Fermi Large Area Telescope as a Galactic Supernovae Axionscope

In a Galactic core-collapse supernova (SN), axionlike particles (ALPs) could be emitted via the Primakoff process and eventually convert into $\gamma$ rays in the magnetic field of the Milky Way. From a data-driven sensitivity estimate, we find that, for a SN exploding in our Galaxy, the Fermi Large Area Telescope (LAT) would be able to explore the photon-ALP coupling down to $g_{a\gamma }\simeq 2\times {10}^{-13}{\mathrm{GeV}}^{-1}$ for an ALP mass $m_a\lesssim {10}^{-9}\mathrm{eV}$. These values are out of reach of next generation laboratory experiments. In this event, the Fermi LAT would probe large regions of the ALP parameter space invoked to explain the anomalous transparency of the Universe to $\gamma$ rays, stellar cooling anomalies, and cold dark matter. If no $\gamma$-ray emission were to be detected, Fermi-LAT observations would improve current bounds derived from SN 1987A by more than 1 order of magnitude.

Figure 1

Expected limits on ALP parameters from a SN explosion. The gray shaded region shows the constraints from SN 1987A. Left: Limits for different progenitor masses and GMF models. Right: Limits for different potential SN positions.

Figure 2

Comparison of the expected Fermi-LAT sensitivity (dark green) with limits (red) and sensitivities of future experiments (green). The QCD axion band is shown in gray. The transparency anomaly hint could be explained by ALPs within the blue shaded region. ALPs with parameters below the dashed line could constitute the entire dark matter.