Fig.1: Left: The UHE sky surveyed by LHAASO using the first 308 days 1/2-KM2A data yielding 12 UHE sources. Right: The UHE sky surveyed by LHAASO using the first 933 days data yielding 43 UHE sources.
Fig.2: Sensitivities of very-high-energy and ultra-high-energy gamma-ray astronomical instruments as functions of gamma-ray energy, E.
UHE gamma rays are produced through two mechanisms. One mechanism involves the decay of neutral pions resulting from collisions between cosmic rays and interstellar gas, where the energy of gamma rays is approximately 1/10 of the cosmic ray energy. UHE gamma-rays correspond to cosmic ray energies above 1 PeV, making them a crucial tool for investigating the origins of PeV cosmic rays within the Milky Way. The other mechanism includes inverse Compton scattering of high-energy electrons with low-energy photons. In the UHE range, the scattering cross-section decreases rapidly as the frequency of the seed photons increases. As a result, the primary seed photons for UHE gamma rays are photons from the cosmic microwave background (CMB) radiation, with the typical photon energy being around one-third of the electron energy.
When UHE gamma rays travel through the universe, they are primarily influenced by the absorption effect of CMB photons, restricting their visibility to within 1 Mpc. Consequently, observations of UHE gamma rays mainly concentrate on studying astrophysical sources within the Milky Way. The known types of astrophysical sources emitting UHE gamma rays include the following.
Pulsar Wind Nebulae (PWNe): PWNe are powered by energetic pulsars, which consist of electrons and positrons expelled from the magnetosphere. These particles form a cold, ultra- relativistic wind and are further accelerated at the termination shock when the pulsar wind interacts with the surrounding medium. PWNe are known as one of the most efficient electron factories in the Galaxy. A significant number of the detected UHEsources are linked to energetic pulsars, suggesting that these UHE sources may be PWNe. The Crab Nebula is a well-established example of a PWN. Recently, LHAASO has detected gamma-ray emissions from the Crab Nebula reaching at least 1.1 PeV, indicating that the accelerator within the Crab Nebula must operate with exceptionally high efficiency to counter the substantial energy losses. The acceleration efficiency needs to reach 16% of the theoretical upper limit, surpassing that of the supernova blast wave by a factor of 1,000 and challenging the standard paradigm of electron acceleration in high-energy astrophysics.
Young Massive Star Cluster:Young massive stars in a dense cluster, which produce powerful stellar winds, have the potential to create multiple shocks capable of accelerating cosmic ray (CR) protons to energies exceeding 1 PeV. Recently, LHAASO discovered a massive UHE gamma-ray bubble in the Cygnus star-forming region, with several photons surpassing 1 PeV within the structure, and the highest energy recorded at 2.5 PeV. This discovery suggests the presence of a super cosmic ray accelerator within the bubble, continuously boosting high-energy cosmic ray particles with energies of up to 20 PeV and releasing them into interstellar space. The massive star cluster, Cygnus OB2, located near the center of the bubble, emerges as the most promising candidate for this super cosmic ray accelerator.
Supernova remnant (SNR): SNRs, the spherical shock waves expanding in the interstellar medium (ISM) following the explosion of massive stars, have long been considered the most promising sources of Galactic cosmic rays (CRs). Detecting gamma-rays above 100 TeV from SNRs could provide insights into the acceleration limits of SNRs at the highest energies. Evidence supporting a SNR as a PeVatron is crucial for understanding the origin of CRs in the knee region. Recently, LHAASO detected UHE gamma-ray emissions from the interacting SNR W51C, with a maximum energy reaching up to 200 TeV. This can be explained by the π0-decay process resulting from collisions between CRs and the surrounding gas. These findings suggest that W51C is capable of accelerating CR protons well beyond 100 TeV, reaching energies of up to 400 TeV.
Micro-quasar: A micro-quasar is a binary system comprising a compact object (either a black hole or a neutron star) that accretes matter from a companion star. This compact system can exhibit characteristics similar to quasars, including the presence of relativistic jets. LHAASO has detected gamma-rays with energies exceeding 100 TeV from the jets of the micro-quasar SS 433, providing insights into the classification of micro-quasars as PeVatrons.
UHE gamma-rays can also serve as a potent tool for exploring new physics. Through the observation of PeV photons, the LHAASO collaboration has established the most stringent lower limit for Lorentz invariance, a fundamental principle in modern physics. Additionally, by studying diffuse UHE gamma rays originating from regions outside the Galactic plane, the LHAASO collaboration has imposed the most robust constraints to date on the lifetime of heavy dark matter particles with masses ranging between 105 and 109 GeV.
References:
[1] Cao et al. Ultra-High-Energy Gamma-Ray Astronomy, Annual Review of Nuclear and Particle Science, 73:341-363 (2023)
[2] LHAASO collaboration, Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources, Nature, 594:33-36 (2021)
[3] LHAASO collaboration, Peta-electron volt gamma-ray emission from the Crab Nebula, Science, 373:425-430 (2021)
[4] LHAASO collaboration, The First LHAASO Catalog of Gamma-Ray Sources, The Astrophysical Journal Supplement Series, 271:25 (2024)
[5] LHAASO collaboration, An ultrahigh-energy γ -ray bubble powered by a super PeVatron, Science Bulletin, 69:449-457 (2024)
[6] LHAASO collaboration, Constraints on Heavy Decaying Dark Matter from 570 Days of LHAASO Observations, Physical Review Letters, 129:261103 (2022)