The Electron-Thermal Neutron Detector Array (ENDA) is located at the Large High Altitude Air Shower Observatory (LHAASO, 4410 m.a.s.l.). It aims to measure the composition-resolved energy spectrum of cosmic rays, particularly in the knee region. Currently, 64 detectors, referred to as ENDA-64, have been deployed and have been operational for over a year. This paper presents the current status of ENDA-64, including the detector’s operational performance, neutron spectrum measurement, and the lateral distributions of secondary particles.

In this paper, we report the detection of the very-high-energy (VHE, 100 GeV < E < 100 TeV) and ultra-high-energy (UHE, E > 100 TeV) γ-ray emissions from the direction of the young star-forming region W43, observed by the Large High Altitude Air Shower Observation (LHAASO). The extended γ-ray source was detected with a significance of ∼16 σ by KM2A and ∼17 σ by WCDA, respectively. The angular extension of this γ-ray source is about 0.5 degrees, corresponding to a physical size of about 50 pc. We discuss the origin of the γ-ray emission and possible cosmic ray acceleration in the W43 region using multi-wavelength data. Our findings suggest that W43 is likely another young star cluster capable of accelerating cosmic rays (CRs) to at least several hundred TeV.

The diffuse Galactic gamma-ray emission is a very important tool used to study the propagation and interaction of cosmic rays in the Milky Way. In this Letter, we report the measurements of the diffuse emission from the Galactic plane-covering Galactic longitudes from 15° to 235° and latitudes from -5° to +5°, in an energy range of 1 to 25 TeV-made with the Water Cherenkov Detector Array (WCDA) of the Large High Altitude Air Shower Observatory. After the sky regions of known sources are masked, the diffuse emission is detected with 24.6σ and 9.1σ significance in the inner Galactic plane (15°<l<125°, |b|<5°) and outer Galactic plane (125°<l<235°, |b|<5°), respectively. The WCDA spectra in both regions can be well described by a power-law function, with spectral indices of -2.67±0.05_{stat} in the inner region and -2.83±0.19_{stat} in the outer region, respectively. Combined with the Square Kilometer Array (KM2A) measurements at higher energies, a clear softening of the spectrum is found in the inner region, with change of spectral indices by ∼0.5 at a break energy around 30 TeV. The fluxes of the diffuse emission are higher by a factor of 1.5-2.7 than the model prediction assuming local cosmic ray spectra and the gas column density, which are consistent with those measured by the KM2A. Along the Galactic longitude, the spatial distribution of the diffuse emission shows deviation from that of the gas column density. The spectral shape of the diffuse emission may vary in different longitude regions. The WCDA measurements bridge the gap between the low-energy measurements by space detectors and the ultra-high-energy observations by KM2A and other experiments. These results suggest that improved modeling of the wideband diffuse emission is required.

Introduction:Electron-Neutron Detector Array (ENDA) is designed to measure thermal neutrons produced by hadronic interactions between cosmic ray extensive air showers (EAS) and the surrounding environment as well as electrons around the cores of EAS. ENDA is located within Large High Altitude Air Shower Observatory (LHAASO). ENDA was expanded from an initial 16 detectors to 64 detectors in April 2023, so called ENDA-64, and has been running alongside LHAASO. The stability and consistency of neutron detection are crucial for laying a solid foundation for subsequent data analysis and physical results. Methods:We obtain the stability by studying variations of event rate and thermal neutron rate in each cluster and the consistency by comparing distribution of number of thermal neutrons between clusters. Additionally, we investigate the specific influences of the rainy and dry seasons, as well as the presence or absence of sand cubes under the detectors, to examine the environmental factors affecting neutron measurement performance. Results:The calibration results indicate good consistency in thermal neutron detection across the clusters, with the maximum inconsistency of 6.85%. The maximum instability of event rate and thermal neutron rate over time are 4.68% and 11.0% respectively. The maximum inconsistency between the clusters without the sand cubes is 18%. The use of sand cubes is effective in protecting the target material from rainwater, and the sand cubes help the cluster to increase collection of neutrons generated by EAS events.

