Cosmic-ray acceleration and escape from post-adiabatic supernova remnants

We study production of secondary cosmic rays (CR) in supernova remnants (SNR). The model includes reacceleration of already existing in interstellar medium secondary CR particles as well as creation of secondary CRs in nuclear collisions of accelerated protons with gas nuclei and their subsequent acceleration by SNR shock. It is shown that production secondary CRs in SNRs produces considerable effect in their resultant energy spectrum making it essentially flatter above 10 GeV. It is shown that calculated energy spectra of antiprotons, positrons and secondary nuclei Li, Be, B are well consistent experimental data obtained in recent experiments PAMELA, Fermi and AMS-02.

The idea of Bhat et al. (1985) that the bulk of cosmic rays below 10 GeV are accelerated in supernova remnants suggests that cosmic rays should also exhibit intensity variations on a scale comparable with the linear size of a representative remnant. Following the general spirit of shock-wave acceleration models. The authors carry out Monte Carlo simulations to predict what this scale should be and then proceed to present corroborative evidence from an autocorrelation analysis of the COS B and SAS II gamma -ray data for the latitude range mod b mod =10-20 degrees ('near Galaxy') and mod b mod <or approximately=5 degrees ('far Galaxy').

Cosmic ray studies with the Fermi Gamma-ray Space Telescope Large Area Telescope

Context. Supernova remnants are known as sources of galactic cosmic rays for their non-thermal emission of radio waves, X-rays, and gamma-rays. However, the observed soft broken power-law spectra are hard to reproduce within standard acceleration theory based on the assumption of Bohm diffusion and steady-state calculations. Aims. We point out that a time-dependent treatment of the acceleration process together with a self-consistent treatment of the scattering turbulence amplification is necessary. Methods. We numerically solve the coupled system of transport equations for cosmic rays and isotropic Alfvenic turbulence. The equations are coupled through the growth rate of turbulence determined by the cosmic-ray gradient and the spatial diffusion coefficient of cosmic rays determined by the energy density of the turbulence. The system is solved on a co-moving expanding grid extending upstream for dozens of shock radii, allowing for the self-consistent study of cosmic-ray diffusion in the vicinity of their acceleration site. The transport equation for cosmic rays is solved in a test-particle approach. Results. We demonstrate that the system is typically not in a steady state. In fact, even after several thousand years of evolution, no equilibrium situation is reached. The resulting time-dependent particle spectra strongly differ from those derived assuming a steady state and Bohm diffusion. Our results indicate that proper accounting for the evolution of the scattering turbulence and hence the particle diffusion coefficient is crucial for the formation of the observed soft spectra. In any case, the need to continuously develop magnetic turbulence upstream of the shock introduces non-linearity in addition to that imposed by cosmic-ray feedback.

RX J1713.7–3946 is a unique supernova remnant (SNR) emitting very high energy \(\gamma \)-rays and X-rays, which are likely emitted by cosmic ray particles accelerated in the SNR via diffusive shock acceleration. It has been suggested that the SNR’s shockwave interacts with the dense gas clumps around the SNR, affecting the synchrotron X-ray radiation and cosmic ray acceleration. However, to date, there has not been a quantitative study to clarify the connection among the X-rays, gas clumps, and accelerated cosmic ray electrons. In this Chapter, we show a strong connection between synchrotron X-rays and the surrounding gas distribution by comparing the spatial distribution among CO, HI, and X-rays over the entire SNR. The present work is aimed at better understanding the SNR shock interstellar gas interaction and thereby establishing the origin of the distribution of the synchrotron X-rays in the SNR RX J1713.7–3946.

