Observation of Multi-TeV Gamma Rays from the Crab Nebula using the Tibet Air Shower Array

Area of Science:

• High-energy astrophysics
• Gamma-ray astronomy

Background:

• The Tibet experiment operates an air shower array at Yangbajing (4300 m altitude).
• A new high-density array, built in 1996, enables sensitivity to gamma-ray air showers at energies as low as 3 TeV.

Purpose of the Study:

• To observe the Crab Nebula in multi-TeV gamma-rays.
• To determine the gamma-ray energy spectrum of the Crab Nebula above 3 TeV.

Main Methods:

• Utilized the new high-density air shower array at the Tibet experiment.
• Detected gamma-ray air showers originating from the Crab Nebula.
• Employed scintillation detectors in a conventional air shower array setup.

Main Results:

• A statistically significant signal (5.5 sigma) was detected from the Crab Nebula in multi-TeV gamma-rays.
• The gamma-ray energy spectrum was determined above 3 TeV, partially overlapping with imaging atmospheric Cherenkov telescope observations.
• The spectrum is described by a power-law fit: dJ(E)/dE = (4.61+/-0.90)x10^-12 (E/3 TeV)^-2.62+/-0.17 cm^-2 s^-1 TeV^-1.

Conclusions:

• This study reports the first observation of gamma-ray signals from point sources using a conventional air shower array with scintillation detectors.
• The results provide valuable data on the high-energy emission from the Crab Nebula.