Gamma-ray Observations Research Articles

The results of analysis of Rich Galaxy Clusters from CfA2 Redshift Survey spatial distribution

Preliminary results of the investigation of the properties of 7 galaxy cluster from CfA2 redshift survey are discussed in the presented article. Clusters 933, 142, 1046, and 1652 have several peculiarities of the spatial distributions of galaxies. The distributions on absolute magnitude and luminosity represent two areas for clusters 933, 88, 142, 1046, 1101. The investigation of the spatial distribution and other characteristics of 88, 1101, 1046, 142, 933, 1242 and 1652 galaxy clusters allow concluding gravitational lensing effect. Galaxies from these areas are paired accordingly its spectral characteristics and position. Redshifts of these clusters are in the region 0.002 – 0.022. Preliminary results of analysis allow us concluding compact objects or dark matter blobs as lenses. Moreover, groups #933, #142, #1046 and #1652 reveals high-energy γ-associations on Fermi/LAT 10-Year Point Source Catalog 4FGL-DR2 (4FGL J1144.9 + 1937, 4FGL J0152.2 + 3714, 4FGL J1230.8 + 1223 and 4FGL J1653.8 + 3945); and sources 4FGL J0123.1+3421, 4FGL J1259.5+2332, 4FGL J1353.2+3740 are located inside sizes of clusters #88, #1101 and #1242. Also several anomalies of spatial dynamic of galaxies in these clusters were separated. Joint observations of such clusters by orbital gamma-ray observatories with high angular resolution and ground-based Cherenkov air-shower experiments could possibly clarify the type of gravitational lensing and processes of particle acceleration in these objects especially highest energy of emitted gammas. Thus we propose including these and similar clusters in the programs of observations of the planned experiment GAMMA-400 (Gamma Astronomical Multifunctional Modular Apparatus) with angular resolution ~0.01° at Eγ = 100 GeV. Also now it is discussed coordination of multiwavelength observations program of Cherenkov Telescope Array (CTA) and GAMMA-400 objects list for observations. Moreover, common observations of such clusters by orbital gamma-ray telescopes with high angular resolution and ground-based Cherenkov air-shower experiments could possibly clarify the type of gravitational lenses.

Galactic 26Al traces metal loss through hot chimneys

ABSTRACT Radioactive 26Al is an excellent tracer for metal ejection in the Milky Way, and can provide a direct constraint on the modelling of supernova feedback in galaxy evolution. Gamma-ray observations of the 26Al decay line have found high velocities and hence require a significant fraction of the Galactic 26Al in the hot component. At the same time, meteoritic data combined with simulation results suggest that a significant amount of 26Al makes its way into stars before decay. We investigated the distribution into hot and cold channels with a simulation of a Milky-Way-like galaxy with massive-star feedback in superbubbles and with ejecta traced by 26Al. About 30–40 per cent of the ejecta remain hot, with typical cooling times of the order Gyr. 26Al traces the footpoints of a chimney-fed outflow that mixes metals turbulently into the halo of the model galaxy on a scale of at least 50 kpc. The rest diffuses into cold gas ≲ 104 K, and may therefore be quickly available for star formation. We discuss the robustness of the result by comparison to a simulation with a different global flow pattern. The branching ratio into hot and cold components is comparable to that of longer term average results from chemical evolution modelling of galaxies, clusters, and the intracluster medium.

Optimization and Estimation of Parameters for a Compton Camera Consisting of the DSSD Scatterer and the GAGG Absorber with the Monte Carlo Simulation

With the advent of new rare-earth-based scintillators, a variety of Compton cameras with novel features in detector material and structure have been developed for the application to either molecular imaging or gamma-ray observation with space telescopes. In this study, we investigated the performance of a Compton camera consisting of a DSSD as a scatterer and an array-type GAGG scintillator as an absorber. We mainly focused on how to optimize the performance of the Compton camera in imaging resolution as well as in efficiency and thereby determined the optimal geometrical configuration with the Monte Carlo simulation using a GEANT4 toolkit. The energy discrimination levels were set in the simulation so that the general experimental environments could be taken into account. We studied different configurations with a figure of merit accounted for efficiency and resolution as a function of the gamma-ray energy, the pixel size of the absorber, the source-to-scatterer distance, the scatterer-to-absorber distance and the scatterer-to-absorber angle in this study. The figure of merit reached its maximum when the source-to-scatterer distance was 3 cm, the scatterer-to-absorber distance was 5 cm and the scatterer-to-absorber angle, that is, the absorber with 256 pixels was located at an angle of 15° with respect to the scatterer for 356-keV gamma rays used in this study. We also investigated how the event rates for Compton scattering obtained by counting the scattering events were contributed to the reconstructed image and caused unwanted imaging artifacts for the optimal configuration. Finally, calculations were performed by using the gamma-ray tracking method with inclusion of two-fold events occurring within the absorber and showed increase in imaging efficiency by 25%.

