The observation of the Crab Nebula with LHAASO-KM2A for the performance study

TL;DR

This paper reports the first observation of the Crab Nebula using LHAASO-KM2A, demonstrating its performance in detecting very high energy gamma rays above 10 TeV and measuring its energy spectrum.

Contribution

The study presents the first gamma-ray detection of the Crab Nebula with LHAASO-KM2A, validating detector performance and extending observations into the 10-250 TeV range.

Findings

Gamma-ray signals detected from Crab Nebula in 10-100 TeV and >100 TeV ranges.

Energy spectrum fits a single power-law, consistent with previous measurements.

Demonstrated detector capabilities including angular resolution and background rejection.

Abstract

As a sub-array of the Large High Altitude Air Shower Observatory (LHAASO), KM2A is mainly designed to cover a large fraction of the northern sky to hunt for gamma-ray sources at energies above 10 TeV. Even though the detector construction is still underway, a half of the KM2A array has been operating stably since the end of 2019. In this paper, we present the pipeline of KM2A data analysis and the first observation on the Crab Nebula, a standard candle in very high energy gamma-ray astronomy. We detect gamma-ray signals from the Crab Nebula in both energy ranges of 10$-$100 TeV and $>$100 TeV with high significance, by analyzing the KM2A data of 136 live days between December 2019 and May 2020. With the observations, we test the detector performance including angular resolution, pointing accuracy and cosmic ray background rejection power. The energy spectrum of the Crab Nebula in the energy range 10-250 TeV fits well with a single power-law function dN/dE =(1.13$\pm$0.05$_{stat}$$\pm$0.08$_{sys}$)$\times$10$^{-14}$$\cdot$(E/20TeV)$^{-3.09\pm0.06_{stat}\pm0.02_{sys}}$ cm$^{-2}$ s$^{-1}$ TeV$^{-1}$. It is consistent with previous measurements by other experiments. This opens a new window of gamma-ray astronomy above 0.1 PeV through which ultrahigh-energy gamma-ray new phenomena, such as cosmic PeVatrons, might be discovered.