Sensitivity estimation of LHAASO-WCDA for observing GLE events

[1] We made an effort to understand the associations and relationships between ground level enhancement (GLE) events and solar flares for the time period of 1986–2006. Our results show that, on average, the GLE event–associated solar flare (∼0.2 × 10−4 W/m2) is much stronger than the non-GLE-associated solar flare (∼0.3 × 10−5 W/m2). The findings have also been supported by the solar flare indices that, on average, the GLE event–associated solar flare index (∼35.01) is much higher than the non-GLE-associated solar flare index (∼4.88). However, this association does not seem to precisely imply that GLEs can occur because of a solar flare, so we examined cross correlations between GLE events and simultaneous solar flares. We found that most (∼78%) of the highest correlations (r > 0.8) took place during an X class flare. There is no clear indication that the more the time lag, the less or more is the correlation or vice versa. Overall, 50% of the high correlations took place at higher time lag (≥65 min), and ∼36% of the high correlations took place at lower time lag (≤40 min), while the rest (∼14%) of the correlations were abruptly high and low at medium time lag (>40 and <65 min). On the basis of the results of cross correlations, we suggest that the intensive portions of solar flares should be responsible for causing GLEs and that the direct proportionality of the time-integrated intensive portion of a flare with the impulsive phase of a GLE event seems to be the main property for comprehending the mechanism.

The quasi-biennial oscillation of 1.7 years in ground level enhancement events

The LHAASO~J2018+3651 region is one of the brightest sources in the sky at TeV energies. Photons with energies up to ∼0.27 PeV from this region have been detected with the Large High Altitude Air Shower Observatory (LHAASO) and here we present a detailed study of this region using more data from LHAASO. This analysis resolves the region into six sources: LHAASO~J2018+3641, LHAASO~J2019+3649, LHAASO~J2021+3657, LHAASO~J2016+3712, LHAASO~J2013+3610 and LHAASO~J2028+3655. An investigation of the morphology and spectrum for LHAASO~J2018+3641, LHAASO~J2019+3649 and LHAASO~J2021+3657 are the focus and we find that the spectrum above TeV deviates significantly from a single power-law, and is best described by a power-law with exponential cutoff. The three sources may be associated with PSR~J2021+3651 and its X-ray pulsar wind nebula. Additionally, LHAASO~J2016+3712 is tentatively associated with the evolved supernova remnant CTB 87.

Introduction to Large High Altitude Air Shower Observatory (LHAASO)

Status of the large high altitude air shower observatory project

We investigate the extinction coefficients of the surface atmospheric aerosol over the Large High Altitude Air Shower Observatory (LHAASO), located at the Haizi Mountain, Daocheng County, China. To this end, we utilize the Longtin model, Mie scattering theory, and experimental data obtained by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Our theoretical calculations show that the total extinction coefficients of the atmospheric aerosol at the wavelength of 200–500 nm are inversely proportional to the laser wavelength, and influenced by the wind speed. From July 2015 to October 2016, the extinction coefficient of the surface atmospheric aerosols at 532 nm wavelength reached 0.04 km−1 with no wind, while it increased to 0.1 km−1 with gusts. In this period, the extinction coefficients of the surface atmospheric aerosol at 532 nm wavelength, obtained by the CALIPSO, change from 0.01 to 0.07 km−1, which is less than the values obtained the theoretical calculation and larger than the average of Tibetan Plateau in 2006−2016. These calculations and experimental evidence provide important arguments to the model of atmospheric aerosol to be applied in the calibration of LHAASO. Our results suggest that the extinction coefficients over LHAASO require further study, including research on the size distribution, shape, concentration of aerosols particles, wind dependence, relative humidity dependence, etc.

The Large High Altitude Air Shower Observatory (LHAASO) is a mega-scale dual-task facility designed to study cosmic rays and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-rays. Online computing system of LHAASO supports its online operation and computation. Physical phenomena such as cosmic rays occur unpredictably and therefore require the online computing system to run uninterruptedly. LHAASO is large and the environment is harsh, so the online computing system is subject to failure. Once a system fails, maintenance personnel are required to quickly analyze the cause of the failure and repair it. The fault detection and diagnosis software (FADD) is designed to quickly detect and analyze system faults. The software implements comprehensive monitoring of each component of LHAASO’s online computing system (computing nodes, switches, and data flow software) and collects real-time status information. When a fault occurs, FADD can quickly analyze the cause of the fault and provide alarm information to the on-call staff as soon as possible. In addition, it can also analyze historical data within a specified period and generate data reports as needed. FADD is designed to fully consider the characteristics of large-scale high-energy physics experiments and satisfy the requirements of high throughput and high efficiency by using a distributed architecture. The software consists of the following layers: information collection layer, data analysis layer, and result layer, and contains metrics detection software, fault monitoring module, fault diagnosis module, and other functional modules. FADD has been applied to LHAASO and can diagnose operational faults quickly and accurately, helping to reduce the burden on maintenance personnel.