Orbital Foregrounds for Ultra-short Duration Transients

Abstract Reflections from objects in Earth orbit can produce subsecond, star-like optical flashes similar to astrophysical transients. Reflections have historically caused false alarms for transient surveys, but the population has not been systematically studied. We report event rates for these orbital flashes using the Evryscope Fast Transient Engine, a low-latency transient detection pipeline for the Evryscopes. We select single-epoch detections likely caused by Earth satellites and model the event rate as a function of both magnitude and sky position. We measure a rate of sky−1 hr−1, peaking at m g = 13.0, for flashes morphologically degenerate with real astrophysical signals in surveys like the Evryscopes. Of these, sky−1 hr−1 are bright enough to be visible to the naked eye in typical suburban skies with a visual limiting magnitude of V ≈ 4. These measurements place the event rate of orbital flashes orders of magnitude higher than the combined rate of public alerts from all active all-sky fast-timescale transient searches, including neutrino, gravitational-wave, gamma-ray, and radio observatories. Short-timescale orbital flashes form a dominating foreground for untriggered searches for fast transients in low-resolution, wide-angle surveys. However, events like fast radio bursts with arcminute-scale localization have a low probability (∼10−5) of coincidence with an orbital flash, allowing optical surveys to place constraints on their potential optical counterparts in single images. Upcoming satellite internet constellations, like SpaceX Starlink, are unlikely to contribute significantly to the population of orbital flashes in normal operations.

Interpreting correlated observations of cosmic rays and gamma-rays from Centaurus A with a proton blazar inspired model

ABSTRACT The nearest active radio galaxy Centaurus (Cen) A is a gamma-ray emitter in GeV–TeV energy scale. The high energy stereoscopic system (HESS) and non-simultaneous Fermi–Large Area Telescope observation indicate an unusual spectral hardening above few GeV energies in the gamma-ray spectrum of Cen A. Very recently the HESS observatory resolved the kilo parsec (kpc)-scale jets in Centaurus A at TeV energies. On the other hand, the Pierre Auger Observatory (PAO) detects a few ultrahigh energy cosmic ray (UHECR) events from Cen-A. The proton blazar inspired model, which considers acceleration of both electrons and hadronic cosmic rays in active galactic nuclei (AGN) jet, can explain the observed coincident high-energy neutrinos and gamma-rays from Ice-cube detected AGN jets. Here, we have employed the proton blazar inspired model to explain the observed GeV–TeV gamma-ray spectrum features including the spectrum hardening at GeV energies along with the PAO observation on cosmic rays from Cen-A. Our findings suggest that the model can explain consistently the observed electromagnetic spectrum in combination with the appropriate number of UHECRs from Cen A.

Cherenkov Telescope Array sensitivity to branon dark matter models

In the absence of a clear hint of dark matter (DM) signals in the GeV regime so far, heavy, \U0001d4aa(TeV) DM candidates are gradually earning more and more attention within the community. Among others, extra-dimensional brane-world models may produce thermal DM candidates with masses up to 100 TeV . These heavy DM candidates could be detected with the next generation of very-high-energy gamma-ray observatories such as the Cherenkov Telescope Array (CTA) . In this work, we study the sensitivity of CTA to branon DM via the observation of representative astrophysical DM targets, namely dwarf spheroidal galaxies. In particular, we focus on Draco and Sculptor, two well-known dwarfs visible from the Northern and Southern Hemisphere, respectively. For each of these targets, we simulated 300 h of CTA observations and studied the sensitivity of both CTA-North and CTA-South to branon annihilations using the latest publicly available instrument response functions and most recent analysis tools. We computed annihilation cross section values needed to reach a 5σ detection as a function of the branon mass. Additionally, in the absence of a predicted DM signal, we obtained 2σ upper limits on the annihilation cross section. These limits lie 1.5–2 orders of magnitude above the thermal relic cross section value, depending on the considered branon mass. Yet, CTA will allow to exclude a significant portion of the brane tension-mass parameter space in the 0.1–60 TeV branon mass range, and up to tensions of ∼ 10 TeV . More importantly, CTA will significantly enlarge the region already excluded by AMS and CMS, and will provide valuable complementary information to future SKA radio observations. We conclude that CTA will possess potential to constrain brane-world models and, more in general, TeV DM candidates.

