Various theoretical models predict the existence of extended γ-ray halos around normal galaxies that could be produced by interactions of cosmic rays with the circumgalactic medium or by annihilation or decay of hypothetical dark matter particles. Observations of M31, the closest massive galaxy, also corroborate this possibility. In this Letter, we describe our search for gamma-ray emission from the galaxies within 15 Mpc at energies higher than 2 GeV and try to assess its spatial extension. We use the latest catalog of local galaxies and apply a simple yet robust method of aperture photometry. By imposing the mass, energy, and spatial cuts, we selected a set of 16 late-type galaxies and found a statistically significant excess above the background level: a p value of 3.7×10^{-7} at E>2  GeV, reaching maximal significance of p-val=2.3×10^{-8} for a subset of front-converted events with E>2  GeV, where the angular resolution is higher. More importantly, our analysis shows that this excess can be ascribed to an extended source with a radius ∼0.3° rather than a pointlike one. This, for D=15  Mpc, corresponds to a physical halo radius of r_{h}=80  kpc. In contrast, six early-type galaxies, which satisfied the same cuts, showed no excess. Our results are supported by the stacking likelihood analysis technique, which significantly (5.6σ) detected an extended excess. The difference between the late- and early-type galaxies and a rather irregular shape of the extended source that we found, indicate that this high-energy emission is more likely caused by the interactions of cosmic rays with the circumgalactic medium, in preference to dark matter annihilation and decay processes.

The origin of the diffuse gamma-ray background in the range from hundreds keV to several MeV is not known conclusively. From current models and observations it is believed that, at least partially, this background is formed by blazars and remnants of supernovae (SN) of type Ia in distant galaxies. However, these contributions are not sufficient to reproduce the observed level of the signal. In this work we propose another source which could contribute to this background, namely the jets of active galactic nuclei (AGN). The composition of jets is not known, but there are observational hints that the fraction of positrons there is substantial. Positrons are partially evacuated to the intergalactic medium and partially mix with the circumgalactic medium and annihilate there comparatively quickly. Using the AGN luminosity function, we estimated the positron production rate and the contribution of the positron annihilation to the cosmic background below 511 keV. We also estimated the analogous contribution from positron annihilation within SN Ia remnants in distant galaxies. The contribution of AGNs is estimated to be a factor of 5 - 10 smaller than the observed background intensity, and the contribution from SNe is yet smaller by one order of magnitude. Nevertheless, the contribution of AGNs appeared to be larger than the contribution of blazars estimated from Swift-BAT and Fermi-LAT observations. The main uncertainty in our model is the fraction of positrons remaining in the circumgalactic medium which makes our estimation an upper limit.

We report an estimation of the injected mass composition of ultrahigh energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extragalactic magnetic fields (EGMFs), the results are consistent with a relatively heavy injected composition at E similar to 10 EeV that becomes lighter up to E similar to 100 EeV, while the composition at E > 100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extragalactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

We report an estimation of the injected mass composition of ultrahigh energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extragalactic magnetic fields (EGMFs), the results are consistent with a relatively heavy injected composition at E∼10  EeV that becomes lighter up to E∼100  EeV, while the composition at E>100  EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extragalactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

We use a new method to estimate the injected mass composition of ultrahigh cosmic rays (UHECRs) at energies higher than 10 EeV. The method is based on comparison of the energy-dependent distribution of cosmic ray arrival directions as measured by the Telescope Array (TA) experiment with that calculated in a given putative model of UHECR under the assumption that sources trace the large-scale structure (LSS) of the Universe. As we report in the companion Letter, the TA data show large deflections with respect to the LSS which can be explained, assuming small extragalactic magnetic fields (EGMF), by an intermediate composition changing to a heavy one (iron) in the highest energy bin. Here we show that these results are robust to uncertainties in UHECR injection spectra, the energy scale of the experiment and galactic magnetic fields. The assumption of weak EGMF, however, strongly affects this interpretation at all but the highest energies E > 100 EeV, where the remarkable isotropy of the data implies a heavy injected composition even in the case of strong EGMF. This result also holds if UHECR sources are as rare as 2 x 10(-5) Mpc(-3), that is the conservative lower limit for the source number density.