The Large High Altitude Air Shower Observatory (LHAASO) is a hybrid extensive air shower (EAS) array with an area of about 1 𝑘𝑚2 at an altitude of 4410 m a.s.l. in Sichuan province, China. It contains three sub-detectors: 1 𝑘𝑚2 array (KM2A) composed of 5195 electromagnetic particle detectors (ED) and muon detectors (MD); water Cerenkov detector array (WCDA) and 18 Wide Field-of-view Cerenkov Telescope Array (WFCTA). One of the main scientific goals is measuring the individual energy spectra of cosmic rays from ~30 TeV to a couple of EeV. The construction of the whole observatory will be completed by the end of 2021. In this paper, a new energy reconstruction method and result based on KM2A simulated nuclei events will be shown.

Large High Altitude Air Shower Observatory (LHAASO) is a composite cosmic ray observatory consisting of three types of detectors: KM2A, WCDA and WFCTA. One of the main scientific objectives of LHAASO is to measure the cosmic ray spectrum and composition precisely. With hybrid detection of WCDA and WFCTA, one can further study the Knee of cosmic rays from 100TeV to 10PeV. The original components of cosmic rays can be effectively distinguished with four parameters provided by WFCTA, WCDA and MDA, respectively. The energy reconstruction resolution of proton shower is about 20% reconstructed only by WFCTA. Combining with the energy flow of shower core provided by WCDA, a better energy resolution is expected.

Diffuse γ-ray emission between 10 and 1000 TeV from the Galactic plane was recently measured by the Large High Altitude Air Shower Observatory (LHAASO). These observations will help tremendously in constraining the propagation and interaction of cosmic rays in the Milky Way. Additionally, new measurements of cosmic-ray spectra reach a very high precision of up to 100 TeV energies, revealing multiple spectral structures of various species. In this work, based on up-to-date measurements of local cosmic-ray spectra and a simplified propagation setup, we confront a model prediction of diffuse γ-ray emission with measurements of diffuse γ-rays. To better constrain the low-energy part of the model, we analyze the 14.6 yr of Fermi Large Area Telescope (Fermi-LAT) data to extract the Galactic diffuse emission between 1 and 500 GeV from the same sky regions of LHAASO, after subtracting the contribution from known sources and the isotropic diffuse γ-ray background. The joint Fermi-LAT and LHAASO spectra thus cover a very wide energy range from 1 GeV to 1 PeV with small gaps from 0.5 to 10 TeV. Compared with the prediction, we find that clear excesses between several GeV and ∼60 TeV of the diffuse emission exist. Possible reasons to explain the excesses may include unresolved sources or more complicated propagation models. We illustrate that an exponential cutoff power-law component with an index of −2.40 and a cutoff energy of ∼30 TeV is able to account for such excesses.

The Large High Altitude Air Shower Observatory (LHAASO) experiment, fully operational since July 2021, is a cutting-edge facility for detecting cosmic rays and gamma rays across a wide energy range. This paper offers an overview of the current status and recent significant scientific achievements of the LHAASO collaboration. These include the gamma-ray astronomy, charged cosmic ray physics and new physics.

The Large High Altitude Air Shower Observatory (LHAASO) is located in Haizi Mountain, Daocheng County, Sichuan Province, with an average altitude of 4410 m. The Wide Field of view Cherenkov Telescope Array (WFCTA) is one of its three observation arrays. Laser calibration system for WFCTA consists of 2 nitrogen lasers and 1 Nd:YAG laser system. During the operation of WFCTA, the outdoor temperature may dropped from 17 °C to about -30 °C, which will bring serious challenges to the operation of laser calibration system operating in the field. In order to ensure the stable working environment and remote operation of the laser, we have developed the LHAASO-WFCTA laser temperature control, remote control and data monitoring system. The fluctuation of temperature in the box is within 1 °C around the target. The laser temperature control system was officially put into operation in Sept. 2020. In this paper, the design scheme, fabrication scheme, comparison of heat dissipation coefficient and test of temperature control system are introduced. The working data shows that both nitrogen molecule lasers and Nd:YAG laser temperature control system can achieve stable and long-term operation in high-altitude extreme field.

