Large High Altitude Air Shower Observatory Research Articles

Constraints on Lorentz invariance violation from the LHAASO observation of GRB 221009A

In some quantum gravity (QG) theories, Lorentz symmetry may be broken above the Planck scale. The Lorentz invariance violation (LIV) may induce observable effects at low energies and be detected at high energy astrophysical measurements.The Large High Altitude Air Shower Observatory (LHAASO) has detected the onset, rise, and decay phases of the afterglow of GRB 221009A, covering a wide energy range of photons approximately from 0.2 to 13 TeV. This observation provides an excellent opportunity to study the Lorentz invariance violation effect. In this study, we simultaneously utilize the data from the KM2A and WCDA detectors of LHAASO, and apply two event by event methods, namely the pair view method and maximum likelihood method, to investigate LIV. We obtain stringent constraints on the QG energy scale. For instance, through the maximum likelihood method, we determine the 95% confidence level lower limits to be E QG,1 > 14.7 (6.5) × 1019 GeV for the subluminal (superluminal) scenario of n = 1, and E QG,2 > 12.0 (7.2) × 1011 GeV for the subluminal (superluminal) scenario of n = 2. We find that the rapid rise and slow decay behaviors of the afterglow can impose strong constraints on the subluminal scenario, while the constraints are weaker for the superluminal scenario.

LHAASO J2108+5157 as a molecular cloud illuminated by a supernova remnant

Context. The search for Galactic PeVatrons – astrophysical accelerators of cosmic rays to PeV energies – has entered a new phase in recent years with the discovery of the first ultra-high-energy (UHE, E > 100 TeV) γ-ray sources by the High Altitude Water Cherenkov (HAWC) observatory and Large High Altitude Air Shower Observatory (LHAASO). Establishing whether the emission is leptonic or hadronic in nature, however, requires multi-wavelength data and modelling studies. Among the currently known UHE sources, LHAASO J2108+5157 is an enigmatic source without clear association to a plausible accelerator, yet spatially coincident with molecular clouds. Aims. We aim to investigate the scenario of a molecular cloud illuminated by cosmic rays accelerated in a nearby supernova remnant (SNR) as an explanation for LHAASO J2108+5157. We aim to constrain the required properties of the SNR as well as which of the clouds identified in the vicinity is the most likely association. Methods. We used a model for cosmic-ray acceleration in SNRs, their transport through the interstellar medium, and subsequent interaction with molecular material to predict the corresponding γ-ray emission. The parameter space of SNR properties was explored to find the most plausible parameter combination that can account for the γ-ray spectrum of LHAASO J2108+5157. Results. In the case that a SNR is illuminating the cloud, we find that it must be young (< 10 kyr) and located within 40–60 pc of the cloud. A SN scenario with a low Sedov time is preferred, with a maximum proton energy of 3 PeV assumed. No SNRs matching these properties are currently known, although an as yet undetected SNR remains feasible. The Galactic CR sea is insufficient to solely account for the observed flux, such that a PeVatron accelerator must be present in the vicinity.

Optimization of performance of the KM2A full array using the Crab Nebula* *Supported by the National Key R & D Program of China (2018YFA0404201, 2018YFA0404202, 2018YFA0404203, 2018YFA0404204), the National Natural Science Foundation of China (12022502, 12205314, 12105301, 12261160362, 12105294, U1931201), the Youth Innovation Promotion Association CAS (2022010) and in Thailand by the National Science and Technology Development Agency (NSTDA), and

The full array of the Large High Altitude Air Shower Observatory (LHAASO) has been in operation since July 2021. For its kilometer-square array (KM2A), we optimized the selection criteria for very high and ultra-high energy γ-rays using data collected from August 2021 to August 2022, resulting in an improvement in significance of the detection in the Crab Nebula of approximately 15%, compared with that of previous cuts. With the implementation of these new selection criteria, the angular resolution was also significantly improved by approximately 10% at tens of TeV. Other aspects of the full KM2A array performance, such as the pointing error, were also calibrated using the Crab Nebula. The resulting energy spectrum of the Crab Nebula in the energy range of 10-1000 TeV are well fitted by a log-parabola model, which is consistent with the previous results from LHAASO and other experiments.

