Ultra High Energy Γ-ray Research Articles

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.

An ultrahigh-energy [formula omitted]-ray bubble powered by a super PeVatron

We report the detection of a γ-ray bubble spanning at least 100deg2 in ultra-high energy (UHE) up to a few PeV in the direction of the star-forming region Cygnus X, implying the presence super PeVatron(s) accelerating protons to at least 10 PeV. A log-parabola form with the photon index Γ(E)=(2.71±0.02)+(0.11±0.02)×log10(E/10TeV) is found fitting the gamma-ray energy spectrum of the bubble well. UHE sources, “hot spots” correlated with very massive molecular clouds, and a quasi-spherical amorphous γ-ray emitter with a sharp central brightening are observed in the bubble. In the core of ∼0.5°, spatially associating with a region containing massive OB association (Cygnus OB2) and a microquasar (Cygnus X-3), as well as previously reported multi-TeV sources, an enhanced concentration of UHE γ-rays is observed with 2 photons at energies above 1 PeV. The general feature of the bubble, the morphology, and the energy spectrum, are reasonably reproduced by the assumption of a particle accelerator in the core, continuously injecting protons into the ambient medium.

Ultrahigh-energy γ-Rays from Past Explosions in Our Galaxy

Abstract The discovery of the sources of ultrahigh-energy photons in our Galaxy by the High-Altitude Water Cherenkov Gamma-ray Observatory and the Large High Altitude Air Shower Observatory (LHAASO) has revolutionized the field of gamma-ray astronomy in the last few years. These emissions are sometimes found in the vicinity of powerful pulsars or supernova remnants (SNRs) associated with giant molecular clouds (GMCs). Inverse Compton emission by shock-accelerated electrons emitted by pulsars and proton–proton interactions of shock-accelerated protons emitted by SNRs with cold protons in molecular clouds are often identified as the causes of these emissions. In this paper we have selected two ultrahigh-energy photon sources, LHAASO J2108+5157 and LHAASO J0341+5258, which are associated with GMCs, but no powerful pulsar or SNR has been detected in their vicinity. We have proposed a scenario where shock-accelerated electrons and protons are injected in the local environment of these sources from past explosions, which happened thousands of years ago. We show that the observed ultrahigh-energy photon flux can be explained with the secondary gamma rays produced by the time-evolved relativistic electron and proton spectra.

EAS Arrays at High Altitudes Start the Era of UHE γ-ray Astronomy

The evolution of extensive air shower detection as a technique for γ-ray astronomical instrumentation for the last three decades is reviewed. The first discoveries of galactic PeVatrons by the Large High Altitude Air Shower Observatory demonstrate the importance of this technique in ultra-high energy γ-ray astronomy. Utilizing this technique, the origins of high energy cosmic rays may be discovered in the near future.

Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources.

The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 1015 electronvolts) indicates the existence of the so-called PeVatrons-cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays1. The principal signature of both electron and proton PeVatrons is ultrahigh-energy (exceeding 100TeV) γ radiation. Evidence of the presence of a proton PeVatron has been found in the Galactic Centre, according to the detection of a hard-spectrum radiation extending to 0.04PeV (ref. 2). Although γ-rays with energies slightly higher than 0.1PeV have been reported from a few objects in the Galactic plane3-6, unbiased identification and in-depth exploration of PeVatrons requires detection of γ-rays with energies well above 0.1PeV. Here we report the detection of more than 530 photons at energies above 100 teraelectronvolts and up to 1.4PeV from 12 ultrahigh-energy γ-ray sources with a statistical significance greater than seven standard deviations. Despite having several potential counterparts in their proximity, including pulsar wind nebulae, supernova remnants and star-forming regions, the PeVatrons responsible for the ultrahigh-energy γ-rays have not yet been firmly localized and identified (except for the Crab Nebula), leaving open the origin of these extreme accelerators.

