High-energy Gamma-ray Sources Research Articles

The influence of the Sun and Moon on the observation of very high energy gamma-ray sources using EAS arrays

Abstract With great advance of ground-based extensive air shower array, such as LHAASO and HAWC, many very high energy (VHE) gamma-ray sources have been discovered and are been monitored regardless of the day and the night. Hence, the Sun and Moon would have some compact on the observation of gamma-ray sources, which have not been taken into account in previous analysis. In this paper, the influence of the Sun and Moon on the observation of very high energy gamma-ray sources when they are near the line of sight of the Sun or Moon is estimated. The tracks of all the known VHE sources are scanned and several VHE sources are found to be very close to the line of sight of the Sun or Moon during some period. The absorption of very high energy gamma-ray by sunlight is estimated with detailed method and some usefully conclusions are achieved. The main influence is the block of the Sun and Moon on gamma-ray and their shadow on the cosmic ray background. The influence is investigated considering the detector angular resolution and some strategy on data analysis are proposed to avoid the underestimation of the gamma-ray emission.

High energy gamma-ray sources in the VVV survey - II. The AGN counterparts

ABSTRACT We identified Active Galactic Nuclei (AGN) candidates as counterparts to unidentified gamma-ray sources (UGS) from the Fermi-LAT Fourth Source Catalogue at lower Galactic latitudes. Our methodology is based on the use of near- and mid-infrared photometric data from the VISTA Variables in the Vía Láctea (VVV) and Wide-field Infrared Survey Explorer (WISE) surveys. The AGN candidates associated with the UGS occupy very different regions from the stars and extragalactic sources in the colour space defined by the VVV and WISE infrared colours. We found 27 near-infrared AGN candidates possibly associated with 14 Fermi-LAT sources using the VVV survey. We also found 2 blazar candidates in the regions of 2 Fermi-LAT sources using WISE data. There is no match between VVV and WISE candidates. We have also examined the Ks light curves of the VVV candidates and applied the fractional variability amplitude (σrms) and the slope of variation in the Ks passband to characterise the near-infrared variability. This analysis shows that more than 85 per cent of the candidates have slopes in the Ks passband >10−4 mag/day and present σrms values consistent with a moderate variability. This is in good agreement with typical results seen from type-1 AGN. The combination of YJHKs colours and Ks variability criteria was useful for AGN selection, including its use in identifying counterparts to Fermi γ-ray sources.

Homogeneous fine classification of Markarian galaxies based on SDSS medium-resolution spectroscopy

Context. Markarian (Mrk) galaxies, revealed due to their ultraviolet excess are very famous; they play a significant role in many astro-physical problems and come in a variety of types: Seyferts (Sys), low-ionization narrow emission-line regions (LINERs), starbursts (SBs), blue compact dwarf galaxies (BCDGs), as well as some quasars (QSOs) and blazars. They also appear as sources of non-optical radiation, such as gamma-ray, X-ray, ultraviolet (UV), infrared (IR), and radio, including some extremely high-energy gamma-ray sources (Mrk 421, Mrk 501) and very high-luminosity IR galaxies (Mrk 231). Aims. The classifications of Mrk galaxies for activity types have been carried out based on old mostly poor-quality and inhomoge-neous spectra (relatively low-resolution photographic spectra and low S/N spectra). The Sloan Digital Sky Surveys (SDSS) provides ~90% of the existing medium-resolution spectra, and is a relevant homogeneous database for classification of galaxies. Our aim was to obtain homogeneous classifications of Mrk galaxies by means of the SDSS spectra. Methods. Based on our work on the classification of newly revealed objects from various recent catalogs, we developed a fine classification scheme for activity types based on the homogeneous database of the SDSS spectroscopy. This scheme was applied to Mrk galaxies having spectra in the SDSS (779 out of 1544, 50.45%). Results. As a result, 779 Mrk galaxies now have homogeneous optical spectral classification that can be used for further studies and statistics of their physical and spatial properties. We revealed 2 QSOs, 49 classical Seyferts (broad-line Seyferts 1, BLS1s), 17 narrow-line Seyferts 1 (NLS1s), 4 Sy2s, 12 LINERs, 11 active galactic nuclei (AGN) without a definite type, 31 Composite spectrum objects, and 533 HII/Starbursts. Due to low-quality spectra, some objects were left without definite activity types and were classified as emission or absorption galaxies.

