Cosmic Rays In Supernova Remnants Research Articles

Limits on compression of cosmic rays in supernova remnants

ABSTRACTThe spectral shape of the gamma-ray emission observed for dynamically old supernova remnants that interact with molecular clouds triggered an exciting scenario of adiabatic compression and farther re-acceleration of Galactic cosmic rays (GCRs) in radiative shells of the remnants, which was extensively discussed and applied to various sources over recent years. Indeed, the observed gamma-ray spectrum from a number of remnants strongly resembles the expected spectrum of the gamma-ray emission from the compressed population of GCRs. In the following we discuss the feasibility of this scenario and show that it is very unlikely that compressed GCRs could produce sufficient amount of gamma-rays and that the observed spectral shape is putting strong limits on the allowed compression factors. Further, absence of curvature in featureless power-law spectra of evolved supernova remnants at radio wavelengths is strongly disfavouring the compression scenario for electrons and hence for hadrons. Our calculations show that the contribution of compressed electrons to the observed radio flux could reach at most $\sim 10\,$ per cent.

Long-term Evolution of Nonthermal Emission from Type Ia and Core-collapse Supernova Remnants in a Diversified Circumstellar Medium

The contribution of galactic supernova remnants (SNRs) to the origin of cosmic rays (CRs) is an important open question in modern astrophysics. Broadband nonthermal emission is a useful proxy for probing the energy budget and production history of CRs in SNRs. We conduct hydrodynamic simulations to model the long-term SNR evolution from explosion all the way to the radiative phase (or 3 × 105 yr at maximum) and compute the time evolution of the broadband nonthermal spectrum to explore its potential applications on constraining the surrounding environments, as well as the natures and mass-loss histories, of the SNR progenitors. A parametric survey is performed on the ambient environments separated into two main groups, namely, a homogeneous medium with a uniform gas density and one with the presence of a circumstellar structure created by the stellar wind of a massive red supergiant progenitor star. Our results reveal a highly diverse evolution history of the nonthermal emission closely correlated to the environmental characteristics of an SNR. Up to the radiative phase, the roles of CR reacceleration and ion−neutral wave damping on the spectral evolution are investigated. Finally, we make an assessment of the future prospect of SNR observations by the next-generation hard X-ray space observatory FORCE and predict what we can learn from their comparison with our evolution models.

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

Context. Supernova remnants are known to accelerate cosmic rays on account of their nonthermal emission of radio waves, X-rays, and gamma rays. Although there are many models for the acceleration of cosmic rays in supernova remnants, the escape of cosmic rays from these sources has not yet been adequately studied. Aims. We aim to use our time-dependent acceleration code RATPaC to study the acceleration of cosmic rays and their escape in post-adiabatic supernova remnants and calculate the subsequent gamma-ray emission from inverse-Compton scattering and Pion decay. Methods. We performed spherically symmetric 1D simulations in which we simultaneously solved the transport equations for cosmic rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in a volume large enough to keep all cosmic rays in the simulation. The transport equations for cosmic rays and magnetic turbulence were coupled via the cosmic-ray gradient and the spatial diffusion coefficient of the cosmic rays, while the cosmic-ray feedback onto the shock structure can be ignored. Our simulations span 100 000 years, thus covering the free-expansion, the Sedov–Taylor, and the beginning of the post-adiabatic phase of the remnant’s evolution. Results. At later stages of the evolution, cosmic rays over a wide range of energy can reside outside of the remnant, creating spectra that are softer than predicted by standard diffusive shock acceleration, and feature breaks in the 10 − 100 GeV-range. The total spectrum of cosmic rays released into the interstellar medium has a spectral index of s ≈ 2.4 above roughly 10 GeV which is close to that required by Galactic propagation models. We further find the gamma-ray luminosity to peak around an age of 4000 years for inverse-Compton-dominated high-energy emission. Remnants expanding in low-density media generally emit more inverse-Compton radiation, matching the fact that the brightest known supernova remnants – RCW86, Vela Jr., HESS J1731−347 and RX J1713.7−3946 – are all expanding in low density environments.