Synchrotron versus Compton Interpretations for Extended X‐Ray Jets

A widely discussed explanation for the origin of the X-ray emission observed from knots in extended quasar jets with the Chandra X-Ray Observatory is Compton-scattered cosmic microwave background radiation by electrons with Lorentz factors γ' ~ 102. This model faces difficulties in terms of total energy requirements and in explaining the spatial profiles of the radio, optical, and X-ray knots in sources such as PKS 0637-752, 3C 273, or PKS 1127-145. These difficulties can be resolved in the framework of one- and two-component synchrotron models. We propose a model in which the broadband radio-to-X-ray synchrotron emission in quasar jets is powered by collimated beams of ultrahigh energy neutrons and γ-rays formed in the subparsec-scale jets. The decay of the neutral beam in the intergalactic medium drives relativistic shocks to accelerate nonthermal electrons of the ambient medium. A second synchrotron component arises from the injection of leptons with Lorentz factors ≫107 that appear in the extended jet in the process of decay of ultrahigh energy γ-rays. This approach could account for qualitative differences in the extended X-ray jets of Fanaroff-Riley (FR) 1 and 2 galaxies. Detection of high-energy neutrinos from blazars and core-dominated quasars will provide strong evidence for this model.

Image and non-image parameters of atmospheric Cherenkov events: a comparative study of their γ-ray/hadron classification potential in ultrahigh energy regime

In this exploratory simulation study, we compare the event-progenitor classification potential of a variety of measurable parameters of atmospheric Cherenkov pulses which are produced by ultrahigh energy γ-ray and hadron progenitors and are likely to be recorded by the TACTIC (TeV atmospheric Cherenkov telescope with imaging camera) array of atmospheric Cherenkov telescopes. The parameters derived from Cherenkov images include Hillas, fractal and wavelet moments, while those obtained from non-image Cherenkov data consist of pulse profile rise time and base width and the relative ultraviolet to visible light content of the Cherenkov event. It is shown by a neural-net approach that these parameters, when used in suitable combinations, can bring about a proper segregation of the two event types, even with modest sized data samples of progenitor particles.

First results on characterization of Cherenkov images through combined use of Hillas, fractal and wavelet parameters

Based on Monte Carlo simulations using the CORSIKA code, it is shown that Cherenkov images produced by ultrahigh energy γ-rays and cosmic ray nuclei (proton, Neon and Iron) are fractal in nature. The resulting multifractal and wavelet moments when employed in association with the conventional Hillas parameters as inputs to a properly-trained artificial neural network are found to provide a more efficient primary characterization scheme than the one based on the use of Hillas or fractal parameters alone.

High statistics search for ultrahigh energy γ-ray emission from Cygnus X-3 and Hercules X-1

We have carried out a high statistics $(2\ifmmode\times\else\texttimes\fi{}{10}^{9}$ events) search for ultrahigh energy \ensuremath{\gamma}-ray emission from the x-ray binary sources Cygnus X-3 and Hercules X-1. Using data taken with the CASA-MIA detector over a five year period (1990--1995), we find no evidence for steady emission from either source. The derived 90 % C.L. upper limit to the steady integral flux of $\ensuremath{\gamma}$ rays from Cygnus X-3 is $\ensuremath{\Phi}(E>115$TeV)$<6.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ photons ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ sec${}^{\ensuremath{-}1}$, and from Hercules X-1 it is $\ensuremath{\Phi}(E>115\mathrm{TeV})<8.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ photons cm${}^{\ensuremath{-}2}$ sec${}^{\ensuremath{-}1}$. These limits are more than two orders of magnitude lower than earlier claimed detections and are better than recent experiments operating in the same energy range. We have also searched for transient emission on time periods of one day and $0.5$ h and find no evidence for such emission from either source. The typical daily limit on the integral $\ensuremath{\gamma}$-ray flux from Cygnus X-3 or Hercules X-1 is ${\ensuremath{\Phi}}_{\mathrm{daily}}(E>115\mathrm{TeV})<2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ photons cm${}^{\ensuremath{-}2}$ sec${}^{\ensuremath{-}1}$. For Cygnus X-3, we see no evidence for emission correlated with the $4.8$ h x-ray periodicity or with the occurrence of large radio flares. Unless one postulates that these sources were very active earlier and are now dormant, the limits presented here put into question the earlier results, and highlight the difficulties that possible future experiments will have in detecting $\ensuremath{\gamma}$-ray signals at ultrahigh energies.

