Acceleration Of Cosmic-ray Electrons Research Articles

Radio relics radio emission from multishock scenario

ABSTRACT Radio relics are giant (∼Mpc) synchrotron sources that are believed to be produced by the (re)acceleration of cosmic ray electrons (CRe) by shocks in the intracluster medium. In this numerical study, we focus on the possibility that some radio relics may arise when electrons undergo diffusive shock acceleration at multishocks in the outskirts of merging galaxy clusters. This multishock (MS) scenario appears viable to produce CRe that emit visible synchrotron emission. We show that electrons that have been shocked multiple times develop an energy spectrum that significantly differs from the power-law spectrum expected in the case of a single shock scenario. As a consequence, the radio emission generated by CRe that shocked multiple times is higher than the emission produced by CRe that are shocked only once. In the case explored in this paper, the radio emission produced in the two scenarios differ by one order of magnitude. In particular in the MS scenario, the simulated relic follows a KGJP spectral shape, consistent with observation. Furthermore, the produced radio emission is large enough to be detectable with current radio telescopes (e.g. LOFAR, JVLA).

Morphology of radio relics – II. Properties of polarized emission

ABSTRACTRadio relics are diffuse radio sources in galaxy clusters that are associated with merger shock waves. Detailed observations of radio relics in total intensity and in polarization show complex structures on kiloparsec scales. The relation between the observed features and the underlying morphology of the magnetic field is not clear. Using 3D magneto-hydrodynamical simulations, we study the polarized emission produced by a shock wave that propagates through a turbulent medium that resembles the intracluster medium. We model the polarized synchrotron emission on the basis of diffusive shock acceleration of cosmic ray electrons. We find that the synchrotron emission produced in a shocked turbulent medium can reproduce some of the observed features in radio relics. Shock compression can give rise to a high polarization fraction at the shock front and a partial alignment of the polarization E-vectors with the shock normal. Our work confirms that radio relics can also be formed in an environment with a tangled magnetic field. We also discuss the effect of Faraday rotation intrinsic to the source, and how our results depend on the angular resolution of observations.

Shock waves in the magnetized cosmic web: the role of obliquity and cosmic ray acceleration

ABSTRACT Structure formation shocks are believed to be the largest accelerators of cosmic rays in the Universe. However, little is still known about their efficiency in accelerating relativistic electrons and protons as a function of their magnetization properties, i.e. of their magnetic field strength and topology. In this work, we analysed both uniform and adaptive mesh resolution simulations of large-scale structures with the magnetohydrodynamical grid code enzo, studying the dependence of shock obliquity with different realistic scenarios of cosmic magnetism. We found that shock obliquities are more often perpendicular than what would be expected from a random 3D distribution of vectors, and that this effect is particularly prominent in the proximity of filaments, due to the action of local shear motions. By coupling these results to recent works from particle-in-cell simulations, we estimated the flux of cosmic ray protons in galaxy clusters, and showed that in principle the riddle of the missed detection of hadronic γ-ray emission by the Fermi-LAT can be explained if only quasi-parallel shocks accelerate protons. On the other hand, for most of the cosmic web the acceleration of cosmic ray electrons is still allowed, due to the abundance of quasi-perpendicular shocks. We discuss quantitative differences between the analysed models of magnetization of cosmic structures, which become more significant at low cosmic overdensities.

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.

The challenge of turbulent acceleration of relativistic particles in the intra-cluster medium

Acceleration of cosmic-ray electrons (CRe) in the intra-cluster medium (ICM) is probed by radio observations that detect diffuse, megaparsec-scale, synchrotron sources in a fraction of galaxy clusters. Giant radio halos are the most spectacular manifestations of non-thermal activity in the ICM and are currently explained assuming that turbulence, driven during massive cluster–cluster mergers, reaccelerates CRe at several giga-electron volts. This scenario implies a hierarchy of complex mechanisms in the ICM that drain energy from large scales into electromagnetic fluctuations in the plasma and collisionless mechanisms of particle acceleration at much smaller scales. In this paper we focus on the physics of acceleration by compressible turbulence. The spectrum and damping mechanisms of the electromagnetic fluctuations, and the mean free path (mfp) of CRe, are the most relevant ingredients that determine the efficiency of acceleration. These ingredients in the ICM are, however, poorly known, and we show that calculations of turbulent acceleration are also sensitive to these uncertainties. On the other hand this fact implies that the non-thermal properties of galaxy clusters probe the complex microphysics and the weakly collisional nature of the ICM.