X-Ray Polarimetry of Magnetic Turbulence and Cosmic Rays in Supernova Remnants

Young supernova remnants (SNRs) are effective accelerators of charged particles. Particle acceleration is accompanied by the amplification of turbulent magnetic fields. The structure of SNR synchrotron intensity and polarization maps is sensitive to features of this field, especially in the X-ray energy band. Numerical modeling of synchrotron SNR maps is performed for different models of magnetic turbulence. The capabilities of a new-generation IXPE polarimeter designed to study magnetic turbulence in the vicinity of SNRs are discussed.

Very high energy astrophysics with the SHALON Cherenkov telescopes

Abstract The base of experiments using the imaging atmospheric Cherenkov telescopes is that the Cherenkov light emitted from the particles of extensive air shower created by the primary gamma-ray is collected by a mirror reflector and then detected by a pixelized PMT camera. SHALON are the imaging atmospheric Cherenkov telescopes created for the gamma-ray astronomy at the energies of 800 GeV to 100 TeV. Telescopes are located in the Tien-Shan mountains at the altitude of 3340 m a.s.l. The telescope systems have characteristics sufficient to record precise information about the shower structure in the energy range under consideration. SHALON experiment has been operating since 1992 and covers the wide astroparticle physics topics including an acceleration and origin of cosmic rays in supernova remnants , the physics of relativistic flaring objects like a black holes and active galactic nuclei as well as the long-term studies of the different type objects.

Probing maximum energy of cosmic rays in SNR through gamma rays and neutrinos from the molecular clouds around SNR W28

The galactic cosmic rays are generally believed to be originated in supernova remnants (SNRs), produced in diffusive shock acceleration (DSA) process in supernova blast waves driven by expanding SNRs. One of the key unsettled issue in SNR origin of cosmic ray model is the maximum attainable energy by a cosmic ray particle in the supernova shock. Recently it has been suggested that an amplification of effective magnetic field strength at the shock may take place in young SNRs due to growth of magnetic waves induced by accelerated cosmic rays and as a result the maximum energy achieved by cosmic rays in SNR may reach the knee energy instead of ∼ 200 TeV as predicted earlier under normal magnetic field situation. In the present work we investigate the implication of such maximum energy scenarios on TeV gamma rays and neutrino fluxes from the molecular clouds interacting with the SNR W28. The authors compute the gamma-ray and neutrino flux assuming two different values for the maximum energy reached by cosmic rays in the SNR, from CR interaction in nearby molecular clouds. Both protons and nuclei are considered as accelerated particles and as target material. Our findings suggest that the issue of the maximum energy of cosmic rays in SNRs can be observationally settled by the upcoming gamma-ray experiment the Large High Altitude Air Shower Observatory (LHAASO). The estimated neutrino fluxes from the molecular clouds are , however, out of reach of the present/near future generation of neutrino telescopes.

Contribution of Supernova Remnants to CRs Secondary to Primary ratios

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 generation of 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 with experimental data obtained in recent experiments PAMELA, Fermi and AMS-02.

Possible evidence for the stochastic acceleration of secondary antiprotons by supernova remnants

The antiproton-to-proton ratio in the cosmic-ray spectrum is a sensitive probe of new physics. Using recent measurements of the cosmic-ray antiproton and proton fluxes in the energy range of 1-1000 GeV, we study the contribution to the $\bar{p}/p$ ratio from secondary antiprotons that are produced and subsequently accelerated within individual supernova remnants. We consider several well-motivated models for cosmic-ray propagation in the interstellar medium and marginalize our results over the uncertainties related to the antiproton production cross section and the time-, charge-, and energy-dependent effects of solar modulation. We find that the increase in the $\bar{p}/p$ ratio observed at rigidities above $\sim$ 100 GV cannot be accounted for within the context of conventional cosmic-ray propagation models, but is consistent with scenarios in which cosmic-ray antiprotons are produced and subsequently accelerated by shocks within a given supernova remnant. In light of this, the acceleration of secondary cosmic rays in supernova remnants is predicted to substantially contribute to the cosmic-ray positron spectrum, accounting for a significant fraction of the observed positron excess.