Constraints on the Gamma-Ray Emission from Small Solar System Bodies with the Fermi Large Area Telescope Data

All known small solar system bodies have diameters between a few meters and a few thousands of kilometers. Based on the collisional evolution of solar system bodies, a larger number of asteroids with diameters down to ∼2 m is thought to exist. As all solar system bodies, small bodies can be passive sources of high-energy gamma-rays, produced by the interaction of energetic cosmic rays impinging on their surfaces. Since the majority of known asteroids are in orbits between Mars and Jupiter (in a region known as the Main Belt), we expect them to produce a diffuse emission close to the ecliptic plane. In this work, we have studied the gamma-ray emission coming from the ecliptic using the data collected by the Large Area Telescope (LAT) onboard the Fermi satellite. We have fit the results with simulations of the gamma-ray intensity at the source level (calculated with the software FLUKA) to constrain the small solar system bodies population. Finally, we have proposed a model describing the distribution of asteroid sizes and we have used the LAT data to constrain the gamma-ray emission expected from this model and, in turn, on the model itself.

Classification of the Fermi-LAT blazar candidates of uncertain type using extreme gradient boosting

ABSTRACT Machine learning based approaches are emerging as very powerful tools for many applications including source classification in astrophysics research due to the availability of huge high quality data from different surveys in observational astronomy. The Large Area Telescope on board Fermi satellite (Fermi-LAT) has discovered more than 6500 high energy gamma-ray sources in the sky from its survey over a decade. A significant fraction of sources observed by the Fermi-LAT either remains unassociated or has been identified as Blazar Candidates of Uncertain type (BCUs). We explore the potential of eXtreme Gradient Boosting – a supervised machine learning algorithm to identify the blazar subclasses among a sample of 112 BCUs of the 4FGL catalogue whose X-ray counterparts are available within 95 per cent uncertainty regions of the Fermi-LAT observations. We have used information from the multiwavelength observations in IR, optical, UV, X-ray, and γ-ray wavebands along with the redshift measurements reported in the literature for classification. Among the 112 uncertain type blazars, 62 are classified as BL Lacertae objects and six have been classified as flat spectrum radio quasars. This indicates a significant improvement with respect to the multiperceptron neural network based classification reported in the literature. Our study suggests that the gamma-ray spectral index, and IR colour indices are the most important features for identifying the blazar subclasses using the XGBoost classifier. We also explore the importance of redshift in the classification BCU candidates.

Design optimization of the closed-water activation loop at the JSI irradiation facility

A closed-water activation loop is being built at the research reactor TRIGA at the Jožef Stefan Institute, Slovenia, which will serve as a well-defined and stable high energy gamma-ray and neutron source. The main focus of this work is to analyse and optimise the main components of the water activation loop to achieve the highest activity at reasonably low pressure drop. Both neutronic and hydraulic aspects were considered. The results show that the desired configuration consists of a high effective water volume of the inner activation part, outer observation part and narrow transport pipes connecting both parts. The complex snail-shape design of the inner part systematically outperformed other simplified designs by more than a factor of two for the main water activated isotopes (16N, 17N and 19O). The activity values increase strongly with increasing flow rate, but reach saturation at about 0.5 l/s. On this basis, the snail configuration was selected as the main candidate for both the inner activation and outer observation part. To not exceed the pressure drop of 3 bar within the closed-water loop, the nominal water flow rate and the inner diameter of the transport pipes were set to 0.5 l/s and 1.2 cm, respectively.

Probing LHAASO galactic PeVatrons through gamma-ray and neutrino correspondence

ABSTRACT Recently, Large High Altitude Air Shower Observatory (LHAASO) has detected several Galactic point sources of ultra high energy (UHE; Eγ > 100 TeV) gamma-rays. These gamma-rays are possibly created in leptonic or hadronic interactions of cosmic rays (CRs) of PeV energies. In the hadronic channel (p − p interaction), the gamma-rays are accompanied by neutrinos. The detection of neutrinos is therefore crucial in understanding CR acceleration in such objects. To estimate the neutrino flux, we adopt the two LHAASO sources (J2226+6057, J1908+0621) found to be spatially associated with the supernova remnants (SNR G106.3+2.7, SNR G40.5–0.5). For these two sources, the detected TeV-PeV gamma-ray spectra are found to be unusually hard (with spectral index ∼1.8). We develop a model of gamma-ray and neutrino emission based on the above two prototypes. The neutrino fluxes from these two sources are found to be below the IceCube sensitivity, but are detectable in upcoming IceCube-Gen2 and KM3NeT experiments. We further estimate the neutrino fluxes from similar other 10 LHAASO PeVatron sources and most of them are found to be detectable in IceCube-Gen2 and KM3NeT. Finally, we explore our model parameters, in particular the spectral power-law index and estimate the future potential of the neutrino detectors to probe CR acceleration in such Galactic sources.