On 11 September 2021, two small thunderstorms developed over the Telescope Array Surface Detector (TASD) that produced an unprecedented number of six downward terrestrial gamma ray flashes (TGFs) within one-hour timeframe. The TGFs occurred during the initial stage of negative cloud-to-ground flashes whose return strokes had increasingly large peak currents up to -223 kA, 147 GeV energy deposit in up to 25 1.2 km-spaced surface detectors, and intermittent bursts of gamma-rays with total durations up to 717 mu s. The analyses are based on observations recorded by the TASD network, complemented by data from a 3D lightning mapping array, broadband VHF interferometer, fast electric field change sensor, high-speed video camera, and the National Lightning Detection Network. The TGFs of the final two flashes had gamma fluences of similar or equal to 2 x 10(14) and 8x10(14), logarithmically bridging the gap between previous TASD and satellite-based detections. The observations further emphasize the similarity between upward and downward TGF varieties, suggesting a common mechanism for their production. Plain Language Summary Terrestrial Gamma-Ray Flashes (TGFs) are high-energy lightning-produced radiation events that have primarily been detected by satellites, but are increasingly detected by ground-based instrumentation. While the upward satellite-detected events exhibit extremely high numbers of gamma photons, the downward ground-based versions observed by the Telescope Array Surface Detector have, until now, displayed much weaker numbers by orders of magnitude. In this paper, we report observations of downward TGFs obtained at the large-area cosmic ray detector Telescope Array facility in west-central Utah, showing strengths midway between previously detected downward TGFs and upward satellite-detected events. The results indicate that TGFs span a wide range of fluences, a finding further supported by recent airborne observations over storm tops.

Optical emissions associated with Terrestrial Gamma-ray Flashes (TGFs) have recently become important subjects in space-based and ground-based observations since these emissions play crucial roles in understanding the generation of TGFs during thunderstorms. In this paper, we present the first time-resolved leader spectra of the optical component associated with a downward TGF. The TGF was observed at the Telescope Array Surface Detector (TASD) simultaneously with other lightning detectors, including a Lightning Mapping Array (LMA), an INTerFerometer (INTF), a Fast Antenna (FA), and a spectroscopic system. The spectroscopic system recorded leader spectra at 29,900 frames per second (33.44 $\mu$s time resolution), covering a spectral range from 400 nm to 900 nm, with 2.1 nm per pixel. The recordings of the leader spectra began 11.7 ms before the $-18$ kA return stroke and at a height of 2.37 km above the ground. These spectra reveal that optical emissions of singly-ionized nitrogen and oxygen occur between 167 $\mu$s before and 267 $\mu$s after the TGF detection, while optical emissions of neutrals (H I, 656 nm; N I, 744 nm, and O I, 777 nm) occur right at the moment of the detection. The time-dependent spectra reveal differences in the optical emissions of lightning leaders with and without downward TGFs.

We report on an observation of the difference between northern and southern skies of the ultrahigh energy cosmic ray energy spectrum with a significance of ∼8σ. We use measurements from the two largest experimentsx2014the Telescope Array observing the northern hemisphere and the Pierre Auger Observatory viewing the southern hemisphere. Since the comparison of two measurements from different observatories introduces the issue of possible systematic differences between detectors and analyses, we validate the methodology of the comparison by examining the region of the sky where the apertures of the two observatories overlap. Although the spectra differ in this region, we find that there is only a 1.8σ difference between the spectrum measurements when anisotropic regions are removed and a fiducial cut in the aperture is applied.

Various hints for anisotropies in the distribution of arrival directions of ultra-high-energy cosmic rays (UHECRs) have been reported. Still, our poor knowledge about intergalactic and Galactic magnetic fields and about the UHECR mass composition makes it non-trivial to interpret such results in terms of possible models of UHECR sources. In this work, we apply the same analyses that have been performed on the Pierre Auger Observatory and the Telescope Array UHECR data to a variety of Monte Carlo simulations generated according to many different combinations of hypotheses about the sources, composition and magnetic deflections of UHECRs. We find that only some of these models can yield results similar to those obtained with the real data.

Источник BL Lac демонстрировал повышенную активность с августа 2020 до июля 2022 г. Максимум этой активности пришелся на период июль–август 2021 г. В этот период источник достиг исторических максимумов яркости в различных энергетических диапазонах. Наблюдения этого нестандартного максимума проводились сотрудниками ГАИШ Крымской станции ГАИШ и на телескопах Кавказской горной обсерватории (КГО ГАИШ МГУ). Были получены данные фотометрических наблюдений в оптическом диапазоне и ближней инфракрасной области, а также поляриметрические данные в оптике. При исследовании корреляции оптического и гамма-излучения, оптического и ИК-излучения подтвердилась ее высокая степень с почти нулевым запаздыванием – это значит, что области излучения пространственно совпадают либо расположены очень близко друг от друга. Степень линейной поляризации в исследуемый период менялась в широких пределах, достигая значения 20 \({\%}\) , причем наблюдалась антикорреляция степени линейной поляризации с яркостью объекта. Подобный характер изменения наблюдался и в предшествующие вспышки этого объекта. Изменения направления вектора напряженности электрического поля (EVPA) показывают зависимость от скорости изменения яркости объекта. Угол вектора напряженности электрического поля сильно менялся при вспышках, а в моменты медленного изменения интенсивности менялся медленно.

