Unlike the complicated X-ray and radio structure observed in the North Polar Spur area, the southeastern part of the eROSITA bubbles can be reasonably well described as a propagating forward shock, plausibly created by the transient energy release at the Galactic center. In this model, the physical radius of the bubble is R_ ̊m b ̊m kpc , and the age of the outburst is t_ ̊m age ∼ 5-8, ̊m Myr . The latter quantity is plausibly a lower limit on the true age. The visible segment of the shock front (located at a distance of ∼ 10-12, ̊m kpc above the Galactic disk and at a similar distance from the Sun) is currently expanding with a velocity of ∼ 700, -1 through gas with a density of n_e∼ 3 , . Moreover, the abundance of heavy elements in this gas is low, Z∼ 0.1-0.2 10^ -4 ̊m cm^ -3 Z_⊙ (depending on the adopted reference solar abundances). Unlike the constraints derived from the line-of-sight-integrated quantities, these are effectively in situ measurements of the properties of the circumgalactic medium. Given the simplifying assumptions used in deriving the density and abundance, we assigned a systematic uncertainty of factor of 2 to the final estimates. An eventual decisive test for the shock properties can be provided by the velocity measurements of the X-ray-emitting gas with soft X-ray bolometers. The extended forward shock propagating through low-metallicity gas is a favorable site to accelerate very high-energy cosmic rays, which might contribute to the recently discovered proton-rich Galactic cosmic ray component at PeV energies impinging on the Earth's atmosphere.

The structure of the quasi-perpendicular bow shock of the Earth observed by the MMS spacecraft on 31 January 2017 with an Alfvén Mach number of approximately 10 and plasma parameter β of approximately 3, has been simulated using the Maximus hybrid kinetic code. We investigated types of instabilities governing the front structure and showed that in this case both ion Weibel and Alfvén ion cyclotron instabilities can arise at the shock foot simultaneously, thus leading to fast magnetic oscillations with a relative variation close to unity. Some signatures of the mirror instability were found in the near downstream. Simulation also showed that the front structure substantially differ for shock inclination angles of 50° and 75°.

The W50 nebula around microquasar SS 433, powered by supercritical accretion, features two "extended jets" (tens of pc long and a few pc wide) from which polarized x-ray and very high-energy radiation above 100 TeV is detected. Here we present a model of very high-energy particle acceleration in these extended jets. In the "minimalist" model (discussed in Churazov, Khabibullin, and Bykov, 2024), a collimated outflow aligned with the rotation axis is propagating through a more isotropic wind produced by the accretion disk. The observed extended x-ray jets in this model are associated with the formation of strong recollimation magnetohydrodynamics shocks after the collision of the collimated outflow with the isotropic wind termination surface. The spectra of electrons and protons up to PeV energies are simulated with a nonlinear Monte Carlo model of diffusive shock acceleration with turbulent magnetic field amplification. The overall efficiency of the jets power transfer to accelerated protons in this model is above 10% and about 0.5% for electrons above 50 TeV. The magnetic field amplification by Bell's instability due to the electric current of cosmic rays escaping the accelerator produces highly anisotropic magnetic turbulence in the shock downstream. This results in the polarized synchrotron x-ray emission with the photon electric vector predominantly transverse to the jet direction and the degree of polarization above 20%. The model is able to reproduce the observed spectra and intensity profiles of nonthermal x-ray and gamma-ray emission, which are both dominated by the leptonic radiation.

The operation of a small-size Cherenkov gamma-ray telescope TAIGA-IACT with camera on SiPMs OnSemi MicroFJ-60035 has been modelled by multiparticle Monte Carlo (MC) methods. The model implies that telescope camera is equipped with two specific types of filters of 290-590 nm (visible+NUV) and 220-320 nm (MUV+UVB)-bands, each covering half of the camera pixels in some uniform order. This allows one to measure the fraction of UV-radiation in total amount of Cherenkov radiation of an extensive air shower (EAS), that can be used for efficient gamma-hadron separation. The corresponding quality factor takes values up to 5.07 in the 10-100 TeV range depending on the distance to EAS axis and camera orientation.