Identifying Galactic PeVatrons (PeV particle accelerators) from ultrahigh-energy (UHE, >100 TeV)gamma-ray sources plays a crucial role in revealing the origin of Galactic cosmic rays. The UHE source 1LHAASO J1857+0203u is suggested to be associated with HESS J1858+020, which may be attributed to the possible PeVatron candidate supernova remnant (SNR) G35.6-0.4 or H II region G35.6-0.5. We perform detailed analysis on the very-high-energy and UHE gamma-ray emissions toward this region with data from the Large High Altitude Air Shower Observatory (LHAASO). 1LHAASO J1857+0203u is detected with a significance of 11.6 sigma above 100 TeV, indicating the presence of a PeVatron. It has an extent of similar to 0 degrees.18 with a power-law (PL) spectral index of similar to 2.5 at 1-25 TeV and pointlike emission with a PL spectral index of similar to 3.2 above 25 TeV. Using archival CO and H I data, we identify some molecular and atomic clouds that may be associated with the TeV gamma-ray emissions. Our modeling indicates that the TeV gamma-ray emissions are unlikely to arise from clouds illuminated by the protons that escaped from SNR G35.6-0.4. In the scenario in which H II region G35.6-0.5 could accelerate particles to the UHE band, the observed GeV-TeV gamma-ray emission could be well explained by a hadronic model with a PL spectral index of similar to 2.0 and cutoff energy of similar to 450 TeV. However, an origin in an evolved pulsar wind nebula cannot be ruled out.

Ultra-high-energy (UHE), exceeding 100 TeV (10^12 electronvolts), γ-rays manifests extreme particle acceleration in astrophysical sources. Recent observations by γ-ray telescopes, particularly by the Large High Altitude Air Shower Observatory (LHAASO), have revealed a few tens of UHE sources, indicating numerous Galactic sources capable of accelerating particles to PeV (10^15 electronvolts) energies. However, discerning the dominant acceleration mechanisms (leptonic versus hadronic), the relative contributions of specific source classes, and the role of particle transport in shaping their observed emission are central goals of modern UHE astrophysics. Here we report the discovery of a giant UHE γ-ray emitter at -17.5° off the Galactic plane - a region where UHE γ-ray sources are rarely found. The emitter exhibits a distinctive asymmetric shape, resembling a giant "Peanut" spanning 0.45° \times 4.6°, indicative of anisotropic particle distribution over a large area. A highly aged millisecond pulsar (MSP) J0218+4232 is the sole candidate accelerator positionally coincident with the Peanut region. Its association with UHE γ-rays extending to 0.7 PeV, if confirmed, would provide the first evidence of a millisecond pulsar powering PeV particles. Such a finding challenges prevailing models, which posit that millisecond pulsars cannot sustain acceleration to PeV energies. The detection reveals fundamental gaps in understanding particle acceleration, cosmic-ray transport, and interstellar magnetic field effects, potentially revealing new PeV accelerator (PeVatron) classes.

In this study, we present a comprehensive analysis of an unidentified point-like ultra-high-energy (UHE) γ-ray source, designated as 1LHAASO J1740+0948u, situated in the vicinity of the middle-aged pulsar PSR J1740+1000. The detection significance reached 17.1σ (9.4σ) above 25TeV (100TeV). The source energy spectrum extended up to 300TeV, which was well fitted by a log-parabola function with N0 = (1.93±0.23) × 10^-16TeV^-1 cm^-2 s^-2, α = 2.14±0.27, and β = 1.20±0.41 at E0 = 30TeV. The associated pulsar, PSR J1740+1000, resides at a high galactic latitude and powers a bow-shock pulsar wind nebula (BSPWN) with an extended X-ray tail. The best-fit position of the gamma-ray source appeared to be shifted by 0.2^∘ with respect to the pulsar position. As the (i) currently identified pulsar halos do not demonstrate such offsets, and (ii) centroid of the gamma-ray emission is approximately located at the extension of the X-ray tail, we speculate that the UHE γ-ray emission may originate from re-accelerated electron/positron pairs that are advected away in the bow-shock tail.