Annealing of proton radiation damages in Si-PM at room temperature

We report the radiation damage results of two new types of Multi-Pixel Photon Counter (MPPC) under the 200 MeV proton beam, for cosmic gamma-ray observations with CubeSats. The new MPPCs, S13360-6050CS and S14160-6050HS, have recently become commercially available by Hamamatsu Photonics K.K. (HPK). After 100 rad irradiation, the dark current increased by a factor of ∼100 and the threshold energy of gamma-ray detection by CsI scintillator increased, but S14160-6050HS was less degraded. We then confirmed the annealing effects of the dark current and threshold energy for both. The decrease rate of the dark current in both MPPCs were similar to those in the previous works. At 20 °C, the threshold energies of both MPPCs reduced by several tens of percent after irradiation, but below 0 °C, the threshold energy of S14160-6050HS was not significantly lowered. We also confirmed that annealing began before one month post-irradiation.

A crucial test of the phantom closed cosmological model

ABSTRACT We suggest a crucial direct-observational test for measuring distinction between the standard ΛCDM model and recently proposed phantom dark energy positive curvature cosmological model. The test is based on the fundamental distance–flux–redshift relation for general Friedmann models. It does not depend on the CMBR data, on the large-scale structure growth models, and also on the value of the Hubble constant H0. Our crucial test can be performed by future gamma-ray burst observations with THESEUS space mission and by using gravitational-wave standard siren observations with modern advanced LIGO–Virgo and also forthcoming LISA detectors.

Simulating TeV gamma-ray morphologies of shell-type supernova remnants

ABSTRACT Supernova remnant (SNR) shocks provide favourable sites of cosmic ray (CR) proton acceleration if the local magnetic field direction is quasi-parallel to the shock normal. Using the moving-mesh magnetohydrodynamical (MHD) code arepo we present a suite of SNR simulations with CR acceleration in the Sedov–Taylor phase that combine different magnetic field topologies, density distributions with gradients and large-scale fluctuations, and – for our core-collapse SNRs – a multiphase interstellar medium with dense clumps with a contrast of 104. Assuming the hadronic gamma-ray emission model for the TeV gamma-ray emission, we find that large-amplitude density fluctuations of δρ/ρ0 ≳ 75 per cent are required to strongly modulate the gamma-ray emissivity in a straw man’s model in which the acceleration efficiency is independent of magnetic obliquity. However, this causes strong corrugations of the shock surface that are ruled out by gamma-ray observations. By contrast, magnetic obliquity-dependent acceleration can easily explain the observed variance in gamma-ray morphologies ranging from SN1006 (with a homogeneous magnetic field) to Vela Junior and RX J1713 (with a turbulent field) in a single model that derives from plasma particle-in-cell simulations. Our best-fitting model for SN1006 has a large-scale density gradient of ∇n ≃ 0.0034 cm−3 pc−1 pointing from south-west to north-east and a magnetic inclination with the plane of the sky of ≲10°. Our best-fitting model for Vela Junior and RX J1713 adopts a combination of turbulent magnetic field and dense clumps to explain their TeV gamma-ray morphologies and moderate shock corrugations.

Constraining the annihilating dark matter mass by the radio continuum spectral data of the NGC4214 galaxy

Recent gamma-ray and radio observations provide stringent constraints for annihilating dark matter. The current $2\sigma$ lower limits of dark matter mass can be constrained to $\sim 100$ GeV for thermal relic annihilation cross section. In this article, we use the radio continuum spectral data of a nearby galaxy NGC4214 and differentiate the thermal contribution, dark matter annihilation contribution and cosmic-ray contribution. We can get more stringent constraints of dark matter mass and annihilation cross sections. The $5\sigma$ lower limits of thermal relic annihilating dark matter mass obtained are 300 GeV, 220 GeV, 220 GeV, 500 GeV and 600 GeV for $e^+e^-$, $\mu^+\mu^-$, $\tau^+\tau^-$, $W^+W^-$ and $b\bar{b}$ channels respectively. These limits challenge the dark matter interpretation of the gamma-ray, positron and antiproton excess in our Milky Way.