CONTEXT: Supernova Remnants (SNRs) are considered as the primary sources of galactic cosmic rays (CRs), where CRs are assumed to be accelerated by diffusive shock acceleration (DSA) mechanism. The SNR shocks expand in the complex ambient environment, particularly in the core-collapse scenarios as those SNRs evolve inside wind-blown bubbles created by the mass-loss of massive stars. Therefore, the evolution of core-collapse SNRs, as well as CRs acceleration is expected to be considerably different from SNR evolution in a uniform environment. AIM: The aim is to observe the influence of different ambient medium of core-collapse SNR shock on the particle spectra. Furthermore, the interactions of SNR shock with fluctuations in density within the wind-blown bubble generate several transmitted and reflected shocks. So, the impact of SNR shock interactions with different discontinuities, on particle spectra, and finally the effect on emission from the remnant are also the areas of focus. Methods. The hydrodynamic structures of wind-blown bubbles at pre-supernova stages formed by $20M_{\odot}$, $35M_{\odot}$, and $60M_{\odot}$ stars have been used to create the ambient environment for SNRs. Evolution of those stars through different stages from Zero Age Main Sequence (ZAMS) to the pre-supernova stage, results into formation of structurally different wind bubbles. Then, the transport equation for cosmic rays, and hydrodynamic equations have been solved simultaneously in 1-D spherical symmetry. RESULT: The modifications in particle spectra depend on the hydrodynamics and magnetic field structure of SNR ambient medium. We have obtained softer spectra with spectra index close to 2.5 originated during SNR interaction with hot wind bubble and further, magnetic field structure effectively influences the emission morphology of SNR as it governs the transportation of particles.

The hypothesis that supernova remnants are key sources of Galactic cosmic rays gains support from evidence that HESS J1731-347 one of the few Galactic objects capable of accelerating hadronic cosmic rays to TeV energies may harbor an exotic strange quark star rather than a conventional neutron star. This conclusion stems from its unusually low mass and compact radius, which challenge standard neutron star models. If confirmed, such a quark star could generate cosmic rays through the transition from the two-flavor color-superconducting (2SC) phase to the color-flavor-locked (CFL) phase, potentially releasing strangelets, hypothetical strange quark matter (SQM) particles. Detecting these strangelets in cosmic rays would provide groundbreaking evidence for quark matter. The future Cherenkov Telescope Array (CTA), with its unmatched sensitivity and spectral resolution in the very-high-energy (VHE) gamma-ray regime, is uniquely positioned to search for their annihilation or decay signatures. We analyze theoretical predictions for these gamma-ray signals and evaluate CTA’s potential to detect or constrain them. Additionally, we present an in-depth assessment of CTA observations of HESS J1731-347, focusing on spectral features that could confirm strangelet production. A positive detection would not only validate the existence of strange quark stars but also establish a direct link between quark matter and cosmic-ray acceleration, reshaping our understanding of compact objects and high-energy astrophysics.

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.

There are cosmic rays in the current universe. They play various important roles in the current universe. However, we do not understand when, where, how cosmic rays are first accelerated since the Big Bang. We show that supernova remnants of first stars can accelerate the first cosmic rays at z~20. Shock waves of the first supernova remnants are nonrelativistic Weibel mediated shock, so that the coherent length scale of magnetic field fluctuations is much smaller than the gyroradius of the accelerated particles. Therefore, the maximum energy of the first CRs becomes much smaller than that in the current universe, which is about 100 MeV. Furthermore, we discuss cosmic-ray acceleration by accretion shocks due to the cosmological structure formation at z~20. Since the accretion shocks at z~20 propagate not an ionized medium but a neutral medium, they cannot accelerate the first cosmic rays.

We discuss new methods to integrate the cosmic ray (CR) evolution equations\ncoupled to magneto-hydrodynamics (MHD) on an unstructured moving mesh, as\nrealised in the massively parallel AREPO code for cosmological simulations. We\naccount for diffusive shock acceleration of CRs at resolved shocks and at\nsupernova remnants in the interstellar medium (ISM), and follow the advective\nCR transport within the magnetised plasma, as well as anisotropic diffusive\ntransport of CRs along the local magnetic field. CR losses are included in\nterms of Coulomb and hadronic interactions with the thermal plasma. We\ndemonstrate the accuracy of our formalism for CR acceleration at shocks through\nsimulations of plane-parallel shock tubes that are compared to newly derived\nexact solutions of the Riemann shock tube problem with CR acceleration. We find\nthat the increased compressibility of the post-shock plasma due to the produced\nCRs decreases the shock speed. However, CR acceleration at spherically\nexpanding blast waves does not significantly break the self-similarity of the\nSedov-Taylor solution; the resulting modifications can be approximated by a\nsuitably adjusted, but constant adiabatic index. In first applications of the\nnew CR formalism to simulations of isolated galaxies and cosmic structure\nformation, we find that CRs add an important pressure component to the ISM that\nincreases the vertical scale height of disk galaxies, and thus reduces the star\nformation rate. Strong external structure formation shocks inject CRs into the\ngas, but the relative pressure of this component decreases towards halo centres\nas adiabatic compression favours the thermal over the CR pressure.\n