We report the detection of an extended very-high-energy (VHE) γ-ray source coincident with the location of middle-aged (62.4 kyr) pulsar PSR J0248+6021, by using the LHAASO-WCDA data of live 796 d and LHAASO-KM2A data of live 1216 d. A significant excess of γ-ray induced showers is observed both by WCDA in energy bands of 1–25 TeV and KM2A in energy bands of >25 TeV with 7.3σ and 13.5σ, respectively. The best-fit position derived through WCDA data is R.A. = 42.06° ± 0.12° and Dec. = 60.24° ± 0.13° with an extension of 0.69°±0.15° and that of the KM2A data is R.A.= 42.29° ± 0.13° and Dec. = 60.38° ± 0.07° with an extension of 0.37° ±0.07°. No clear extended multiwavelength counterpart of this LHAASO source has been found from the radio band to the GeV band. The most plausible explanation of the VHE γ-ray emission is the inverse Compton process of highly relativistic electrons and positrons injected by the pulsar. These electrons/positrons are hypothesized to be either confined within the pulsar wind nebula or to have already escaped into the interstellar medium, forming a pulsar halo.

The ultra-high-energy (UHE) gamma-ray source 1LHAASO J0007+7303u is positionally associated with the composite SNR CTA1 that is located at high Galactic Latitude b ≈ 10.5°. This provides a rare opportunity to spatially resolve the component of the pulsar wind nebula (PWN) and supernova remnant (SNR) at UHE. This paper conducted a dedicated data analysis of 1LHAASO J0007+7303u using the data collected from December 2019 to July 2023. This source is well detected with significances of 21σ and 17σ at 8–100 TeV and >100 TeV, respectively. The corresponding extensions are determined to be 0.23°±0.03° and 0.17°±0.03°. The emission is proposed to originate from the relativistic electrons accelerated within the PWN of PSR J0007+7303. The energy spectrum is well described by a power-law with an exponential cutoff function dN/dE=(42.4± 4.1)(E 20 TeV)^-2.31± 0.11 exp(-E 110± 25 TeV) TeV−1 cm−2 s−1 in the energy range from 8 to 300 TeV, implying a steady-state parent electron spectrum dN_e/dE_e∝ (E_e 100 TeV)^-3.13± 0.16 exp[(-E_e373± 70 TeV)^2] at energies above ≈ 50 TeV. The cutoff energy of the electron spectrum is roughly equal to the expected current maximum energy of particles accelerated at the PWN terminal shock. Combining the X-ray and gamma-ray emission, the current space-averaged magnetic field can be limited to ≈ 4.5 µG. To satisfy the multi-wavelength spectrum and the γ-ray extensions, the transport of relativistic particles within the PWN is likely dominated by the advection process under the free-expansion phase assumption.

We propose to use existing LHAASO detectors to study of the flavor composition of UHE astrophysical neutrino. Monte Carlo simulations show that large Water Cherenkov Detector Array (WCDA) is a suitable detector to record high energy near horizontal neutrino interactions, due to its cell structure and rather large size. Signature of such events is very clear thus allows us even to determine the neutrino flavor interacting in the WCDA. The mass of this detector is about 300 kt of water and it is not enough to accumulate many high-energy neutrino events in a near future. Nevertheless, it is running detector and we could develop suitable algorithms for this program and make a search for these events regarding WCDA as a prototype for future full-scale experiment. Moreover, existing of square km EAS array (Km2A) with many muon detectors makes it possible to have a powerful veto against a background flux of near-horizontal atmospheric muons.

The Water Cherenkov Detector Array (WCDA) is one of the components of Large High Altitude Air Shower Observatory (LHAASO) and can monitor any sources over two-thirds of the sky for up to 7 h per day with >98 per cent duty cycle. In this work, we report the detection of two outbursts of the Fanaroff-Riley I radio galaxy NGC 1275 that were detected by LHAASO-WCDA between 2022 November and 2023 January with statistical significance of 5.2 sigma and 8.3 sigma. The observed spectral energy distribution in the range from 500 GeV to 3 TeV is fitted by a power law with the best-fitting spectral index of alpha = -3.37 +/- 0.52 and -3.35 +/- 0. 29, respectively. The outburst flux above 0.5 TeV was (4.55 +/- 4.21) x10(-11) cm(-2) s(-1) and (3.45 +/- 1.78) x10(-11) cm(-2) s(-1), corresponding to 60 per cent and 45 per cent of Crab Nebula flux, respectively. Variation analysis reveals the variability time-scale of days at the TeV energy band. A simple test by one-zone synchrotron self-Compton model reproduces the data in the gamma-ray band well.