Broadband Multiwavelength Study of LHAASO-detected Active Galactic Nuclei

Recently, the Large High Altitude Air Shower Observatory (LHAASO) collaboration presented the first catalog of γ-ray sources using 508 days of LHAASO data, from 2021 March to 2022 September. This catalog contains four blazars and a possible LINER-type active galactic nucleus (AGN) counterpart. In this work, we establish averaged multiwavelength spectral energy distributions (SEDs) by combining data from the Fermi-Large Area Telescope, Swift, Zwicky Transient Facility, and Wide-field Infrared Survey Explorer (WISE) covering the same period as the LHAASO detection. In general, these five AGNs are found in low states at all wavelengths. To study the multiwavelength properties of these AGNs, several jet emission models, including the one-zone leptonic model, the one-zone leptonic and hadronuclear (pp) model, the one-zone proton-synchrotron model, and the spine-layer model, are applied to reproduce their averaged SEDs. We find that the one-zone leptonic model can reproduce most of the SEDs, except for the high-energy tail of the LHAASO spectra of Mrk 421 and Mrk 501. To improve the fitting, emission from pp interactions is favored in the framework of a one-zone model. The spine-layer model, which can be treated as a multizone scenario, can also provide good spectral fits. The influence of different extragalactic background light models on fitting a LHAASO energy spectrum is also discussed.

Lorentz invariance violation from GRB 221009A

The Large High Altitude Air Shower Observatory (LHAASO) reported observation of photons with energies above 10[Formula: see text]TeV from gamma-ray burst GRB 221009A. A suggestion was proposed that this result may contradict our knowledge of special relativity (SR) and the standard model (SM), according to which photons of about 10[Formula: see text]TeV from such a distant object should be severely suppressed because of the absorption by extragalactic background light. As a result, a number of mechanisms have been proposed to solve this potential puzzle, including Lorentz invariance violation (LIV). In this work, we perform a detailed numerical calculation and show the feasibility to constrain LIV of photons from the LHAASO observation of GRB 221009A quantitatively.

Determining the Origin of Very-high-energy Gamma Rays from Galactic Sources by Future Neutrino Observations

Recently, the Large High Altitude Air Shower Observatory (LHAASO) identified 12 gamma-ray sources emitting gamma rays with energies above 100 TeV, making them potential PeV cosmic-ray accelerators (PeVatrons). Neutrino observations are crucial in determining whether the gamma-ray radiation process is of hadronic or leptonic origin. In this paper, we study three detected sources, LHAASO J1908+0621, LHAASO J2018+3651, and LHAASO J2032+4102, which are also the most promising Galactic high-energy neutrino candidate sources with the lowest pretrial p-value based on the stacking searches testing for excess neutrino emission by the IceCube Neutrino Observatory. We study the lepto-hadronic scenario for the observed multiband spectra of these LHAASO sources considering the possible counterpart source of the LHAASO sources. The very-high-energy gamma rays are entirely attributed to the hadronic contribution; therefore, the most optimistic neutrino flux can be derived. Then, we evaluate the statistical significance (p-value) as a function of the observation time of IceCube and the next-generation IceCube-Gen2 neutrino observatory, respectively. Our results tend to disfavor that all gamma rays above 100 GeV from LHAASO J1908+0621 are of purely hadronic origin based on current IceCube observations, but the purely hadronic origin of gamma rays above 100 TeV is still possible. By IceCube-Gen2, the origin of gamma rays above 100 TeV from LHAASO J1908+0621 can be further determined at a 5σ significance level within a running time of ∼3 yr. For LHAASO J2018+3651 and LHAASO J2032+4102, the required running time of IceCube-Gen2 is ∼10 yr (3σ) and ∼10 yr (5σ), respectively. Future observations by the next-generation neutrino telescope will be crucial to understanding the particle acceleration and radiation processes inside the sources.