The detection of very high-energy neutrinos by IceCube experiment supports the existence of a comparable gamma-ray counterpart from the same cosmic accelerators. Under the likely assumption that the sources of these particles are of extragalactic origin, the emitted photon flux would be significantly absorbed during its propagation over cosmic distances. However, in the presence of photon mixing with ultra-light axion-like-particles (ALPs), this expectation would be strongly modified. Notably, photon-ALP conversions in the host galaxy would produce an ALP flux which propagates unimpeded in the extragalactic space. Then, the back-conversion of ALPs in the Galactic magnetic field leads to a diffuse high-energy photon flux. In this context, the recent detection of the diffuse high-energy photon flux by the Large High Altitude Air Shower Observatory (LHAASO) allows us to exclude at the 95% CL an ALP-photon coupling g_{agamma } > rsim 3.9–7.8 times 10^{-11}~mathrm {GeV^{-1}} for m_{a}lesssim 4times 10^{-7}~textrm{eV}, depending on the assumptions on the magnetic fields and on the original gamma-ray spectrum. This new bound is complementary with other ALP constraints from very-high-energy gamma-ray experiments and sensitivities of future experiments.

The origin of sub-TeV gamma rays detected by the \textit{Fermi}-Large Area Telescope (LAT) from the \textit{Fermi} Bubbles (FBs) at the Galactic center is still uncertain. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce the observed gamma rays. Recently the High Altitude Water Cherenkov (HAWC) observatory reported an absence of gamma-ray excess from the Northern FB at $b\gtrsim 6^\circ$ Galactic latitude, which resulted in flux upper limits in the energy range of $1.2-126$ TeV. These upper limits are consistent with the gamma-ray spectrum measured by \textit{Fermi}-LAT at $|b|\ge 10^\circ$, where an exponential cutoff at energies $\gtrsim 100$ GeV is evident. However, the FB gamma-ray spectrum at $|b|\le 10^\circ$, without showing any sign of cutoff up to around 1 TeV in the latest results, remains unconstrained. The upcoming Cherenkov Telescope Array (CTA) will perform a Galactic center survey with unprecedented sensitivity in the energy between 20 GeV and 300 TeV. In this work, we perform both morphological and classic on/off analyses with planned CTA deep central and extended survey and estimate the sensitivity of CTA to the FB hadronic gamma-ray flux models that best fit the spectrum at $|b|\le 10^\circ$ and whose counterpart neutrino flux model best fits the optimistic neutrino spectrum from IceCube Neutrino Observatory. We also perform sensitivity analysis with a future ground-based Cherenkov detector the Large High Altitude Air Shower Observatory (LHAASO). We find that CTA will be able to discover or constrain the FB gamma-ray flux at $|b|\le 10^\circ$ in the $\approx$ 200 GeV -- 100 TeV range with planned observation strategy, while LHAASO may constrain emission in the $\approx 100$~GeV -- 100~TeV range if $\lesssim 10\%$ systematic uncertainties can be achieved.

Ground-level enhancement (GLE) events are often associated with large gradual solar events such as fast coronal mass ejections (CMEs), but not all fast CMEs lead to GLE events. Is there a type of coordinated CME that could produce GLEs with larger intensity and higher energies than those in the normal fast isolated CMEs? Here we propose a twin-shock scenario driven by the twin CME coordinately, in which the posterior shock catches up with the preceding shock and has a pileup collision. In the present study, we chose the first GLE event of the solar cycle 24 occurring on 2012 May 17 as an example to investigate the probable association with the twin-shock scenario. We use a dynamic Monte Carlo method to examine the energy spectrum with relevance to the GLE event. In the twin-shock scenario, the seed energetic particles produced by the normal preceding shock can be injected into the posterior shock for reacceleration efficiently. As a result, we obtain the detailed energy spectrum of the solar energetic particles (SEPs) with different behaviors at the related episodes of the twin-shock evolution. Therefore, we predict that the pileup collision of the twin shock would dominate a concave energy spectral slope in the 2012 May 17 SEP event.