Transparency of the Sun to Gamma Rays due to Axionlike Particles

It is shown that the Sun can become partially transparent to high energy photons in the presence of a pseudoscalar. In particular, if the axion interpretation of the PVLAS result were true, then up to 2% of GeV energy gamma rays might pass through the Sun, while an even stronger effect is expected for some axion parameters. We discuss the possibilities of observing this effect. Present data are limited to the observation of the solar occultation of 3C 279 by the Energetic Gamma-Ray Experiment Telescope in 1991; 98% C.L. detection of a nonzero flux of gamma rays passing through the Sun is not yet conclusive. Since the same occultation happens every October, future experiments, e.g., the Gamma-Ray Large Area Space Telescope, are expected to have better sensitivity.

Figure 1

Electron density and magnetic field near the surface of the Sun and into the convection zone.

Figure 2

Axion parameter space. For the shaded region, GeV photon flux through the Sun detectable with GLAST at the solar occultations of 3C 279 is expected. The ellipse corresponds to PVLAS-motivated parameters [4]. The values of $M$ above the thick line are disfavored by CAST [5].

Figure 3

Hillas plot of the magnetic field of objects vs their size. The oscillation length at maximal mixing for the PVLAS coupling $M={10}^5\mathrm{GeV}$ is plotted as a function of magnetic field (diagonal line). The right hand axis shows the photon energy required for maximal mixing as a function of magnetic field assuming the PVLAS coupling and mass $m={10}^{-3}\mathrm{eV}$.

Figure 4

The probability of a photon emerging when a GeV PVLAS axion enters the Sun with an impact parameter 0.95 $R_{\odot }$ Upper curve: unrealistic case with photon cross section neglected. The two resonant regions can be clearly seen. Lower curve: the same but including photon interactions.

Figure 5

The probability of a GeV photon emerging for pseudoscalar mass $m={10}^{-3}\mathrm{eV}$ and different inverse coupling scales $M$. The dotted line bounds the maximal depths still transparent for GeV photons.