Effects of Thunderstorms Electric Field on Cosmic Ray Secondary Photons at LHAASO

The Large High Altitude Air Shower Observatory (LHAASO) is a complex of extensive air shower (EAS) detector arrays, located at Mt. Haizi (29°21' N, 100°08' E) at an altitude of 4410 m a. s. l., Daocheng, Sichuan Province, China. Information of primary cosmic rays can be obtained by using data from secondary particles measured at LHAASO, with photons constituting the majority among these secondary particles. During thunderstorms, the atmospheric electric field can change the information of photons on the ground by affecting secondary charged particles (mainly positrons and electrons). In this paper, Monte Carlo simulations have been performed to investigate the effects of near-ground thunderstorm electric fields on cosmic ray secondary photons at LHAASO. A simple model with a vertical and uniform atmospheric electric field in a layer of atmosphere is used in our simulations. During thunderstorms, the number and energy of photons are found to significantly change and strongly depend on the electric field strength. In a field of -1000 V/cm (below the threshold of the RREA process), the number of photons occurs an increase of 23%. Also, the spectrum of photons softens, and the increased number with energy less than 2 MeV exceeds 29%. In an electric field of -1700 V/cm (above the threshold of the RREA process), the number of photons experiences exponential growth, with an increase of 279%. The spectrum of photons becomes softer compared with that at -1000 V/cm, and the increased number with energy less than 2 MeV is more than 361%. It is consistent with the theory of RREA. For these phenomena of photons at LHAASO, the main factor is that positrons/electrons increase due to the acceleration of negative electric field on electrons, with increase of 65% in -1000 V/cm and 992% in -1700 V/cm, and the spectrum of positrons/electrons becomes soften. Newborn free positrons/electrons may undergo bremsstrahlung and deposit part of their energy into photons, causing the change of number and energy of photons to follow roughly the same pattern as positrons/electrons. The simulation results can provide information for understanding the variations of the data detected by LHAASO during thunderstorms and the acceleration mechanisms of secondary charged particles caused by an atmospheric electric field.

Porting LHAASO WFCTA simulation job to ARM computing cluster

With the advancement of many large-scale high-energy physics experiments, the amount of data to be processed and analyzed has significantly increased. For example, since the start of the Large High Altitude Air Shower Observatory (LHAASO) experiment in 2020, their simulation jobs have been running on an Intel X86 cluster, producing only a fraction of the planned data for the first phase due to limited CPU resources. Therefore, it is necessary to explore and expand other computing service devices. We built an application ecosystem based on the ARM architecture to support offline data processing for high-energy physics. The main work includes porting the offline software based on LHAASO experiments to run on ARM machines, formulating data transfer and job scheduling strategies in the ARM cluster, and evaluating performance and power consumption in both Intel X86 and ARM clusters. The results show that the LHAASO simulation jobs can run correctly on the ARM computing cluster. The singlecore performance of Intel X86 CPUs is better than ARM CPUs, but for the entire server with a multicore architecture, ARM servers perform better.

XkitS：A computational storage framework for high energy physics based on EOS storage system

Large-scale high-energy physics experiments generate scientific data at the scale of petabytes or even exabytes, requiring high-performance data IO for processing. However, in large computing centers, computing and storage devices are typically separated. Large-scale data transfer has become a bottleneck for some data-intensive computing tasks, such as data encoding and decoding, compression, sorting, etc. The time spent on data transfer can account for 50% of the entire computing task. The larger the amount of data accessed, the more significant this cost becomes. One attractive solution to address this problem is to offload a portion of data processing to the storage layer. However, modifying traditional storage systems to support computation offloading is often cumbersome and requires a broad understanding of their internal principles. Therefore, we have designed a flexible software framework called XkitS, which builds a computable storage system by extending the existing storage system EOS. This framework is deployed on the EOS FTS storage server and offloads computational tasks by invoking the computing capabilities (CPU, FPGA, etc.) on FTS. Currently, it has been tested and applied in the data processing of the Large High Altitude Air Shower Observatory (LHAASO), and the results show that the time spent on data decoding using the computable storage technology is half of that using the original method.