W50 is a radio nebula around the hyperaccreting Galactic microquasar SS433. We focus on one peculiar feature of W50, that is, on a pair of so-called extended X-ray jets (EXJs). These jets have a size of similar to 20 pc and a sharp inner boundary, and their spectra are well represented by a featureless X-ray continuum. We argue that EXJs could be an outcome of a powerful anisotropic wind produced by a supercritical accretion disk. In the simplest version of this model, the wind itself consists of two components. The first component is a nearly isotropic outflow that subtends most of the solid angle as seen from the compact source and creates the quasi-spherical part of the W50 nebula. The second component is a more collimated wind that is aligned with the rotation axis of the binary system (polar wind). The isotropic outflow passes through the termination shock, and its increased thermal pressure creates a sequence of recollimation shocks in the polar wind, giving it the appearance of an extended X-ray structure. In this model, the EXJ continuum spectrum is due to synchrotron emission of electrons that are accelerated at the shocks that arise in the polar wind. At variance with many other studies, the EXJ structures in this model are not directly related to the highly collimated and precessing 0.26 c baryonic jets. Instead, the EXJ and the ears of W50 are produced by the part of the wind whose Eddington-level kinetic luminosity is confined to a half-opening angle of 5-10 degrees. This is not necessarily a recollimated version of the 0.26 c jets.

We report the discovery of a faint radio filament near PSR J0538+2817 in the NVSS, CGPS, and the Rapid ASKAP Continuum Survey data. This pulsar is plausibly associated with the supernova that gave rise to the Spaghetti Nebula (Simeis 147). The structure is one-sided and appears to be almost aligned (within 17 degrees) with the direction of the pulsar's proper motion, but in contrast to the known cases of pulsar radio tails, it is located ahead of the pulsar. At the same time, this direction is also approximately (within 5 degrees) perpendicular to the axis of the extended non-thermal X-ray emission around the pulsar. No X-ray or optical emission is detected from the filament region, although the end point of the radio filament appears to be adjacent to a filament of H_α emission. We speculate that this structure might represent a filament connecting pulsar wind nebula with the ambient interstellar medium filled with relativistic electrons escaping the pulsar nebula, i.e. a radio analogue of X-ray filaments of Guitar and Lighthouse PWNs and filaments of non-thermal radio emission in the Galactic Center.

Simeis 147 (S147, G180.0-01.7, "Spaghetti nebula") is a supernova remnant (SNR) extensively studied across the entire electromagnetic spectrum, from radio to giga-electronvolt gamma-rays, except in X-rays. Here, we report the first detection of significant X-ray emission from the entire SNR using data of the extended ROentgen Survey Imaging Telescope Array (eROSITA) onboard the Russian-German Spektrum Roentgen Gamma (SRG). The object is located at the Galactic anticenter, and its 3 degrees size classifies it among the largest SNRs ever detected in X-rays. By employing similar to 15 years of Fermi-LAT data, our study confirms the association of the remnant with a spatially coincident diffuse giga-electronvolt excess, namely 4FGL J0540.3+2756e or FGES J0537.6+2751. The X-ray emission is purely thermal, exhibiting strong O, Ne, and Mg lines; whereas it lacks heavier-Z elements. The emission is mainly confined to the 0.5-1.0 keV band; no significant emission is detected above 2.0 keV. Both a collisional plasma model in equilibrium and a model of nonequilibrium collisional plasma can fit the total spectrum. While the equilibrium model - though statistically disfavored - cannot be excluded by X-ray fitting, only the absorption column of the nonequilibrium model is consistent with expectations derived from optical extinction data. Adopting an expansion in a homogeneous medium of typical interstellar medium (ISM) density, the general SNR properties are broadly consistent with an expansion model that yields an estimated age of similar to 0.66-2x10(5) yr, that is a rather old age. The preference for an X-ray-emitting plasma in nonequilibrium, however, adds to the observational evidence that favors a substantially younger age. In a companion paper, we explore an SNR-in-cavity scenario, resulting in a much younger age that alleviates some of the inconsistencies of the old-age scenario.