The connection between supernova remnants and the Galactic magnetic field: An analysis of quasi-parallel and quasi-perpendicular cosmic-ray acceleration for the axisymmetric sample

The mechanism for acceleration of cosmic rays in supernova remnants (SNRs) is an outstanding question in the field. We model a sample of 32 axisymmetric SNRs using the quasi-perpendicular and quasi-parallel cosmic-ray-electron (CRE) acceleration cases. The axisymmetric sample is defined to include SNRs with a double-sided, bilateral morphology, and also those with a one-sided morphology where one limb is much brighter than the other. Using a coordinate transformation technique, we insert a bubble-like model SNR into a model of the Galactic magnetic field. Since radio emission of SNRs is dominated by synchrotron emission and since this emission depends on the magnetic field and CRE distribution, we are able to simulate the SNRs emission and compare this to data. We find that the quasi-perpendicular CRE acceleration case is much more consistent with the data than the quasi-parallel CRE acceleration case, with G327.6+14.6 (SN1006) being a notable exception. We propose that SN1006 may be a case where both quasi-parallel and quasi-perpendicular acceleration are simultaneously at play in a single SNR.

STOCHASTIC ACCELERATION OF GALACTIC COSMIC RAYS BY COMPRESSIBLE PLASMA FLUCTUATIONS IN SUPERNOVA SHELLS

A theory of 2-stage acceleration of Galactic cosmic rays in supernova remnants is proposed. The first stage is accomplished by the supernova shock front, where a power-law spectrum is established up to a certain cutoff energy. It is followed by stochastic acceleration with compressible waves/turbulence in the downstream medium. With a broad spectrum for the compressible plasma fluctuations, the rate of stochastic acceleration is constant over a wide range of particle momentum. In this case, the stochastic acceleration process extends the power-law spectrum cutoff energy of Galactic cosmic rays to the knee without changing the spectral slope. This situation happens as long as the rate of stochastic acceleration is faster than 1/5 of the adiabatic cooling rate. A steeper spectrum of compressible plasma fluctuations that concentrate their power in long wavelengths will accelerate cosmic rays to the knee with a small bump before its cutoff in the comic-ray energy spectrum. This theory does not require a strong amplification of the magnetic field in the upstream interstellar medium in order to accelerate cosmic rays to the knee energy.

Gamma-ray astronomy with H.E.S.S

H.E.S.S. is an array of imaging atmospheric Cherenkov telescopes for ground based gamma-ray astronomy. Consisting of four telescopes with 107m2 mirror area each, operational since 10 years, and a new 600m2 telescope in the center of the array, it uses Cherenkov light emitted by gamma-ray induced particle showers in the atmosphere to investigate high-energy phenomena in the universe. H.E.S.S. has discovered of the order of 60% of all very-high energy gamma-ray sources. Recent results cover physics topics like the acceleration of cosmic rays in supernova remnants, the interaction of cosmic rays with molecular clouds, or the acceleration and the emission processes in binary systems. Other topics covered are the physics of relativistic flows in pulsar wind nebulae and extra-galactic objects like AGN or radio galaxies as well as exotic physics like the search for axions.

Gamma rays from cosmic rays in supernova remnants

Context: Cosmic rays are thought to be accelerated at supernova remnant (SNR) shocks, but conclusive evidence is lacking. Aims: New data from ground-based gamma-ray telescopes and the Large Area Telescope on the Fermi Gamma-ray Space Telescope are used to test this hypothesis. A simple model for gamma-ray production efficiency is compared with measured gamma-ray luminosities of SNRs, and the GeV to TeV fluxes ratios of SNRs are examined for correlations with SNR ages. Methods: The supernova explosion is modeled as an expanding spherical shell of material that sweeps up matter from the surrounding interstellar medium (ISM). The accumulated kinetic energy of the shell, which provides the energy available for nonthermal particle acceleration, changes when matter is swept up from the ISM and the SNR shell decelerates. A fraction of this energy is assumed to be converted into the energy of cosmic-ray electrons or protons. Three different particle radiation processes---nuclear pion-production interactions, nonthermal electron bremsstrahlung, and Compton scattering---are considered. Results: The efficiencies for gamma-ray production by these three processes are compared with gamma-ray luminosities of SNRs. Our results suggest that SNRs become less gamma-ray luminous at >~ 10^4 yr, and are consistent with the hypothesis that supernova remnants accelerate cosmic rays with an efficiency of ~10% for the dissipation of kinetic energy into nonthermal cosmic rays. Weak evidence for an increasing GeV to TeV flux ratio with SNR age is found.