Positron annihilation line at 511 keV is a known component of the gamma-ray diffuse emission. It is believed to be produced in the Galaxy, but there could be possible extragalactic contribution as well. E.g., positrons can be produced in jets of active galactic nuclei (AGN) and after that accumulate and gradually annihilate in hot gaseous halos around galaxies. In this work we test this hypothesis in application to an individual object-the Andromeda galaxy (M31) which is close and has a supermassive black hole in its center, which powered an AGN before. We compute the growth history of the supermassive black hole in M31, relate it to the evolution of jet luminosity and estimate the positron content in its halo. We calculate the 511 keV photon flux due to positron annihilation which should be observed at Earth and find the value of around 10(-4) photon cm(-2) s(-1). It is very close to the observational limits (<10(-4) photon cm(-2) s(-1)) set by the INTEGRAL/SPI in the assumption of the point source, so further observations would be able to constrain leptonic models of the jets and propagation of cosmic rays in the circumgalactic medium of large spiral galaxies.

The source BL Lac exhibited an enhanced activity from August 2020 to July 2022. This activity peaked in July–August 2021. In this period the source reached historic brightness maxima in various energy ranges. Observations of this nonstandard maximum were carried out by the SAI staff of the Crimean Station of SAI and with the telescopes of the Caucasus Mountain Observatory (CMO SAI MSU). Optical and near-infrared photometric data as well as optical polarimetric data were obtained. When investigating the correlation between the optical and gamma-ray fluxes and between the optical and infrared fluxes, its high degree with an almost zero lag was confirmed—this means that the emission regions spatially coincide or are very close together. The degree of linear polarization in the investigated period changed in a wide range, reaching 20 % , with an anticorrelation between the degree of linear polarization and the brightness of the object having been observed. A similar pattern of variation was also observed in the preceding flares of this object. The changes in the electric vector position angle (EVPA) show a dependence on the rate of change of the object’s brightness. The EVPA changed greatly during flares and slowly at the times of a slow change in the intensity.

Fast radio bursts (FRBs) were discovered only in 2007. However, the number of known events and sources of repeating bursts grows very rapidly. In the near future, the number of events will be ≳104 and the number of repeaters ≳100. Presently, there is a consensus that most of the sources of FRBs might be neutron stars (NSs) with large magnetic fields. These objects might have different origin as suggested by studies of their host galaxies which represent a very diverse sample: from regions of very active star formation to old globular clusters. Thus, in the following decade we expect to have a very large sample of events directly related to extragalactic magnetars of different origin. This might open new possibilities to probe various aspects of NS physics. In the review we briefly discuss the main directions of such future studies and summarize our present knowledge about FRBs and their sources.

Positron annihilation line at 511 keV is a known component of the gamma-ray diffuse emission. It is believed to be produced in the Galaxy, but there could be possible extragalactic contribution as well. E.g., positrons can be produced in jets of active galactic nuclei (AGN) and after that accumulate and gradually annihilate in hot gaseous halos around galaxies. In this work we test this hypothesis in application to an individual object – the Andromeda galaxy (M31) which is close and has a supermassive black hole in its center, which powered an AGN before. We compute the growth history of the supermassive black hole in M31, relate it to the evolution of jet luminosity and estimate the positron content in its halo. We calculate the 511 keV photon flux due to positron annihilation which should be observed at Earth and find the value of around \(10^{-4}\) photon cm\({}^{-2}\)s\({}^{-1}\). It is very close to the observational limits (\(10^{-4}\)photon cm\({}^{-2}\)s\({}^{-1}\)) set by the INTEGRAL/SPI in the assumption of the point source, so further observations would be able to constrain leptonic models of the jets and propagation of cosmic rays in the circumgalactic medium of large spiral galaxies.

