Very High Energy (VHE) gamma ray astronomy is a fascinating and rapidly evolving field of astrophysics that focuses on the study of gamma rays with energies above 100 GeV. These extremely energetic gamma rays provide valuable insights into some of the most extreme and energetic phenomena in the universe, such as active galactic nuclei, gamma-ray bursts and search of signatures of dark matter annihilation etc.  

     Ground based VHE gamma ray astronomy has led to numerous important discoveries. These observatories operated by detecting the air showers produced as high energy gamma rays interact with the Earth's atmosphere, allowing astronomers to reconstruct the direction and the energy of the incoming gamma rays. By studying the properties of very high energy gamma-ray sources, such as their spectra, variability, and spatial distribution, astronomers can gain insights into the physical processes responsible for producing these energetic photons and the environments in which they originate.

     LHAASO is a gound based complex array, one of China’s major national scientific and technological facilities, aiming to conduct cosmic ray and gamma ray observation and research. Based on its wide field of view, high duty cycle and very high sensitivity, LHAASO can monitor 1/7 of the entire sky at any given time and to scan 2/3 of the entire sky in every single day. Among them the Water Cherenkov Detector Array (WCDA) and the KiloMeter Square Array (KM2A) have been established as the most sensitive VHE/UHE gamma-ray full sky survey instruments in the world, respectively. With the data accumulation, the sensitivity of the instruments will be further improved, more VHE/UHE gamma ray radiation sources will be coming soon.

Fig. 1 Big events at VHE/UHE gamma-ray astronomy

Fig. 2  THE VHE sky surveyed map by LHAASO using 508 WCDA data yielding 69 sources, among them 32 new sources.

Fig. 3 The detection of Brightest Of All Time GRB221009A: left panel is energy flux light curve; right panel is intrinsic VHE spectra corrected for EBL correction.

Reference:

[1] WHIPPLE collaboration, Observation of TeV Gamma rays from the crab nebula using the atmospheric Cerenkov imaging technique, APJ, 342:379-395 (1989)

[2] WHIPPLE collaboration, Detection of TeV Photons from the active galaxy Markarian 421, Nature, 358:477-478 (1992)

[3] H.E.S.S. collaboration, High-energy particle acceleration in the shell of a supernova remanent, Nature, 432:75-77 (2004)

[4] MAGIC collaboration, Teraelectronvolt emission from the gamma-ray burst GRB 190114C, Nature, 575:455-458 (2019)

[5] LHAASO collaboration, Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources, Nature, 594:33-36 (2021)

[6] LHAASO collaboration, A tera-electron volt afterglow from a narrow jet in an extremely bright gamma-ray burst, Science, 380:1390-1396 (2023)

[7] LHAASO collaboration, Very high-energy gamma-ray emission beyond 10 TeV from GRB 221009A, Science  Advance, 9, eadj2778 (2023)

[8] LHAASO collaboration, The First LHAASO Catalog of Gamma-Ray Sources, The Astrophysical Journal Supplement Series, 271:25 (2024)