Current generation of ground based gamma-ray telescopes observed dozens of sources of photons above 100 TeV. Supernova remnants, pulsar wind nebulae, young stellar clusters and superbubbles are considered as possible sites of PeV-regime particles producing the radiation. Another possible source of PeV particles could be gamma-ray binary systems. In these systems, a strong relativistic outflow from a compact object (neutron star or black hole) collides with the dense wind from a massive companion early-type star. Gamma-ray binaries are observed from radio to high energy gamma-rays as luminous non-thermal sources. Apart from acceleration of very high energy leptons producing most of the non-thermal radiation, these systems may also efficiently accelerate protons. We present here the results of numerical simulation of the PeV-regime proton acceleration in gamma-ray binaries. The simulation is based on relativistic MHD modeling of local flows of magnetized plasma in the region of interaction of two colliding winds. We then inject 0.1 PeV protons into the system and directly follow their trajectories to demonstrate that they are accelerated to energies above PeV. High magnetization of the wind of the young massive star providing a Gauss range field in the winds interaction region is of paramount importance for the acceleration of protons above PeV. The maximum energies of protons accelerated by colliding winds in gamma-ray binaries can significantly exceed the energy of the pulsar potential’s drop, which limits from above the energy of particles accelerated by an isolated pulsar.

Gamma-ray bursts (GRBs) are the phenomena of rapid energy release of enormous power associated with the collapse or merging of stars. As a result of internal processes, populations of nonthermal accelerated particles radiating in a wide energy range are formed in them. A number of observations have shown that photons with energies up to tens of TeV are detected from some GRBs. However, due to the great energy losses of radiating particles, the explanation of this high-energy radiation in terms of standard radiation mechanisms runs into great difficulties. In this paper, based on the model of adiabatic expansion for the GRB afterglow phase, we investigate the influence of magnetic inhomogeneities on the spectra within the electron and proton synchrotron radiation mechanism by taking into account the Compton scattering of synchrotron photons. We show that the magnetic inhomogeneity effect can increase the maximum energies of the synchrotron radiation from electrons and protons several fold without affecting the maximum energies of the Compton photons being produced in the Klein–Nishina regime.

The core collapse of massive stars and compact relativistic star mergers are accompanied by a rapid release of an enormous amount of energy, of the order of the rest energy of a star. Supernovae and gamma-ray bursts associated with these processes are observed almost every day by modern telescopes. Radiation from such sources is observed across the entire electromagnetic spectrum. Neutrinos from supernova 1987A and gravitational waves from relativistic star mergers have been detected. Along with rapidly variable and transient events, relativistic compact remnants of a collapsed star & ETH; accreting black holes and fast rotating pulsars & ETH; demonstrate high X-ray and gamma-ray luminosity for significantly longer times. The Crab Nebula and pulsars in gamma-ray binaries are excellent galactic laboratories that allow studying relativistic winds acting as cosmic high-energy particle accelerators. The study of physical processes leading to the conversion of the gravitational and rotational energy of relativistic objects into powerful electromagnetic radiation and high-energy neutrino fluxes provides unique opportunities for testing fundamental physical laws under extreme conditions unattainable in laboratory experiments on Earth. In this paper, we briefly review the results of observations and modeling of nonthermal processes in cosmic sources of high-energy radiation and discuss the prospects for advances in these studies.