Analysis of Moon shadow from 2021-2022 LHAASO WCDA Data

The Large High Altitude Air Shower Observatory (LHAASO) is a major ground-based array of cosmic-ray (CR) detectors in the Sichuan Province in China. The Water Cherenkov Detector Array (WCDA) is an essential component of LHAASO, which is designed to measure ~1-50 TeV CRs and gamma rays. As observed on Earth, the Moon blocks some CRs to create a shadow. Although CRs are mostly protons, WCDA can probabilistically identify some constituents (mostly electrons and photons) of CRs which produce electromagnetic (EM) showers in the atmosphere, using the “compactness” (C) of the showers as the criterion for which the higher C indicates the higher probability of an event being EM. The observed CR Moon shadow positions must be shifted from that of the actual Moon due to the deflection of charged particles by the Earth’s magnetic field. We analyze the LHAASO WCDA data from March 2021 to December 2022 to study the properties of the CR Moon shadows for events with C < 10 and C > 10, such as the shifted position and the size of the shadow at different energies. We found that i) CR shadows’ shifts decrease when energy increases, and ii) the higher the energy, the size of the CR shadows are more compact. This analysis is useful for detector calibration, event selection study, and understanding CR trajectories in the geomagnetic field.

Could the TeV emission of starburst galaxies originate from pulsar wind nebulae?

ABSTRACT While the GeV γ-ray emission of starburst galaxies (SBGs) is commonly thought to arise from hadronic interactions between accelerated cosmic rays and interstellar gas, the origin of the TeV γ-ray emission is more uncertain. One possibility is that a population of pulsar wind nebulae (PWNe) in these galaxies could be responsible for the TeV γ-ray emission. In this work, we first synthesize a PWNe population in the Milky Way and assess their contribution to the γ-ray emission of the Galaxy, using a time-dependent model to calculate the evolution of the PWN population. Such a synthetic PWN population can reproduce the flux distribution of PWNe identified in the Milky Way given a distribution of the initial state of the pulsar population. We then apply it to starburst galaxies and quantitatively calculate the spectral energy distribution of all PWNe in the SBGs NGC 253 and M82. We propose that TeV γ-ray emission in starburst galaxies can be dominated by PWNe for a wide range of parameter space. The energetic argument requires that ηe × vSN &amp;gt; 0.01 yr−1, where ηe is the fraction of spin-down energy going to electrons and vSN is the supernova rate. By requiring the synchrotron emission flux of all PWNe in the galaxy not to exceed the hard X-ray measurement of NGC 253, we constrain the initial magnetic field strength of PWNe to be $\lesssim 400\,\mu{\rm G}$. Future observations at higher energies with the Large High Altitude Air Shower Observatory (LHAASO) or the next-generation neutrino observatory IceCube-Gen2 will help us to understand the origin of the TeV γ-ray emission in SBGs better.

A new large area MCP-PMT for high energy detection

20-inch Large area photomultiplier tube based on microchannel plate (MCP-PMT) is newly developed in China. It is widely used in high energy detection experiments such as Jiangmen Underground Neutrino Observatory (JUNO), China JinPing underground Laboratory (CJPL) and Large High Altitude Air Shower Observatory (LHAASO). To overcome the poor time performance of the existing MCP-PMT, a new design of large area MCP-PMT is proposed in this paper. Three-dimensional models are developed in CST Studio Suite to validate its feasibility. Effects of the size and bias voltage of the focusing electrodes and MCP configuration on the collection efficiency (CE) and time performance are studied in detail using the finite integral technique and Monte Carlo method. Based on the simulation results, the optimized operating and geometry parameters are chosen. Results show that the mean ratio of photoelectrons landing on the MCP active area is 97.5%. The acceptance fraction of the impinging photoelectrons is close to 100% due to the emission of multiple secondary electrons when hitting the MCP top surface. The mean transit time spread (TTS) of the photoelectrons from the photocathode is 1.48 ns.