A limit to the rate of ultra high energy γ-rays in the primary cosmic radiation

Abstract An upper limit to the flux of Ultra High Energy (UHE) γ-rays in the primary cosmic radiation is obtained through the data of the electromagnetic and the muon detectors of the EAS-TOP Extensive Air Shower array (Campo Imperatore, National Gran Sasso Laboratories, atmospheric depth 810g cm−2). The search is performed by selecting Extensive Air Showers (EAS) with low muon content. For EAS electron sizes Ne > 6.3 · 105, no showers are observed with the core located inside a fiducial area and no muons recorded in the 140 m2 muon detector, during a live time of 8440 h. The 90% c.l. upper limit to the relative intensity of γ-ray with respect to cosmic ray (c.r.) primaries is I γ I c.r. −5 , at primary energy E0 ≥ 1015 eV: this limit is lower than reported in previous measurements.

Constraints on massive unstable dark matter from ultrahigh-energy γ-ray flux

We ruled out regions in parameter space of the mass, lifetime and branching ratio for radiative decay of massive unstable dark matter particles based on upper limits to the ultrahigh-energy cosmic diffuse γ-ray spectrum. The lifetime of massive unstable particles is shown to be no shorter than ∼ 1017 s if they exist at a density equal to the cosmological critical density. The branching ratio for radiative decay of such massive unstable particles is less than 10-11 at mass of 1 GeV and 10-2 at 105 GeV.

Ultra-high-energy cosmic rays in the Magellanic Clouds

Ultra-high-energy (UHE) γ-rays have been detected from several X-ray binaries. UHE γ-rays from sources in the LMC will interact in the microwave background producing electrons which emit synchrotron radiation in the X-ray and γ-ray bands. This radiation might be observable.

A study of the intergalactic magnetic field using extragalactic ultra-high-energy gamma-ray sources

The possibility of measuring the intergalactic magnetic field between an extragalactic ultra-high-energy (UHE) γ-ray source and our Galaxy is studied. The absorption of UHE γ-rays in the 3 K microwave background changes its character at a magnetic field strength of 10^-12^ G. In stronger magnetic fields, UHE γ-rays with E<SUB>gamma</SUB>_ ~ 10^15^ eV are effectively absorbed. But for weaker field strengths, UHE γ-rays are converted to lower energy γ-rays by the cascading of UHE γ-rays and electrons in the 3 K microwave background. These effects make the UHE γ-ray energy spectrum at energies E<SUB>gamma</SUB>_ &gt;= 10^14^ eV very different for B &gt; 10^-12^ and B &lt;= 10^-12^ G fields. From observations of an extragalactic UHE γ-ray source, the upper or lower bound of the magnetic field strength can be obtained.

Time Variation of Ultra High Energy -Ray from SN1987A

If the neutron star exists in the remnant of SN1987A, there is a possibility to check the acceleration activity of the pulsar in the very early phase. As indicated by the observation of Cyg X-3, the pulsar may generate a proton beam with energy up to 1017 eV, which interacts with the ejected matter and produces ultra high energy (UHE) γ-rays as well as UHE neutrinos with energy up to 1016 eV. Luminosity of UHE γ-rays will take a maximum at 5.5 months (3.8 months) from February 23rd, if the mass ejected by the supernova explosion with energy ∼1.4 ×1017 ergs/g is 7 M⊙ (15 M⊙ and if the UHE protons are not confined in the ejecta by the turbulent magnetic fields. If they are completely trapped, the maximum epoch will be prolonged by the factor about 3. The proton beam luminosity will be limited to 7 ×1041 ergs/s at most if muons generated by the UHE neutrons are not detected in the underground detectors.

