Discovery of Gamma Rays from the Quiescent Sun with HAWC

We report the first detection of a TeV $\gamma$-ray flux from the solar disk ($6.3\sigma$), based on 6.1 years of data from the High Altitude Water Cherenkov (HAWC) observatory. The 0.5–2.6 TeV spectrum is well fit by a power law, $dN/dE=A(E/1\mathrm{TeV}{)}^{-\gamma }$, with $A=(1.6\pm 0.3)\times {10}^{-12}{\mathrm{TeV}}^{-1}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ and $\gamma =3.62\pm 0.14$. The flux shows a strong indication of anticorrelation with solar activity. These results extend the bright, hard GeV emission from the disk observed with Fermi-LAT, seemingly due to hadronic Galactic cosmic rays showering on nuclei in the solar atmosphere. However, current theoretical models are unable to explain the details of how solar magnetic fields shape these interactions. HAWC’s TeV detection thus deepens the mysteries of the solar-disk emission.

Figure 1

Results for the example of bin $B3$. Left: significance maps in Sun-centered coordinates for 6.1 years of data, smoothed with a 1° top-hat function for visual clarity. The green circle illustrates the true point spread function. Right: angular profiles (steps of 0.15° from the Sun) of the fractional deviation from background. The black dashed line shows the projection of the best-fit 2D Gaussian model fitted to the shadow, with the shaded band indicating the total uncertainty in the model. The top row shows the cosmic-ray dominated data. The middle row shows the events that survive the gamma-hadron separation cuts. The bottom row is after subtracting the measured cosmic-ray shadow (see top-row data) from the middle-row data, leaving a $\gamma$-ray excess at the position of the Sun (marked by a cross).

Figure 2

Results for the combination of bins $B2$, $B3$, and $B4$. Top: the full 6.1-year data. Middle: solar-maximum period. Bottom: solar-minimum period.

Figure 3

Spectrum of the solar disk. The 6.1-year spectrum by HAWC is shown by the red solid line. The 90% CL upper limit at 7 TeV is indicated with the red arrow. The spectrum at the solar minimum is indicated by the dashed line. The shaded bands show statistical uncertainties. The solar maximum flux at 1 TeV is shown as the $1\sigma$ upper limit. The Fermi-LAT spectra over the full solar cycle [7] (orange) and at the solar minimum [6] (blue) are also shown. The gray dashed line shows the theoretical maximum on the $\gamma$-ray spectrum [5].