We show here that highly polarized x-ray synchrotron radiation from young supernova remnants (SNRs) can be modeled within the framework of diffusive shock acceleration (DSA) and nonlinear magnetic turbulence generation. Cosmic ray acceleration by SNR shocks to very high energies requires efficient magnetic turbulence amplification in the shock precursor. As the strong turbulence generated by Bell's instability far upstream from the viscous subshock convects through the subshock, nonlinear dynamical effects on the large amplitude, compressible fluctuations produce a downstream layer filled with strong anisotropic turbulence with predominantly radial magnetic fields. The synchrotron radiation from shock accelerated electrons in the turbulent downstream layer has a high degree of polarization shown to be consistent with recent observations of young SNRs by the Imaging X-ray Polarimetry Explorer (IXPE) taking into account high-energy electron losses and line-of-sight integration in a spherical remnant. In the case of our model of Tycho's SNR, the measured x-ray radiation constrains the thickness of the energy containing interval and the amplitude of cosmic ray driven magnetic turbulence, as well as the maximal energy of accelerated protons. The preferential direction of the x-ray polarization depends sensitively on the SNR shock velocity and the ambient density. A fast shock in a region with high enough density is a favorable place to produce tangential polarization of synchrotron radiation, i.e., a dominantly radial magnetic field. A unique feature of our model is the sensitive dependence of the degree and direction of x-ray polarization on the spatial overlap between regions of amplified magnetic turbulence and TeV electron populations. While this overlap occurs on scales orders of magnitude below the resolution of IXPE, , its polarization measurement allows testing of turbulent plasma processes on unprecedented scales.

We argue that the North Polar Spur (NPS) and many less prominent structures are formed by gaseous metal-rich plumes associated with star-forming regions (SFRs). The SFRs located at the tangent to the 3-5 kpc rings might be particularly relevant to the NPS. A multi-temperature mixture of gaseous components and cosmic rays rises above the Galactic disk under the action of their initial momentum and buoyancy. Eventually, the plume velocity becomes equal to that of the ambient gas, which rotates with different angular speeds than the stars in the disk. As a result, the plumes acquire characteristic bent shapes. An ad hoc model of plumes' trajectories shows an interesting resemblance to the morphology of structures seen in the radio continuum and X-rays.

Fast blue optical transients (FBOTs) represent a new class of highly energetic sources observed from radio to X-rays. High luminosity, light curves and spectra of the sources can be understood if they are associated with supernova-like or tidal disruption events. Radio observations of the transient sources revealed a mildly relativistic expansion of some of the remnants. The high power and mildly relativistic shock velocities are providing favorable conditions for very high energy particle acceleration. In this paper we present a model of particle acceleration in mildly relativistic magnetohydrodynamic (MHD) outflow of the transient source. To construct the non-thermal radiation and cosmic ray spectra in a broad range of energies we combined the microscopic particle-in-cell simulations of electron and proton injection at mildly relativistic shock with Monte Carlo technique for high energy particle transport and acceleration. The model allows describing the observed non-thermal radio and X-ray emission of CSS161010, and predicts that such sources can accelerate cosmic rays to energies above 10 PeV, and possible upper limit of maximum energy is 100 PeV. With the expected event rate of FBOTs they can contribute to the very high energy cosmic rays population in galaxies.

The Simeis 147 nebula (S147) is particularly well known for a spectacular net of H-alpha-emitting filaments. It is often considered one of the largest and oldest (similar to 10(5) yr) cataloged supernova remnants in the Milky Way, although the kinematics of the pulsar PSR J0538+2817 suggests that this supernova remnant might be a factor of three younger. The former case is considered in a companion paper, while here we pursue the latter. Both studies are based on the data of SRG/eROSITA All-Sky Survey observations. Here, we confront the inferred properties of the X-ray emitting gas data with the scenario of a supernova explosion in a low-density cavity, such as a wind-blown-bubble. This scenario assumes that a similar to 20 M-circle dot progenitor star has had a low velocity with respect to the ambient interstellar medium, and so stayed close to the center of a dense shell created during its main-sequence evolution till the moment of the core-collapse explosion. The ejecta first propagate through the low-density cavity until they collide with the dense shell, and only then does the reverse shock go deeper into the ejecta and power the observed X-ray emission of the nebula. The part of the remnant inside the dense shell remains non-radiative till this point, plausibly in a state with T-e

Mildly-relativistic outflows with shocks of velocities 0.1-0.7c were deduced from multiwavelength observations of powerful fast transient sources. These outflows are associated with merging relativistic objects, relativistic supernovae and fast blue optical transients. Relativistic magneto-hydrodynamic (RMHD) models of these objects rely on the equation of state of the fluid, which is a collisionless plasma with a contribution of non-thermal components. In this paper, we present kinetic simulations of mildly-relativistic shocks with Particle-in-Cell and Monte-Carlo techniques to derive the adiabatic index of plasma in the shock downstream directly from the particle distributions, which can be implemented into the RMHD models.