Photon–photon pair production and the opacity of SN1987A to TeV and PeV γ-rays

Supernovae have long been considered as likely sites of cosmic-ray acceleration. Interaction of newly accelerated cosmic-ray nuclei with target material within the expanding supernova is expected to produce an observable flux from SN1987A of very high-energy and possibly ultra high-energy γ-rays in the TeV and PeV (1015 eV) ranges, and several experiments are being constructed to detect this radiation. The presence of intense infrared emission from the supernova itself will, however, make some regions of SN1987A opaque to TeV and PeV γ-rays due to pair-production interactions. Observations at these energies, combined with a knowledge of which regions of SN1987A could be contributing γ-rays may thus give information about the nature and location of particle accelerators in supernovae. Here I discuss the important question of photon–photon pair-production interactions and calculate from which regions of SN1987A may be observed TeV and PeV γ-rays.

Limits on hadronic cosmic ray production by young pulsars

Since the discovery of supernova 1987A (Shelton) several authors1,2 have noted that it may provide an excellent opportunity to observe the cosmic ray output of a young pulsar through the 'beam dump' that the supernova ejecta provide. It has been suggested3 that neutrino emission from p–p collisions is possible immediately, and that ultra-high energy γ-ray emission might be possible after several months, when the supernova remnant becomes transparent to them. In this letter we argue that the cosmic abundances of Li, Be and B set significant constraints on the cosmic ray proton production in the young (t≤1 yr) remnant, and, in particular, rule out neutrinos from shock-accelerated protons in the ejecta at currently detectable levels.

Cygnus X-3 and Other Ultra-High-Energy γ-Ray Sources

Recently, several binary X-ray sources have been found to be sources of ultra high energy γ-ray emission. Air shower observations indicate photon energies &gt;∼ 1015 eV. We review the current status of observations from the source Cygnus X-3, and compare this data with that from the sources Hercules X-1, Vela X-1, and LMC X-4. Current theoretical models for the production of γ-rays and the acceleration of high energy particles are discussed and the consequences for the evolution of such systems are examined.

Gamma-Ray Astronomy at the Highest Energies

AbstractIn 1983 ultra high energy γ-rays were detected from Cygnus X-3. This was of particular interest since it is the first identifiable source of cosmic rays. Since then, there has been an explosion of interest in the field with more than twenty air shower arrays planned or being used to search for γ-rays at energies above 1014 eV. Observational techniques for both very high energy and ultra high energy γ-ray astronomy are described and existing and proposed experiments reviewed with particular reference to their geographical locations. The results obtained so far are summarised together with some of the models proposed to explain them.

First observation of ultra-high-energy γ rays from LMC X-4

Ultra-high-energy γ rays (>1015 eV) have recently been observed from Cygnus X-3 (refs 1, 2) and from Vela X-1 (ref. 3). The discovery that some neutron star binary X-ray sources are emitting ultra-high-energy (UHE) γ rays has led to considerable interest and speculation as to the nature of the cosmic particle accelerators responsible for the γ rays4–7 and to the notion that much of the galactic cosmic radiation above 1016 eV may originate from such objects7–9. Data from the University of Adelaide air-shower array at Buckland Park taken over a 3-year period have been analysed to search for evidence of UHE γ-ray emission from additional neutron-star binary X-ray sources having known orbital periods. We present here the result of this search and report the detection of UHE γ rays from LMC X-4 above 1016 eV with an integral flux of (4.6±1.7)×10−11 photons m−2 s−1. This is the first extragalactic object to be positively detected at these energies.

Evidence for an increasing 59-s period in the X-ray emission from Geminga

Einstein Observatory and Exosat X-ray data from 1E0630 + 178 reveal a ∼50% periodic emission at ≥59 s. This confirms the earlier SAS 2 and COS B findings on 2CG195 + 4 at E> 50 MeV, and provides the temporal signature for the identification of Geminga. The observations, spanning over a decade (1973–83), also suggest a very high positive period derivative, P, which has become still higher between 1979 and 1983. A simple source scenario is proposed, based on a binary system containing a compact object responsible for the high energy emission, and also taking into account recent ultra high energy γ-ray measurements.