Search for Periodicities in High Energy AGNs with a Time Domain Approach

This paper investigates a new methodology to search for periods in light curves of high-energy gamma-ray sources such as active galactic nuclei (AGNs). High-energy light curves have significant stochastic components, making period detection somewhat challenging. In our model, periodic terms, drifts of the light curves, and random walk with a correlation between flux points due to colored noise are taken into account independently. The parameters of the model are obtained directly from a Markov Chain Monte Carlo minimization. The time periods found are compared to the output of the publicly available Agatha program. The search method is applied to high-energy periodic AGN candidates from the Fermi-LAT catalog. The significance of periodic models over pure noise models is discussed. Finally, the variability of the period and amplitude of oscillating terms is studied on the most significant candidates.

Measurement of the Gamma-Ray Energy Spectrum beyond 100 TeV from the HESS J1843–033 Region

HESS J1843–033 is a very high energy gamma-ray source whose origin remains unidentified. This work presents, for the first time, the energy spectrum of gamma rays beyond 100 TeV from the HESS J1843–033 region using the data recorded by the Tibet air shower array and its underground muon detector array. A gamma-ray source with an extension of 0.°34 ± 0.°12 is successfully detected above 25 TeV at (α, δ) = (281.°09 ± 0.°10, −3.°76 ± 0.°09) near HESS J1843–033 with a statistical significance of 6.2σ, and the source is named TASG J1844–038. The position of TASG J1844–038 is consistent with those of HESS J1843–033, eHWC J1842–035, and LHAASO J1843–0338. The measured gamma-ray energy spectrum in 25 TeV < E < 130 TeV is described with (E/40 TeV)−3.26±0.30 TeV−1 cm−2 s−1, and the spectral fit to the combined spectra of HESS J1843–033, LHAASO J1843–0338, and TASG J1844–038 implies the existence of a cutoff at 49.5 ± 9.0 TeV. Associations of TASG J1844–038 with SNR G28.6–0.1 and PSR J1844–0346 are also discussed in detail for the first time.

The Gamma-ray Window to Intergalactic Magnetism

One of the most promising ways to probe intergalactic magnetic fields (IGMFs) is through gamma rays produced in electromagnetic cascades initiated by high-energy gamma rays or cosmic rays in the intergalactic space. Because the charged component of the cascade is sensitive to magnetic fields, gamma-ray observations of distant objects such as blazars can be used to constrain IGMF properties. Ground-based and space-borne gamma-ray telescopes deliver spectral, temporal, and angular information of high-energy gamma-ray sources, which carries imprints of the intervening magnetic fields. This provides insights into the nature of the processes that led to the creation of the first magnetic fields and into the phenomena that impacted their evolution. Here we provide a detailed description of how gamma-ray observations can be used to probe cosmic magnetism. We review the current status of this topic and discuss the prospects for measuring IGMFs with the next generation of gamma-ray observatories.

Simulation of possibility of detecting gamma rays emitted from crab by Alborz-I array

Crab Nebula, as an active source of high energy gamma-rays, is a common standard source used for calibrating different gamma-ray telescopes and observatories. Since gamma rays with energies above 100 TeV from crab is reported, it is possible to detect gamma rays emitted from this nebula by a suitable ground based detector array. Alborz-I array is designed to study cosmic rays with energies around the knee of cosmic ray spectrum. In this paper, it is shown that the current location and configuration of Alborz-I, makes it impossible to detect Crab gamma rays. Our simulation shows that the average rate of gamma particle detection reaches to something above 1 gamma particle in 2 years, which is very difficult to distinguish from cosmic ray background. Finally some proposals to constructing an array with capability of detecting Crab gamma rays are presented.