Two Candidate Pulsar TeV Halos Identified from Property-similarity Studies

Teraelectronvolt halos have been suggested to be a common phenomenon associated with middle-aged pulsars. Based on our recent work on the middle-aged pulsar J0631+1036, which is the only known source positionally coincident with a hard teraelectronvolt γ-ray source and likely powers the latter as a teraelectronvolt halo, we select three candidate teraelectronvolt halos from the first Large High Altitude Air Shower Observatory (LHAASO) catalog of γ-ray sources. The corresponding pulsars, given by the positional coincidences and property similarities, are PSR J1958+2846, PSR J2028+3332, and PSR J1849-0001. We analyze the gigaelectronvolt γ-ray data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope for the first two pulsars, as the last is γ-ray quiet. We remove the pulsed emissions of the pulsars from the source regions from timing analysis, and determine that there are no residual gigaelectronvolt emissions in the regions as any possible counterparts to the teraelectronvolt sources. Considering the previous observational results for the source regions and comparing the two pulsars to Geminga (and Monogem), the LHAASO-detected teraelectronvolt sources are likely the pulsars’ respective teraelectronvolt halos. We find that the candidate and identified teraelectronvolt halos, including that of PSR J1849-0001, have luminosities at 50 TeV (estimated from the differential fluxes) approximately proportional to the spin-down energy of the pulsars, and the ratios of the former to the latter are ∼6 × 10−4.

Galactic Diffuse γ-Ray Emission from GeV to PeV Energies in Light of Up-to-date Cosmic-Ray Measurements

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.

Measurement of Ultra-High-Energy Diffuse Gamma-Ray Emission of the Galactic Plane from 10TeV to 1PeV with LHAASO-KM2A.

The diffuse Galactic γ-ray emission, mainly produced via interactions between cosmic rays and the interstellar medium and/or radiation field, is a very important probe of the distribution, propagation, and interaction of cosmic rays in the MilkyWay. In this Letter, we report the measurements of diffuse γ rays from the Galactic plane between 10TeV and 1PeV energies, with the square kilometer array of the Large High Altitude Air Shower Observatory (LHAASO). Diffuse emissions from the inner (15°<l<125°, |b|<5°) and outer (125°<l<235°, |b|<5°) Galactic plane are detected with 29.1σ and 12.7σ significance, respectively. The outer Galactic plane diffuse emission is detected for the first time in the very- to ultra-high-energy domain (E>10 TeV). The energy spectrum in the inner Galaxy regions can be described by a power-law function with an index of -2.99±0.04, which is different from the curved spectrum as expected from hadronic interactions between locally measured cosmic rays and the line-of-sight integrated gas content. Furthermore, the measured flux is higher by a factor of ∼3 than the prediction. A similar spectrum with an index of -2.99±0.07 is found in the outer Galaxy region, and the absolute flux for 10≲E≲60 TeV is again higher than the prediction for hadronic cosmic ray interactions. The latitude distributions of the diffuse emission are consistent with the gas distribution, while the longitude distributions show clear deviation from the gas distribution. The LHAASO measurements imply that either additional emission sources exist or cosmic ray intensities have spatial variations.

Properties and performance of nitrogen laser systems for calibration of LHAASO-WFCTA

The Wide Field of view Cherenkov Telescope Array (WFCTA) of the Large High Altitude Air Shower Observatory (LHAASO), located in Haizishan National Natural Reserve, Sichuan Province, China, is designed to perform nearly calorimetric measurements of extensive air showers induced by cosmic rays with energies between 1013eV–1018eV. The end-to-end calibration method will be employed in the WFCTA and nitrogen lasers have been proven to work properly in our application at the altitude of 4410 m a.s.l. for the first time.Due to extremely harsh environment, especially outdoor temperature as low as about −30 °C during operation of WFCTA, a high-stability thermotank for lasers was designed to stabilize temperature and humidity, with a thermal stability of 1 °C. In order to precisely control the beam pointing, a high-precision 3D Lifting Rotating Platform (HiRoP) with a home-made standard zero monitoring system is developed. To eliminate the error of the beam pointing by manual controls, an automatic scanning program was adopted to control the beam pointing when scanning the full field-of-view (FOV) of WFCTA. Both the on-site and remote controls are applied for the nitrogen laser systems and ∼13 million laser shots have been recorded by WFCTA from Sept. 2020 to May 2022. In this paper, the design, properties, operation and the on-site performance of the nitrogen laser systems at LHAASO are reported.