Very Long Baseline Interferometry Observations of the Proposed Radio Counterpart of an EGRET Source

We present high-resolution radio interferometric imaging observations of the radio source NVSS J182659+343113 (hereafter J1826+3431), the proposed radio counterpart of the γ-ray source, 3EG J1824+3441 detected by the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory satellite. We analyzed eight epochs of archival multi-frequency very long baseline interferometry data. We imaged the asymmetric core–jet structure of the source, and detected apparent superluminal motion in the jet. At the highest observing frequency, 15.3 GHz, the core shows high brightness temperature indicating Doppler boosting. Additionally, the radio features undergo substantial flux density variability. These findings strengthen the previous claim of the association of the blazar J1826+3431 with the possible γ-ray source, 3EG J1824+3441.

Using Convolutional Neural Networks for Muon detection in WCD tank

The aim of this paper is to study the possibility of improving the gamma/hadron discrimination in extensive air showers. For this purpose, the identification of hadronic extensive air showers is carried out by means of the detection of muons in water Cherenkov detectors (WCDs). Machine learning algorithms have proven to be useful in a wide variety of fields, and due to their outstanding performance in problems involving complex data, Convolutional Neural Networks (CNNs) have been used in the analysis of the signals measured by the WCDs. Taking simulated events, different approaches were proposed attending to the balance of the classes in the training stage. The results obtained are promising and show that machine learning algorithms provide a powerful tool for muon detection and gamma/hadron discrimination to be considered in future gamma-rays detectors like The Southern Wide-field Gamma-ray Observatory (SWGO) to be built in South America.

Galactic cosmic ray propagation models using Picard

We will give an overview of recent developments in numerical modelling of CR transport in our Galaxy with the PICARD code. Corresponding numerical models aim at reproducing CR spectra and also diffuse gamma-ray emission from the Galaxy by solving the cosmic-ray transport problem in a three-dimensional model of the Galaxy. We address the impact of different transport physics processes on the flux and distribution of diffuse Galactic gamma rays: we investigate CR transport using different cosmic-ray source distribution models ranging from smooth source distributions to localised sources related to high-energy gamma-ray observations. Additionally, we consider anisotropic cosmic-ray diffusion in an improved Galactic magnetic field model. The choice of changing the different transport parameters is most readily visible in the inverse-Compton channel, which shows features not present in commonly-used axisymmetric transport models.

Particle identification capability of Plastic scintillator tiles equipped with SiPMs for the High Energy cosmic-Radiation Detection (HERD) facility

The High Energy Cosmic Radiation Detection (HERD) facility onboard the future Chinese Space Station (CSS) will be able to detect charged cosmic rays and gamma rays from few GeV to PeV energies, giving a valuable contribution in several scientific topics, such as dark matter searches in astrophysical objects, the study of cosmic ray chemical composition and high energy gamma-ray observations. The entire instrument is supposed to be surrounded by a plastic scintillator detector (PSD), which will be used to discriminate charged from neutral particles in order to correctly identify gamma-rays and nuclei. One configuration proposed and studied for the HERD PSD detector consists in a segmented plastic scintillator with a squared-tile geometry, coupled to Silicon Photomultipliers (SiPMs). SiPMs provide similar or even better performances to the standard photomultiplier tubes (PMTs) with lower power consumption and cost benefits. In 2018, beam test campaigns were performed at CERN PS and SPS to test two prototypes of plastic scintillator tiles, equipped with a set of SiPMs. One was tested with a beam of electrons and pions and another prototype with an ion beam. The results will be presented, showing the capabilities of these prototypes to detect charged particles with very high efficiency and to measure the charge of heavier nuclei.

Gamma-ray flares from relativistic magnetic reconnection in the jet of the quasar 3C 279

Spinning black holes in the centres of galaxies can release powerful magnetised jets. When the jets are observed at angles of less than a few degrees to the line-of-sight, they are called blazars, showing variable non-thermal emission across the electromagnetic spectrum from radio waves to gamma rays. It is commonly believed that shock waves are responsible for this dissipation of jet energy. Here we show that gamma-ray observations of the blazar 3C 279 with the space-borne telescope Fermi-LAT reveal a characteristic peak-in-peak variability pattern on time scales of minutes expected if the particle acceleration is instead due to relativistic magnetic reconnection. The absence of gamma-ray pair attenuation shows that particle acceleration takes place at a distance of ten thousand gravitational radii from the black hole where the fluid dynamical kink instability drives plasma turbulence.