The problem of identification of ultra-high-energy cosmic ray (UHECR) sources is greatly complicated by the fact that even the highest energy cosmic rays may be deflected by tens of degrees in the galactic magnetic fields. We show that arrival directions of UHECRs from several nearest active galaxies form specific patterns in the sky, which can be effectively recognized by convolutional neural networks. We use one of the recently developed convnet implementations for images defined on the sphere to train the classifier that is able to detect patterns that can be present in the experimental data. We calculate the minimal detectable from-source event fractions for several realistic source candidates and discuss the method limitations.

The Telescope Array (TA) Cosmic Ray Observatory is the largest cosmic ray detector in the northern hemisphere. TA was built to study ultra-high-energy cosmic rays (UHECRs), cosmic rays with energies above 1018 eV. TA is a hybrid detector, employing two distinct detection methods: a surface detector array and a set of fluorescence telescopes. We will present a measurement of the cosmic ray energy spectrum for energies above 1017.5 eV using only the fluorescence telescopes. A novel weather classification scheme using machine learning was used to select data parts with good weather to ensure the quality of the fluorescence data. The data from the Black Rock Mesa (BRM) and Long Ridge (LR) fluorescence telescope sites were analyzed separately in monocular mode, with the calculated fluxes combined into a single spectrum. The 10-year monocular combined cosmic ray energy spectrum is observed to be in excellent agreement with previous measurements from the northern hemisphere. We present fits of the combined spectrum to a series of broken power law models. The thrice-broken power law was observed to be the best fit considering the Poisson deviance per degrees of freedom. The three breaks suggest an additional feature of the spectrum between the previously observed Ankle feature at 1018.7 eV and the GZK suppression at 1019.8 eV.

ABSTRACTHigh-energy radiation of young pulsar wind nebulae (PWNe) is known to be variable. This is exemplified by the Crab nebula, which can undergo both rapid brightenings and dimmings. Two pulsars in the Large Magellanic Cloud, PSR J0540–6919 and PSR J0537–6910, are evolutionally close to Crab, so one may expect the same kind of variability from the PWNe around them. In this work, we search for flaring activity in these PWNe in gamma-rays using the data from the Fermi Large Area Telescope in the range 100 MeV–10 GeV collected from 2008 August to 2021 December. We construct light curves of these sources in the three bands, 100–300 MeV, 300–1000 MeV, and 1–10 GeV, with one-week resolution. We find evidence of flaring activity in all the bands, in contrast with Crab, where no flares at E>1 GeV were observed. Analysis of the flaring episode in the 100–300 and 300–1000 MeV bands indicates that the flux of one of the PWNe could grow by a factor of ≈5–10, and the statistical significance of the flare detection reaches 6σ. We are not confident about which of the two PWNe flared because of their proximity in the sky. However, in the 1–10 GeV band, where the angular resolution of LAT is better, we find several episodes of enhanced brightness in both the PWNe. We check possible contaminants which could be responsible for the observed variability but find their contribution not to be relevant.

In this paper, we present the first high-speed video observation of a cloud-to-ground lightning flash and its associated downward-directed Terrestrial Gamma-ray Flash (TGF). The optical emission of the event was observed by a high-speed video camera running at 40,000 frames per second in conjunction with the Telescope Array Surface Detector, Lightning Mapping Array, interferometer, electric-field fast antenna, and the National Lightning Detection Network. The cloud-to-ground flash associated with the observed TGF was formed by a fast downward leader followed by a very intense return stroke peak current of -154 kA. The TGF occurred while the downward leader was below cloud base, and even when it was halfway in its propagation to ground. The suite of gamma-ray and lightning instruments, timing resolution, and source proximity offer us detailed information and therefore a unique look at the TGF phenomena.

Positron annihilation line at 511 keV is a known component of the gamma-ray diffuse emission. It is believed to be produced in the Galaxy, but there could be possible extragalactic contribution as well. E.g., positrons can be produced in jets of active galactic nuclei (AGN) and after that accumulate and gradually annihilate in hot gaseous halos around galaxies. In this work we test this hypothesis in application to an individual object – the Andromeda galaxy (M31) which is close and has a supermassive black hole in its center, which powered an AGN before. We compute the growth history of the supermassive black hole in M31, relate it to the evolution of jet luminosity and estimate the positron content in its halo. We calculate the 511 keV photon flux due to positron annihilation which should be observed at Earth and find the value of around \(10^{-4}\) photon cm\({}^{-2}\)s\({}^{-1}\). It is very close to the observational limits (\(10^{-4}\)photon cm\({}^{-2}\)s\({}^{-1}\)) set by the INTEGRAL/SPI in the assumption of the point source, so further observations would be able to constrain leptonic models of the jets and propagation of cosmic rays in the circumgalactic medium of large spiral galaxies.