The first scientific paper from the Large High Altitude Air Shower Observatory (LHAASO) project, entitled “Observation of the Crab Nebula with LHAASO-KM2A – a Performance Study,” was officially published in Chinese Physics Con Feb. 3. This represents the start of a journey of discovery in ultra-high energy (UHE) gamma-ray astronomy.

Based on the first five months of data recorded by the one-square-kilometer array (KM2A) of LHAASO, gamma-ray emissions from the standard candle Crab Nebula in the energy range above 10 TeV (1 TeV = 10¹²eV) were measured to verify key properties of the KM2A detector. The results show that the observed statistical significance of gamma-rays from the Crab Nebula at UHE (energies > 100 TeV) is more than 14 standard deviations, which is much better than that from many years of observation by all other experiments. It is clear that LHAASO is already the world's most sensitive instrument for the detection of UHE gamma-rays.

LHAASO is one of China's large S&T infrastructure projects. It is located at an altitude of 4410 m on Haizi Mountain, Daocheng County, Sichuan Province. To achieve various scientific goals, LHAASO is designed with three sub-arrays, among which the KM2A sub-array is designed to detect gamma-rays with energies above 10 TeV. The whole KM2A array will consist of 5195 electromagnetic detectors on the ground and 1188 muon detectors underground.

Principal construction began in November 2017. By the end of December 2019, half of the KM2A array had already been completed. It was immediately put into operation and became the most sensitive detector in the world. In fact, the aforementioned publication is based on data from this first half-array.

In this paper, the methods and process of KM2A data analysis from calibration, reconstruction, and simulation to physical analysis are described in detail. Array detector performance, including angular resolution, pointing accuracy, gamma-ray–background separation capability and other key scientific indicators, were thoroughly tested via the observation of the Crab Nebula.

The measured emission spectrum is quite consistent with previous measurements by other experiments. All these results establish a very important foundation for studying gamma-ray astronomy using KM2A data in the future.

Three-quarters of KM2A has been running stably since December 2020. The entire array will be completed in June 2021.