Observation of the cosmic ray shadow of the Sun with the ANTARES neutrino telescope

The ANTARES detector is an undersea neutrino telescope in the Mediterranean Sea. The search for pointlike neutrino sources is one of the main goals of the ANTARES telescope, requiring a reliable method to evaluate the detector angular resolution and pointing accuracy. This work describes the study of the Sun “shadow” effect with the ANTARES detector. The shadow is the deficit in the atmospheric muon flux in the direction of the Sun caused by the absorption of the primary cosmic rays. This analysis is based on the data collected between 2008 and 2017 by the ANTARES telescope. The observed statistical significance of the Sun shadow detection is $3.7\sigma$, with an estimated angular resolution of $0.59°\pm 0.10°$ for downward-going muons. The pointing accuracy is found to be consistent with the expectations and no evidence of systematic pointing shifts is observed.

Figure 1

Expected statistical significance of the Sun shadow detection during the period from 2008 to 2017 based on MC simulations, as a function of cut values on $\mathrm{\Lambda }$ and $\beta$ (${\mathrm{\Lambda }}_{\mathrm{cut}}$ and ${\beta }_{\mathrm{cut}}$). The red point represents the selected set of cut values (${\mathrm{\Lambda }}_{\mathrm{cut}}=-5.9$ and ${\beta }_{\mathrm{cut}}=1.1°$). The expected significance for the selected set of cut values is $3.4\sigma$.

Figure 2

Distribution of the test statistic $\lambda$ for the two hypotheses, $H_0$ (black curve) and $H_1$ (red curve), obtained for the optimized set of cut values (${\mathrm{\Lambda }}_{\mathrm{cut}}=-5.9$ and ${\beta }_{\mathrm{cut}}=1.1°$). The dashed area represents the fraction of pseudo-experiments (50%) where $H_1$ hypothesis is correctly identified. The colored area corresponds to the expected median significance ($3.4\sigma$) to reject the $H_0$ hypothesis in favor of the $H_1$ hypothesis.

Figure 3

The muon event density as a function of the angular distance $\delta$ from the Sun center based on the data sample taken in period 2008–2017 fitted with Eq. (2) (red line). The shaded area corresponds to the Sun angular radius (0.26°).

Figure 4

The distribution of the test statistic values around the nominal Sun position $O\equiv (0°,0°)$. The minimum value ${\lambda }_{\min }=-13.7$ is found at (0.2°, 0°) point (white dot).

Figure 5

Contours corresponding to different confidence levels (red: 68.27%; yellow: 95.45%; green: 99.73%). The white dot indicates (0.2°, 0°) point for which a minimum value of ${\lambda }_{\min }=-13.7$ is obtained.