The origin of cosmic rays and TeV gamma-ray astronomy

Cosmic rays are accelerated to high energies in Galactic and extragalactic objects like Supernova remnants (SNR) and active galactic nuclei (AGN). How these accelerators work and how e cient they accel- erate di erent types of particles to energies of 10 15 eV or beyond, is 100 years after the discovery of cosmic rays by Victor Hess, still unknown. Gamma rays trace cosmic rays at their site of acceleration and give crucial information on the nature and inner workings of these extreme objects. Gamma rays can be used to find the sources of cosmic rays and to determine their type, age and dynamics. We review in these proceedings the observational techniques and recent findings on gamma-ray emission from Supernova remnants. In 1912, the Austrian physicist Victor F. Hess discovered by means of several balloon ascents the particle popula- tion in the universe of highest energy: cosmic rays. Their energy spectrum can be described by a power law cov- ering 10 order of magnitudes in energy (up to 10 20 eV) and about 30 orders of magnitude in flux. Cosmic rays consist mainly of nuclei like protons, helium and heav- ier types up to iron. Supernova remnants (SNRs) have been the prime candidates as sources of galactic cosmic rays since the 1960s. The working hypothesis is that a 10% conversion of the blast wave's energy into relativis- tic particles during the lifetime of a SNR are enough to fill the Galaxy with cosmic rays up to about 10 15 eV. To- day, there is much unequivocal evidence for the accelera- tion of cosmic rays in SNRs from observations at di er- ent wavelengths: the expansion measurements in X-rays (1); observation of magnetic field amplification as byprod- uct of particle acceleration (2); dynamical measurements of shock positions and shock temperature (3) and non- thermal emission in gamma rays (e.g.(4)). The mechanism of di usive shock acceleration is a solid theoretical base for the production of cosmic rays by SNRs. Gamma rays can be used to trace and identify high- energy particles at or close to their acceleration site. The di erent production mechanisms for gamma rays depend on energy and type of the parent particle population and the surrounding photon, magnetic and matter fields. High- energy protons can produce gamma rays through proton- proton or proton-gamma interaction, where secondary neutral pions decay into high-energy photons. Long-living secondary particles in these interactions can initiate parti- cle cascades where photons can be produced through pair production or synchrotron radiation. High-energy leptons

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

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope provides both direct and indirect measurements of galactic cosmic rays (CR). The LAT high-statistics observations of the 7GeV – 1TeV electron plus positron spectrum and limits on spatial anisotropy constrain models for this cosmic-ray component. On a galactic scale, the LAT observations indicate that cosmic-ray sources may be more plentiful in the outer Galaxy than expected or that the scale height of the cosmic-ray diffusive halo is larger than conventional models. Production of cosmic rays in supernova remnants (SNR) is supported by the LAT γ-ray studies of several of these, both young SNR and those interacting with molecular clouds.

Diffusive and Shock-Drift Acceleration in Relativistic Shocks

It is accepted that the shock acceleration mechanism can explain the non-thermal cosmic rays in supernova remnants, active galactic nuclei and gamma ray bursts. Especially, the importance of relativistic shock acceleration of cosmic rays in extragalactic sources is an important subject of study. In this work we will discuss the shock acceleration mechanism and will review the properties of non-relativistic and relativistic shocks, particularly focusing on relativistic Monte Carlo simulation studies.

Numerical modeling of relativistic particle shock acceleration

The shock acceleration mechanism is invoked to explain non-thermal cosmic rays in Supernova Remnants, Active Galactic Nuclei and Gamma Ray Bursts jets. Espe- cially, the importance of relativistic shock acceleration in ex- tragalactic sources is a recurring theme raising a significant interest in the research community. We will briefly overview the shock acceleration mechanism and we will address the properties of non-relativistic and relativistic shocks, particu- larly focusing on relativistic simulation studies. 1 Outline It is accepted that Super Novae Remnants (SNRs) are plau- sible environments for the acceleration of cosmic-ray parti- cles up to energies of about 10 17 eV, while for the higher en- ergies, Active Galactic Nuclei (AGN) and possibly Gamma Ray Bursts (GRBs) are favorable candidates. It is believed that the source of cosmic rays is plasma colliding at su- personic speeds, where shock waves form along with other instabilities, competing for the dissipation and acceleration mechanisms. It is until now not fully understood to which extent the astrophysical bulk flows are due to leptonic flow (electrons and positrons) or baryonic flow (electrons and ions), and by which exact mechanisms this bulk flow energy can be con- verted into cosmic ray radiation, reaching energies of TeV and beyond. Evidence in form of power-law spectra of the observed cosmic ray radiation over wide energy intervals, favors the Fermi shock acceleration mechanism (i.e. first or- der Fermi acceleration), namely diffusive shock acceleration mechanism, which raises a significant interest in the research community. In the diffusive shock acceleration mechanism