The core-collapse supernova remnant (SNR) Cassiopeia A (Cas A) is one of the brightest galactic radio sources with an angular radius of ∼ 2 . ′ 5 . Although no extension of this source has been detected in the γ -ray band, using more than 1000 days of LHAASO data above ∼0.8 TeV, we find that its spectrum is significantly softer than those obtained with Imaging Air Cherenkov Telescopes (IACTs), and its flux near ∼1 TeV is about 2 times higher. In combination with analyses of more than 16 yr of Fermi-LAT data covering 0.1 GeV–1 TeV, we find that the spectrum above 30 GeV deviates significantly from a single power law and is best described by a smoothly broken power law with a spectral index of 1.90 ± 0.15 stat (3.41 ± 0.19 stat ) below (above) a break energy of 0.63 ± 0.21 stat TeV. Given differences in the angular resolution of LHAASO-WCDA and IACTs, TeV γ -ray emission detected with LHAASO may have a significant contribution from regions surrounding the SNR illuminated by particles accelerated earlier, which, however, are treated as background by IACTs. Detailed modeling can be used to constrain the acceleration processes of TeV particles in the early stage of SNR evolution.

Black holes (BHs), one of the most intriguing objects in the universe, can manifest themselves through electromagnetic radiation initiated by the accretion flow. Some stellar-mass BHs drive relativistic jets when accreting matter from their companion stars, forming microquasars. Non-thermal emission from the radio to teraelectronvolt gamma-ray band has been observed from microquasars, indicating the acceleration of relativistic particles. Here we report detection of four microquasars (SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070) of spectra extending to the ultrahigh-energy (UHE; photon energy [Formula: see text] TeV) band, and one microquasar (Cygnus X-1) with a spectrum approaching 100 TeV, using the Large High Altitude Air Shower Observatory. Notably, the total emission associated with SS 433 cannot be interpreted with a single leptonic component. In the UHE band, its emission is in spatial coincidence with a giant atomic cloud, which is consistent with a hadronic origin. An elongated source is discovered from V4641 Sgr with the spectrum continuing up to 800 TeV. The detection of UHE gamma rays demonstrates that accreting BHs and their environments can operate as extremely efficient accelerators of particles up to 1 PeV, suggesting that microquasars are important contributors to Galactic cosmic rays, especially around the 'knee' region.

The first source catalog of the Large High Altitude Air Shower Observatory (LHAASO) reported the detection of a very high energy gamma-ray source, 1LHAASO J1219+2915. This Letter presents a further detailed study of the spectral and temporal behavior of this pointlike source. The best-fit position of the TeV source (R.A. = 185.°05 ± 0.°04, decl. = 29.°25 ± 0.°03) is compatible with NGC 4278 within ∼0.°03. Variation analysis shows an indication of variability on a timescale of a few months in the TeV band, which is consistent with low-frequency observations. Based on these observations, we report the detection of TeV γ -ray emissions from this low-luminosity active galactic nucleus. The observation by LHAASO's Water Cherenkov Detector Array during the active period has a significance level of 8.8 σ with a best-fit photon spectral index Γ = 2.56 ± 0.14 and a flux f 1–10 TeV = (7.0 ± 1.1 sta ± 0.35 syst ) × 10 −13 photons cm −2 s −1 , or approximately 5% of the Crab Nebula. The discovery of VHE gamma-ray emission from NGC 4278 indicates that compact, weak radio jets can efficiently accelerate particles and emit TeV photons.

We present the first catalog of very-high energy and ultra-high energy gamma-ray sources detected by the Large High Altitude Air Shower Observatory (LHAASO). The catalog was compiled using 508 days of data collected by the Water Cherenkov Detector Array (WCDA) from March 2021 to September 2022 and 933 days of data recorded by the Kilometer Squared Array (KM2A) from January 2020 to September 2022. This catalog represents the main result from the most sensitive large coverage gamma-ray survey of the sky above 1 TeV, covering declination from $-$20$^{\circ}$ to 80$^{\circ}$. In total, the catalog contains 90 sources with an extended size smaller than $2^\circ$ and a significance of detection at $> 5\sigma$. Based on our source association criteria, 32 new TeV sources are proposed in this study. Among the 90 sources, 43 sources are detected with ultra-high energy ($E > 100$ TeV) emission at $> 4\sigma$ significance level. We provide the position, extension, and spectral characteristics of all the sources in this catalog.