The direct measurements of cosmic rays (CRs), after correction for the propagation effects in the interstellar medium, indicate that their source spectra are likely to be significantly steeper than the canonical E −2 spectrum predicted by the standard diffusive shock acceleration (DSA) mechanism. The DSA has long been held responsible for the production of galactic CRs in supernova remnant (SNR) shocks. The γ-ray “probes” of the acceleration spectra of CRs on the spot, inside the SNRs, lead to the same conclusion. We show that the steep acceleration spectrum can be attributed to the combination of (i) spherical expansion, (ii) tilting of the magnetic field along the shock surface, and (iii) shock deceleration. Because of (i) and (ii), the DSA is efficient only on two “polar caps” of a spherical shock where the local magnetic field is within ≃45° to its normal. The shock-produced spectrum observed edge-on steepens with the particle energy because the number of freshly accelerated particles with lower energies continually adds up to a growing acceleration region. We demonstrate the steepening effect by obtaining an exact self-similar solution for the particle acceleration at an expanding shock surface with an arbitrary energy dependence of particle diffusivity κ. We show that its increase toward higher energy steepens the spectrum, which deeply contrasts with the standard DSA spectrum where κ cancels out.

The origin of Galactic Cosmic rays (GCRs), whose spectrum extends to PeV energies, is one of the longest-standing problems in astroparticle physics. One of the main sources of GCRs are regarded to be Supernova remnants (SNRs). While SNRs are known to accelerate protons, so far there is no evidence that SNRs can accelerate CRs to PeV energies. Providing that ~10% of the parent Cosmic ray energy is converted to gamma rays, the gamma-ray spectrum extending up to ~100 TeV would be a signature of a so-called Galactic PeVatron, an object responsible for the production of protons up to the knee of the Cosmic ray spectrum. The current multi-wavelength data indicate that HESS J1809-193 is one of the most promising Galactic PeVatron candidates. So far, no firm identification on the source nature has been established as there are several possible counterparts at lower energies; one of them being SNR G11.0−0.0. We report here the results of an observational campaign performed by the MAGIC telescopes on HESS J1809-193 since 2019 in the very-high-energy gamma-ray domain (E>100 GeV). The data were obtained with the Very Large Zenith Angle (VLZA) technique, which increased the collection area significantly to about one square kilometer. We used ~60 hours of collected VLZA data to explore the spectrum and the morphology of the source at energies above several TeV.

Introduction. High-energy processes in the interstellar medium (ISM) of our Galaxy are accompanied by the acceleration of a small fraction of nuclei and electrons to ultra-relativistic velocities. Accelerated particles – cosmic rays – generate non-thermal radiation. In particular, electrons – synchrotron and inverse Compton radiation (leptonic mechanism), protons and heavier nuclei – high-energy gamma-ray emission as a result of inelastic collisions with ISM atoms, in which, in particular, pions are born and quickly decay into two gamma-photons (hadronic mechanism). Supernova remnants and pulsar-wind nebulae are potential accelerators of Galactic cosmic rays. The work simulates the gamma-ray emission of the unidentified extended gamma-ray source 4FGL J1908.6+0915e within the framework of hadronic and leptonic mechanisms working in the Supernova remnants G42.8+0.6, G043.023+0.762, G043.070+0.558 and their pulsar-wind nebulae, which positionally coincide with the source 4FGL J1908.6+0915e on the sky map. Methods. In order to model the non-thermal radiation (from the radio to the gamma-ray range) of the source 4FGL J1908.6+0915e, the observed spectrum of gamma-ray emission obtained by space-based (Fermi-LAT) and ground-based (H.E.S.S., HAWC) gamma-ray telescopes was approximated by a power-law with exponential cut-off. The NAIMA software code (Zabalza, 2016) was used for calculation the model spectra. The characteristics of potential sources of gamma-ray emission – of the Supernova remnants G42.8+0.6, G043.023+0.762 and G043.070+0.558 – were determined from the correspondence of the model spectra to the approximation of the observed spectrum. Results. It is shown that the observed gamma-ray spectral flux of the source 4FGL J1908.6+0915e can be provided by each of the Supernova remnants G42.8+0.6, G043.023+0.762 and G043.070+0.558 or as a sum of their contributions. At the same time, due to the relative proximity of these remnants to the Earth (3-5 kpc), the required energy of the explosion of these Supernovae corresponds to the typical energy of Galactic Supernovae – about 1051 ergs. Conclusions The unidentified extended source 4FGL J1908.6+0915e coincides in the sky position with the the magnetar SGR1900+14. In the work of Hnatyk et al. (2022) it is shown that it can be a manifestation of a Supernova remnant or a magnetar-wind nebula associated with a magnetar. But the necessary energies of the Supernova associated with the birth of the magnetar turn out to be extremely large – on the order of 1052 ergs due to the large distance to the magnetar – about 12.5 kpc. Such Supernovae – they are called Hypernovae – are rare phenomena in the Galaxy. In this paper, we have shown that other Supernova remnants G42.8+0.6, G043.023+0.762 and G043.070+0.558, which also coincide with the source 4FGL J1908.6+0915e on the sky map, but are at closer distances – 3-5 kpc – can provide the observed spectral flux already at typical energies of Supernova remnants – about 1051 ergs.