THE BROADBAND EMISSION FROM SUPERNOVA REMNANT RX J0852.0-4622

We study the nonthermal emission from SNR RX J0852.0-4622 based on a self-consistent kinetic method that describes the nonlinear shock acceleration of cosmic rays in supernova remnants (SNRs). In this method, the spectrum of accelerated protons in the SNR can be self-consistently calculated, where a proton's maximum momentum is determined by equating the SNR's age with the acceleration time of the proton. At the same time, the spectrum of accelerated electrons is similar to the proton's spectrum if the electron's momentum is much less than the electron's maximum momentum (p e,max), which is estimated by equating the synchrotron loss time to the acceleration time of the electrons, but the cutoff shapes around p e,max are assumed to be exponential. Using the accelerated particle's spectra, we calculate nonthermal photon spectra for different values of some main model parameters such as the SNR's age, an injection parameter, and a background magnetic field. Moreover, we study possible (hadronic or leptonic) origins of very high energy (VHE) γ-ray emission from SNR RX J0852.0-4622. Our results indicate that a hadronic origin of VHE γ-rays from SNR RX J0852.0-4622 seems to be more reasonable although a leptonic origin cannot be ruled out. We suggest that the observations of Fermi LAT for this remnant will help us find the evidence to determine the main emission mechanism.

CTA—A project for a new generation of Cherenkov telescopes

Gamma-rays provide a powerful insight into the non-thermal universe and perhaps a unique probe for new physics beyond the standard model. Current experiments are already giving results in the physics of acceleration of cosmic rays in supernova remnants, pulsar and active galactic nuclei with a 100 sources detected at very-high-energies so far. Despite its relatively recent appearance, very high-energy gamma-ray astronomy has proven to have reached a mature technology with fast assembling, relatively cheap and reliable telescopes. The goal of future installation is to increase the sensitivity by a factor 10 compared to current installations, and enlarge the energy domain from few 10s of GeV to a 100 TeV. Gamma-ray spectra of astrophysical origin are rather soft thus hardly one single size telescope can cover more than 1.5 decades in energy, therefore an array of telescopes of 2–3 different sizes is required. Hereafter, we present design considerations for a Cherenkov Telescope Array (CTA), a project for a new generation of highly automated telescopes for gamma-ray astronomy. The status of the project, technical solutions and an insight in the involved physics will be presented.

Cosmic‐Ray Evolution in Parker‐unstable Galactic Magnetic Fields

We discuss the Parker instability of the Galactic interstellar medium, focusing on the cosmic-ray gas injection by supernova remnants and its propagation, through the diffusion equation. We describe the Parker instability in the thin flux-tube approximation and solve the flux-tube and the diffusion equations numerically. We find that the Parker instability triggered by the injection of cosmic rays in supernova remnants is faster than the Parker instability excited by the small vertical perturbation and that there is no lower limit for the horizontal scale of the raised magnetic loops. We find a very strong dynamo effect and discuss the vertical transport of cosmic rays in the Galactic disk across the large-scale horizontal magnetic field.

Composition of cosmic rays accelerated in supernova remnants

The kinetic theory of regular acceleration of cosmic rays in supernova remnants is used to investigate the expected chemical composition of the rays. It is shown that the shapes of the calculated profiles of the chemical elements making up the cosmic rays are consistent with experiment wherever the results of measurements are available. The acceleration process is accompanied by relative enrichment of the cosmic rays with heavy elements. If the analogous property of the mechanism underlying the injection of superthermal particles into the acceleration regime is taken into account, such enrichment supports the formation of the required composition of cosmic rays in the energy range up to 1014–1015 eV.