First Detection of Photons with Energy beyond 100 TeV from an Astrophysical Source

We report on the highest energy photons from the Crab Nebula observed by the Tibet air shower array with the underground water-Cherenkov-type muon detector array. Based on the criterion of a muon number measured in an air shower, we successfully suppress 99.92% of the cosmic-ray background events with energies $E>100\mathrm{TeV}$. As a result, we observed 24 photonlike events with $E>100\mathrm{TeV}$ against 5.5 background events, which corresponds to a $5.6\sigma$ statistical significance. This is the first detection of photons with $E>100\mathrm{TeV}$ from an astrophysical source.

Figure 1

(a) An event display of the observed photonlike AS of energy 251 TeV. The size and color of each circle represent the logarithmic particle density and the relative timing in each detector, respectively. The arrow head and direction indicate the AS core location and incident direction, respectively. Dots and open squares denote scintillation detectors and underground MDs, respectively. The area enclosed by the dashed line indicates the fiducial area of the AS array. (b) Lateral distribution of the photonlike shower event in (a). The solid circles and curve show the experimental data and fitting result, respectively, by the Nishimura-Kamata-Greisen (NKG) function to the data recorded by detectors more than 10 m apart from the AS axis. The dashed curve and open circles are an extrapolation of the NKG function fitting and the unused data within 10 m from the AS axis, respectively.

Figure 2

Distribution of the number of muons ($\mathrm{\Sigma }{N}_{\mu }$) measured by the MD array as a function of the sum of the particle density ($\mathrm{\Sigma }\rho$) measured by the AS array for (a) photon signals generated by the MC simulation and (b) cosmic-ray events extracted from the real data. The color and the solid lines represent the number of events and the optimized muon-cut condition, respectively. $\mathrm{\Sigma }{N}_{\mu }=0$ is plotted at log$(\mathrm{\Sigma }{N}_{\mu })=-0.8$ on the vertical axis.

Figure 3

Distribution of events as a function of the square of the incident angle (${\varphi }^2$) measured from the Crab direction. The filled circles and the solid histograms stand for the experimental data and the MC simulations with (a) $E>10\mathrm{TeV}$ and (b) $E>100\mathrm{TeV}$, respectively.

Figure 4

Cumulative probability ($P_{\mu }$) of $\mathrm{\Sigma }{N}_{\mu }$ for cosmic-ray events above 251 TeV, which are recorded under similar geometries ($\theta \pm 5°$ and $r_{\mathrm{core}}\pm 30\mathrm{m}$) as the 251 TeV photonlike event in Fig. 1. The vertical dashed line indicates $\mathrm{\Sigma }{N}_{\mu }=2.3$ detected in the 251 TeV photonlike event.

Figure 5

Differential energy spectrum of Crab photons. The red solid circles and open circles show fluxes observed with the Tibet $\mathrm{AS}+\mathrm{MD}$ and Tibet-III array [14], respectively. The magenta arrows with plus mark show the previous upper limits at the 90% confidence level given by the Tibet AS and the $100{\mathrm{m}}^2$ prototype MD [15]. The green squares, open squares, blue solid triangles, and blue crosses show flux points observed with three Cherenkov telescopes: HEGRA [12], MAGIC [29], H.E.S.S. [6], and H.E.S.S. during the gamma-ray flare periods detected by Fermi-LAT in 2013 [28], respectively. The solid curve is a model fit for the IC scattering of various seed photons [12].