Galactic cosmic rays are widely thought to be accelerated at supernova remnants (SNRs). SNR RX J1713.7−3946 is the strong sources of nonthermal radiation, making it one of the most well studied particle accelerators in our Galaxy. From the Chandra measurement of the proper motions in the northwest region of RX J1713.7−3946, the blast-wave shock speed is estimated as 3900 km/s. This relatively fast shock velocity, combined with the standard analytic solutions that describe the hydrodynamical properties of SNR evolution, supports the connection with SN393, and suggests that RX J1713.7−3946 would not have exited the ejecta-dominated phase, implying that the energy of accelerated particles has not reached the maximum yet. We have recently performed hard X-ray observations of RX J1713.7−3946 with NuSTAR (3–79 keV), providing fisrt imaging observations of RX J1713.7−3946 at the hard X-ray band above 10 keV. In preliminary fashion, we present spatially-resolved spectral analysis of the northwest part of this remnant and report the detection of an extremely hard X-ray component with NuSTAR.

Sources and mechanisms of acceleration of the cosmic rays (CR) – a stream of nuclei, electrons and, to a lesser extent, antiparticles with a nonthermal power law energy spectrum extending to energies above 1020 eV, are still unknown. Theoretical calculations and experimental data show that the diffusive acceleration of the CR at the shock wave fronts (first order Fermi mechanism) in Galactic sources, first of all in the Supernova remnants (SNR), may be responsible for the observed CR flux with energies up to E ≤ 1018 eV. In this case, the lepton component of the CR (electrons, positrons) is confidently manifested in the SNR due to synchrotron radiation in a turbulent magnetic field inside the SNR. At the same time, the proton and nuclear component is not effective in synchrotron emission under typical CR parameters and magnetic fields in the SNR. Its presence in the SNR can be revealed due to the hadron mechanism of gamma-ray generation – the decay of neutral pions generated by inelastic collisions of the CR with particles-targets – atoms of interstellar medium inside and in the vicinity of SNR. Now only a few SNRs with a detectable gamma-ray flux of a hadron origin are known, in which the interaction of SNR with molecular clouds is observed, which provides the necessary concentration of particles-targets for CR. In our work we calculate the expected fluxes of such gamma-radiation from the Vela SNR, one of the closest SNR to the Earth, which evolves in a cloudy interstellar medium and is a potential candidate for the hadron mechanism gamma-ray source. The possibility of detecting Vela SNR with modern (Magic, HAWC, H.E.S.S.) and future (CTA) gamma-ray detectors is analyzed. It is shown that the sensitivity of CTA will be sufficient to detect the gamma-radiation generated by the hadron mechanism. Also we analyzed the possibility of registration of localized sources of gamma-radiation within the Vela SNR due to the presence of high gas density clouds. The fluxes from the two most massive clouds within the Vela SNR are calculated and compared with the sensitivity of the modern and future gamma-ray detectors. It is shown that sufficient sensitivity to detect the most massive clouds within the SNR will have the CTA facility if the spectrum of the gamma-rays generated by the hadron mechanism is without cut-off on the energy 10 TeV.