A high-energy gamma-ray source based on neutron capture

In special applications, high-energy gamma rays are needed to calibrate the energy response of a detector. However, the energies of commonly used radioactive gamma-ray sources are lower than 3 MeV. An alternative source is the prompt gamma rays created by the radiative capture of thermal neutrons. In this study, a 5×104 neutrons/s Am–Be neutron source was used to emit fast neutrons that were moderated in paraffin and then absorbed by Ni (nickel), which captured thermal neutrons and generated monochromatic high-energy gamma rays in the range 5–9 MeV. The 8.99 MeV line in this was the strongest. The paraffin and Ni were mixed uniformly to increase the yield of the high-energy gamma rays, and the optimal mass ratio of paraffin to Ni was calculated by GEANT4. According to the results of simulations, a 200×200×200 mm3 neutron-capturing device was built with a mass ratio of paraffin to Ni of 3:7. Finally, spectra of the high-energy gamma rays emitted by the device were measured by a 38.1 mm (diameter) × 38.1 mm (thickness) LaBr3:Ce detector, and most characteristic energy peaks were clearly visible.

A new high energy gamma-ray observatory in the southern hemisphere: The ALPACA experiment

The ALPACA experiment is a new international project between Bolivia and Japan. It is going to consist of an 83,000 m2 surface air-shower array and a 5,400 m2 underground water Cherenkov muon detector array, and the experimental site is at Mt. Chacaltaya plateau at an altitude of 4,740 m. Its main target is to observe 100 TeV gamma rays and explore high-energy gamma-ray sources in the southern sky. This is because such high-energy gamma rays hold the key to identify the origin of cosmic rays at the knee region of the energy spectrum. So far many high-energy gamma-ray sources have been found in the southern sky. They are emitting gamma rays of several tens of TeV, so some of them could be PeVatrons which accelerate cosmic rays to PeV energy region in the Galaxy. By observing them in higher energy region, we will obtain new knowledge of cosmic-ray acceleration to the knee region, and discover new gamma-ray sources. As the prototype experiment of ALPACA, the ALPAQUITA experiment is now under construction. In a MC simulation, we found that ALPAQUITA has the ability of detecting bright gamma-ray sources in the southern hemisphere such as Vela X within 1 year.

A Catalog of High-Energy Gamma-Ray Sources

A Catalog of High-Energy Gamma-Ray Sources

VERITAS Detection of LS 5039 and HESS J1825-137

With 8 h of observations, VERITAS confirms the detection of two very high energy gamma-ray sources. The gamma-ray binary LS 5039 is detected with a statistical significance of 8.8σ. The measured flux above 1 TeV is (2.5±0.4)×10−12cm−2s−1 near inferior conjunction and (7.8±2.8)×10−13cm−2s−1 near superior conjunction. The pulsar wind nebula HESS J1825-137 is detected with a statistical significance of 6.7σ and a measured flux above 1 TeV of (3.9±0.8)×10−12cm−2s−1.

TeV halos are everywhere: Prospects for new discoveries

Milagro and HAWC have detected extended TeV gamma-ray emission around nearby pulsar wind nebulae (PWNe). Building on these discoveries, Linden et al. [1] identified a new source class -- TeV halos -- powered by the interactions of high-energy electrons and positrons that have escaped from the PWN, but which remain trapped in a larger region where diffusion is inhibited compared to the interstellar medium. Many theoretical properties of TeV halos remain mysterious, but empirical arguments suggest that they are ubiquitous. The key to progress is finding more halos. We outline prospects for new discoveries and calculate their expectations and uncertainties. We predict, using models normalized to current data, that future HAWC and CTA observations will detect in total $\sim$50--240 TeV halos, though we note that multiple systematic uncertainties still exist. Further, the existing HESS source catalog could contain $\sim$10--50 TeV halos that are presently classified as unidentified sources or PWN candidates. We quantify the importance of these detections for new probes of the evolution of TeV halos, pulsar properties, and the sources of high-energy gamma rays and cosmic rays.

High-energy Emission from Interacting Supernovae: New Constraints on Cosmic-Ray Acceleration in Dense Circumstellar Environments

Abstract Supernovae (SNe) with strong interactions with circumstellar material (CSM) are promising candidate sources of high-energy neutrinos and gamma-rays and have been suggested as an important contributor to Galactic cosmic rays (CRs) beyond 1015 eV. Taking into account the shock dissipation by a fast velocity component of SN ejecta, we present comprehensive calculations of the nonthermal emission from SNe powered by shock interactions with a dense wind or CSM. Remarkably, we consider electromagnetic cascades in the radiation zone and subsequent attenuation in the pre-shock CSM. A new time-dependent phenomenological prescription provided by this work enables us to calculate gamma-ray, hard X-ray, radio, and neutrino signals, which originate from CRs accelerated by the diffusive shock acceleration (DSA) mechanism. We apply our results to Type IIn SN 2010jl and Type Ib/IIn SN 2014C, for which the model parameters can be determined from the multiwavelength data. For SN 2010jl, the more promising case, by using the the latest Fermi Large Area Telescope Pass 8 data release, we derive new constraints on the CR energy fraction, ϵ p ≲ 0.05–0.1. We also find that the late-time radio data of these interacting SNe are consistent with our model. Further multimessenger and multiwavelength observations of nearby interacting SNe should give us new insights into the DSA in dense environments, as well as pre-SN mass-loss mechanisms.