A minor population of antistars in galaxies has been predicted by some of nonstandard models of baryogenesis and nucleosynthesis in the early Universe, and their presence is not yet excluded by the currently available observations. Detection of an unusually high abundance of antinuclei in cosmic rays can probe the baryogenesis scenarios in the early Universe. Recent report of the AMS-02 collaboration on the tentative detection of a few antihelium nuclei in GeV cosmic rays provided a great hope on the progress in this issue. We discuss possible sources of antinuclei in cosmic rays from antistars which are predicted in a modified Affleck-Dine baryogenesis scenario by Dolgov and Silk (1993). The model allows us to estimate the expected fluxes and isotopic content of antinuclei in the GeV cosmic rays produced in scenarios involving antistars. We show that the flux of antihelium CRs reported by the AMS-02 experiment can be explained by Galactic anti-nova outbursts, thermonuclear anti-SN Ia explosions, a collection of flaring antistars, or an extragalactic source with abundances not violating existing gamma-ray and microlensing constraints on the antistar population.

We present the results of our analysis of the observations of the supernova remnant (SNR) G18.1-0.1 performed with the eROSITA X-ray telescope onboard the SRG observatory within the program of Galactic ridge observations. We have constructed an image that entirely covers the surroundings of the SNR located in the difficult-to-analyze diffuse emission region within the central radian of the Galaxy. The high sensitivity of the X-ray telescope in the 0.5–2.3 keV energy band and its angular resolution $${\sim}16^{\prime\prime}$$ have allowed us to construct for the first time the image and to determine the morphology of the asymmetric extended region of interaction between the ejecta of the supernova G18.1-0.1 and its environment. Our spectral analysis of the diffuse X-ray emission from the SNR, minus the discrete sources, has shown that its spectrum is well fitted by the thermal plasma emission model that takes into account the absorption in the interstellar medium. The plasma temperature $$T=0.49_{-0.13}^{+0.14}$$ keV and $$N_{\rm H}=5.8_{-1.0}^{+1.3}\times 10^{22}$$ cm $${}^{-2}$$ obtained as a result of our analysis are consistent with their previous estimates. An overabundance of Si, whose abundance is $$2.0_{-0.7}^{+1.5}$$ of the solar one, has been detected in the hot plasma spectrum of G18.1-0.1.

ABSTRACT We present the results of current observations of the young compact cluster of massive stars Westerlund 2 (Wd2) with the Mikhail Pavlinsky ART-XC telescope aboard the Spectrum-Roentgen-Gamma (SRG) observatory which we analysed together with the archival Chandra data. In general, Wd2 was detected over the whole electromagnetic spectrum including high-energy gamma rays, which revealed a cosmic ray acceleration in this object to the energies up to tens of TeV. The detection of Wd2 with ART-XC allowed us to perform a joint spectral analysis together with the high resolution Chandra observations of the diffuse emission from a few selected regions in the vicinity of the Wd2 core in the 0.4–20 keV range. To fit the Wd2 X-ray spectrum above a few keV one needs either a non-thermal power-law emission component, or a hot plasma with temperatures ∼ 5 keV. Our magnetohydrodynamic modelling of the plasma flows in Wd2 shows substantially lower electron temperatures in the system and thus the presence of the non-thermal component is certainly preferable. A kinetic model of the particle acceleration demonstrated that the non-thermal component may originate from the synchrotron radiation of multi-TeV electrons and positrons produced in Wd2 in accordance with the TeV photons detection from the source.

We study the nature of strong magnetic field oscillations in the vicinity of the Earth bow shock in regions of low interplanetary magnetic field.A hybrid kinetic model is compared to MMS observations.We show that with a magnetic field about 1 nT the shock front is formed mostly due to the ion Weibel instability, while with a magnetic field about 8.5 nT the impact of this instability is neglidgible.In the latter case the ratio of magnetic variation to the upstream magnetic field is an order less than that in the former.