Probing modified dispersion relations in vacuum with high-energy γ-ray sources: review and prospects

A possible violation of Lorentz Invariance (LIV) in the form of Modified Dispersion Relations (MDR) for photons in vacuum appeared in the late 90s as a possible outcome of some models developed with the goal to provide a full theory of Quantum Gravity (QG). Since then, several ways to probe quantum spacetime from high-energy gamma-ray observations of distant sources have been followed and provide stringent limits on the characteristic energy scale of these QG-related phenomena.In this paper, the use of astrophysical sources for constraining MDR will be described. In particular, strengths and weaknesses of each category of sources as well as temporal effects due to emission mechanisms will be discussed. The latest results obtained from observations of Gamma-Ray Bursts, flaring Active Galactic Nuclei and pulsars will be briefly reviewed. Then, efforts on-going to get more robust constraints on MDR and LIV will be discussed and put in context with the beginning of the Cherenkov Telescope Array operations in the next few years.

The low rate of Galactic pevatrons

Although supernova remnants remain the main suspects as sources of Galactic cosmic rays up to the knee, the supernova paradigm still has many loose ends. The weakest point in this construction is the possibility that individual supernova remnants can accelerate particles to the rigidity of the knee, ~ 106 GV. This scenario heavily relies upon the possibility to excite current driven non-resonant hybrid modes while the shock is still at the beginning of the Sedov phase. These modes can enhance the rate of particle scattering thereby leading to potentially very–high maximum energies. Here we calculate the spectrum of particles released into the interstellar medium from the remnants of different types of supernovae. We find that only the remnants of very powerful, rare core–collapse supernova explosions can accelerate light elements such as hydrogen and helium nuclei, to the knee rigidity, and that the local spectrum of cosmic rays directly constrains the rate of such events, if they are also source of PeV cosmic rays. On the other hand, for remnants of typical core–collapse supernova explosions, the Sedov phase is reached at late times, when the maximum energy is too low and the spectrum at very–high energies is very steep, being mostly produced during the ejecta dominated phase. For typical thermonuclear explosions, resulting in type Ia supernovae, we confirm previous findings that these objects can only produce cosmic rays up to ≲ 105 GeV. The implications for the overall cosmic ray spectrum observed at the Earth and for the detection of PeVatrons by future gamma–ray observatories are discussed.

Constraining primordial black hole fraction at the galactic centre using radio observational data

ABSTRACT Recent gamma-ray and cosmic-ray observations have put strong constraints on the amount of primordial black holes (PBHs) in our universe. In this paper, we use the archival radio data of the inner Galactic Centre to constrain the PBH to dark matter ratio for three different PBH mass distributions including monochromatic, lognormal, and power law. We show that the amount of PBHs only constitutes a very minor component of dark matter at the Galactic Centre for a large parameter space.

Study on the escape timescale of high-energy particles from supernova remnants through thermal X-ray properties

Abstract In the current decade, GeV/TeV gamma-ray observations of several supernova remnants (SNRs) have implied that accelerated particles are escaping from their acceleration sites. However, when and how they escape from the SNR vicinities are yet to be understood. Recent studies have suggested that the particle escape might develop with thermal plasma ages of the SNRs. We present a systematic study on the time evolution of particle escape using thermal X-ray properties and gamma-ray spectra using 38 SNRs associated with GeV/TeV gamma-ray emissions. We conducted spectral fittings on the gamma-ray spectra using exponential cutoff power-law and broken power-law models to estimate the exponential cutoff or the break energies, both of which are indicators of particle escape. Plots of the gamma-ray cutoff/break energies over the plasma ages show similar tendencies to those predicted by analytical/numerical calculations of particle escape under conditions in which a shock is interacting with thin interstellar medium or clouds. The particle escape timescale is estimated as ∼100 kyr from the decreasing trends of the total energy of the confined protons with the plasma age. The large dispersions of the cutoff/break energies in the data may suggest an intrinsic variety of particle escape environments. This might be the cause of the complicated Galactic cosmic ray spectral shape measured on Earth.

Theseus–BTA Cosmological Crucial Tests Using Multimessenger Gamma-Ray Bursts Observations

Modern Multimessenger Astronomy is a powerful instrument for performing cosmological crucial tests of the Standard Cosmological Model in the wide redshift interval up to $z \sim 10$. This is principally important for discussion related to discrepancies between local and global measurements of cosmological parameters. We present a review of multimessenger gamma-ray burst observations currently conducted and planed for THESEUS$-$BTA cooperative program. Such observations give a unique opportunity to test the fundamental foundations of cosmological models: gravitation theory; cosmological principle of homogeneity and isotropy of large-scale distribution of matter; and space expansion paradigm. Important role of various selection effects leading to systematic distortions of true cosmological relations is discussed.