3C 66A (a source located at $z=0.444$) by MAGIC and VERITAS, which should not happen according to conventional models of photon propagation over cosmological distances. Another puzzle is the recent published lower limit to the extragalactic background light intensity at $3.6\mu \mathrm{m}$ (which is almost twice larger as the previous one), which implies very hard spectra for some detected TeV gamma-ray sources located at $z=0.1–0.2$. The consequences that come from this work are testable with the current generation of gamma-ray instruments, namely Fermi (formerly known as GLAST) and imaging atmospheric Cherenkov telescopes like CANGAROO, HESS, MAGIC, and VERITAS.

Figure 1

Sketch of the formalism used in this work, where both mixing inside the source and mixing in the IGMF are considered under the same consistent framework. Photon to axion oscillations (or vice versa) are represented by a crooked line, while the symbols $\gamma$ and $a$ mean gamma-ray photons and axions, respectively. This diagram collects the main physical scenarios that we might identify inside our formalism. Each of them are schematically represented by a line that goes from the source to the Earth.

Figure 2

Example of photon/axion oscillations inside the source or vicinity, and its effect on the source intensity (solid line), which was normalized to 1 in the figure. We used the parameters given in Table  to model the AGN source, but we adopted an ALP mass of $1\mu \mathrm{eV}$. This gives $E_{\mathrm{crit}}=0.19\mathrm{GeV}$. The dot-dashed line represents the maximum (theoretical) attenuation given by Eq. (8), and equal to $1/3$.

Figure 3

Effect of intergalactic photon/axion mixing on photon and ALP intensities versus distance to the source, computed for our fiducial model, i.e. for 3C 279 and those parameters given in Table  but taking $B=1\mathrm{nG}$, and using the Primack EBL model. The black thick solid line represents the total photon intensity, while the blue dotted line is the ALP intensity. The photon intensity as given only by the EBL (i.e. without including photon/axion mixing) is shown as the red dashed line. Top panels: mixing computed for $M_{11}=4\mathrm{GeV}$ and an initial photon energy of 50 GeV (left), 500 GeV (middle), and 2 TeV (right); bottom panels: $M_{11}=0.7\mathrm{GeV}$ and same energies than top panels.

Figure 4

Effect of photon/axion conversions both inside the source and in the IGM on the total photon flux coming from 3C 279 ($z=0.536$) and PKS 2155-304 ($z=0.117$) for two EBL models: Kneiske best fit (dashed line) and Primack (solid line). The expected photon flux without including ALPs is also shown for comparison (dotted line for Kneiske best fit and dot-dashed line for Primack).

Figure 5

Boost in intensity due to ALPs for the Kneiske best fit (dashed line) and Primack (solid line) EBL models, computed using the fiducial model presented in Table  for 3C 279 ($z=0.536$) and PKS 2155-304 ($z=0.117$).

Figure 6

Same as in Figs. 4, 5 but for different values of IGMF. Upper panels: 3C 279 using those parameters listed in Table , only changing $\mathbf{B}$. Lower panels: Same exercise for PKS 2155-304, using the corresponding parameters that can be found in the same Table .

Figure 7

Boost in intensity due to ALPs for the Kneiske best fit and Primack EBL models, computed using the fiducial model presented in Table  for 3C 279 ($z=0.536$) and PKS 2155-304 ($z=0.117$), but with $M_{11}=4\mathrm{GeV}$ and $B=0.1\mathrm{nG}$ (dashed and solid lines for Kneiske best fit and Primack EBL models, respectively) and $B=1\mathrm{nG}$ (dot-dashed and dotted lines).