TeV J2032+4130 - very high energy gamma-ray source of unresolved nature

The Cygnus Region is one of the brightest regions in all ranges of the electromagnetic spectrum and contains a number of potential GeV and TeV emission sources. It includes active star formation regions, pulsars and supernova remnants. Some of the sources have been detected at high and very high energies. One of them discovered due to its proximity to the well-known microquasar Cyg X-3 is the object TeV J2032+4130. This object is still of unresolved nature and is being intensively studied in different energy ranges. The results of twenty-year observations of TeV J2032+4130 by the SHALON experiment are presented in this paper. The collected experimental data on fluxes, spectrum shape and morphology of TeV J2032+413 can help in the future to determine an object type and reveal mechanisms of generation of very high energy emission.

Compact Star Clusters as Sources of Cosmic Rays and Neutrinos

The origin of cosmic rays is one of the most important questions in modern astrophysics. Leading world observatories are engaged in their search in our Galaxy and beyond. Current observations of bright sources of hard X-rays and gamma radiation require precise models of cosmic-ray accelerators and emission generation. Recent detection of astrophysical neutrinos with the energy exceeding 1015 eV achieved by the IceCube Neutrino Observatory and registration by the LOFAR observatory of the light cosmic-ray component at 1016–1017 eV have given rise to a wide discussion of whether its sources are galactic or extragalactic. H.E.S.S. observations of high-energy gamma-ray sources in compact massive clusters Westerlund 1 and Cl*1806-20 triggered a new search for the models of effective accelerators of high-energy cosmic rays, socalled galactic PeVatrons. In this paper, the properties of compact massive young stellar clusters as possible sources of cosmic rays, neutrinos, and gamma emission are discussed. We suppose that an expanding supernova remnant in a compact massive cluster is an effective cosmic-ray accelerator up to the energies of 1015 eV and beyond. Energy spectra of particles being accelerated in the system of colliding shocks generated by a supernova remnant interaction with nearby stellar winds have a broken power-law shape with a strong upturn around several teraelectronvolts and a cutoff energy at tens of petaelectronvolts. These sources provide the maximal emission flux at the maximal energies of the spectrum. Teraelectronvolt sources detected by H.E.S.S. could be related to supernova remnants in Westerlund 1 and Cl*1806-20 and be responsible for the 10% of the IceCube neutrino events. We have also shown that PeVatrons in the systems of colliding shocks can provide up to 80% of the light cosmic-ray component at the energies of 1016–1017 eV detected at the Earth.

VERITAS and Fermi-LAT Observations of TeV Gamma-Ray Sources Discovered by HAWC in the 2HWC Catalog

Abstract The High Altitude Water Cherenkov (HAWC) collaboration recently published their 2HWC catalog, listing 39 very high energy (VHE; &gt;100 GeV) gamma-ray sources based on 507 days of observation. Among these, 19 sources are not associated with previously known teraelectronvolt (TeV) gamma-ray sources. We have studied 14 of these sources without known counterparts with VERITAS and Fermi-LAT. VERITAS detected weak gamma-ray emission in the 1 TeV–30 TeV band in the region of DA 495, a pulsar wind nebula coinciding with 2HWC J1953+294, confirming the discovery of the source by HAWC. We did not find any counterpart for the selected 14 new HAWC sources from our analysis of Fermi-LAT data for energies higher than 10 GeV. During the search, we detected gigaelectronvolt (GeV) gamma-ray emission coincident with a known TeV pulsar wind nebula, SNR G54.1+0.3 (VER J1930+188), and a 2HWC source, 2HWC J1930+188. The fluxes for isolated, steady sources in the 2HWC catalog are generally in good agreement with those measured by imaging atmospheric Cherenkov telescopes. However, the VERITAS fluxes for SNR G54.1+0.3, DA 495, and TeV J2032+4130 are lower than those measured by HAWC, and several new HAWC sources are not detected by VERITAS. This is likely due to a change in spectral shape, source extension, or the influence of diffuse emission in the source region.