Pointing calibration of LHAASO-WFCTA telescopes using bright stars

One of the main scientific objectives of the Large High Altitude Air Shower Observatory (LHAASO) is to perform accurate measurements of the energy spectra for different cosmic ray masses, from a few TeV to 100 PeV. As one of the main sub-arrays of LHAASO, the Wide Field-of-View Cherenkov Telescope Array (WFCTA), which consists of 18 telescopes, can help in achieving this goal. The pointing accuracy of each telescope is crucial for reconstructing energy and determining mass-sensitive parameters. In this study, a method for absolute pointing calibration using ultraviolet bright stars was established. The proposed method can achieve a pointing accuracy of approximately 0.02° by using more than five stars. When more stars are used in the telescope’s field of view, the accuracy of the pointing calibration can be improved, e.g, a pointing accuracy of 0.01° can be achieved when using more than 15 stars. A high-precision inclinometer with a monitor resolution of 0.003° was installed on the camera to monitor the zenith direction of the telescope at every second. After calibration using bright stars, the absolute pointing accuracy of the inclinometer was 0.02°.

γ-Ray and ultra-high energy neutrino background suppression due to solar radiation

The Sun emits copious amounts of photons and neutrinos in an approximately spatially isotropic distribution. Diffuse γ-rays and ultra-high energy (UHE) neutrinos from extragalactic sources may subsequently interact and annihilate with the emitted solar photons and neutrinos respectively. This will in turn induce an anisotropy in the cosmic ray (CR) background due to attenuation of the γ-ray and UHE neutrino flux by the solar radiation. Measuring this reduction, therefore, presents a simple and powerful astrophysical probe of electroweak interactions. In this letter we compute such anisotropies, which at the Earth (Sun) can be at least ≃5×10−3(1)% and ≃1×10−16(2×10−14)% for TeV scale γ-rays and PeV scale UHE neutrinos respectively. We briefly discuss observational prospects for experiments such as the Fermi Gamma-Ray Space Telescope Large Area Telescope (Fermi LAT), High-Altitude Water Cherenkov (HAWC) detector, The Large High Altitude Air Shower Observatory (LHAASO), Cherenkov Telescope Array (CTA) and IceCube. The potential for measuring γ-ray attenuation at orbital locations of other active satellites such as the Parker Solar Probe and James Webb Space Telescope (JWST) is also explored.

Prediction of using LHAASO’s cosmic-ray electron measurements to constrain decaying heavy dark matter

Large High Altitude Air Shower Observatory (LHAASO) is an instrument designed for detecting cosmic rays (CRs) and gamma rays at TeV to PeV energies. The decays of heavy dark matter particles in the Galactic halo may produce high-energy electrons that can be detected by LHAASO. The main background for the LHAASO's CR electron measurements is the hadron residuals due to misidentification of the particle species. In this paper, we estimate the LHAASO's electron background using the known all-particle CR spectrum and the hadron rejection efficiency of LHAASO. With the estimated background, we predict the capability of LHAASO to constrain dark matter (DM) decay lifetime at 95% confidence level for various channels. We find that if neglecting systematic uncertainties, the CR electron measurement by LHAASO can improve the current best results by up to one order of magnitude for DM masses between 100--1000 TeV. However, indirect measurements of CR electrons by ground-based experiments suffer from sizable systematic uncertainties. With the systematic uncertainties included in the calculation, the projected constraints will be largely weakened. So, for using the CR electron observation of LHAASO to constrain the DM parameters, the key point is whether the systematic error can be effectively reduced.

A tera-electron volt afterglow from a narrow jet in an extremely bright gamma-ray burst.

Some gamma-ray bursts (GRBs) have a tera-electron volt (TeV) afterglow, but the early onset of this has not been observed. We report observations with the Large High Altitude Air Shower Observatory (LHAASO) of the bright GRB 221009A, which serendipitously occurred within the instrument's field of view. More than 64,000 photons >0.2 TeV were detected within the first 3000 seconds. The TeV flux began several minutes after the GRB trigger and then rose to a peak ~10 seconds later. This was followed by a decay phase, which became more rapid ~650 seconds after the peak. We interpret the emission using a model of a relativistic jet with half-opening angle of ~0.8°. This is consistent with the core of a structured jet and could explain the high isotropic energy of this GRB.