RADIO EMISSION FROM WEAK SPHERICAL SHOCKS IN THE OUTSKIRTS OF GALAXY CLUSTERS

In Kang (2015) we calculated the acceleration of cosmic-ray electrons and the ensuing radio synchrotron emission at weak spherical shocks that are expected to form in the outskirts of galaxy clusters.There we demonstrated that, at decelerating spherical shocks, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws predicted for constant planar shocks, because the shock compression ratio and the flux of injected electrons decrease in time. In this study, we consider spherical blast waves propagating into a constant density core surrounded by an isothermal halo with a decreasing density profile in order to explore how the deceleration rate of the shock speed affects the radio emission from accelerated electrons. The surface brightness profile and the volume-integrated radio spectrum of the model shocks are calculated by assuming a ribbon-like shock surface on a spherical shell and the associated downstream region of relativistic electrons. If the postshock magnetic field strength is about 7 microgauss, at the shock age of ~50 Myr, the volume-integrated radio spectrum steepens gradually with the spectral index from alpha_{inj} to alpha_{inj}+0.5 over 0.1-10 GHz, where alpha_{inj} is the injection index at the shock positionexpected from the diffusive shock acceleration theory. Such gradual steepening could explain the curved radio spectrum of the radio relic in cluster A2266, which was interpreted as a broken power-law by Trasatti et al. (2014), if the relic shock is young enough so that the break frequency falls in around 1 GHz.

NON-THERMAL X-RAYS AND INTERSTELLAR GAS TOWARD THE γ-RAY SUPERNOVA REMNANT RX J1713.7–3946: EVIDENCE FOR X-RAY ENHANCEMENT AROUND CO AND H I CLUMPS

RX J1713.7-3946 is the most remarkable very-high-energy \gamma-ray supernova remnant which emits synchrotron X-rays without thermal features. We made a comparative study of CO, HI and X-rays in order to better understand the relationship between the X-rays, and the molecular and atomic gas. The results indicate that the X-rays are enhanced around the CO and HI clumps on a pc scale but are decreased inside the clumps on a 0.1 pc scale. Magnetohydrodynamic numerical simulations of the shock interaction with molecular and atomic gas indicate that the interaction between the shock waves and the clumps excite turbulence which amplifies the magnetic field around the clumps (Inoue et al. 2012). We suggest that the amplified magnetic field around the CO and HI clumps enhances the synchrotron X-rays and possibly the acceleration of cosmic-ray electrons.

COSMOLOGICAL MAGNETOHYDRODYNAMIC SIMULATIONS OF GALAXY CLUSTER RADIO RELICS: INSIGHTS AND WARNINGS FOR OBSERVATIONS

Non-thermal radio emission from cosmic ray electrons in the vicinity of merging galaxy clusters is an important tracer of cluster merger activity, and is the result of complex physical processes that involve magnetic fields, particle acceleration, gas dynamics, and radiation. In particular, objects known as radio relics are thought to be the result of shock-accelerated electrons that, when embedded in a magnetic field, emit synchrotron radiation in the radio wavelengths. In order to properly model this emission, we utilize the adaptive mesh refinement simulation of the magnetohydrodynamic evolution of a galaxy cluster from cosmological initial conditions. We locate shock fronts and apply models of cosmic ray electron acceleration that are then input into radio emission models. We have determined the thermodynamic properties of this radio-emitting plasma and constructed synthetic radio observations to compare to observed galaxy clusters. We find a significant dependence of the observed morphology and radio relic properties on the viewing angle of the cluster, raising concerns regarding the interpretation of observed radio features in clusters. We also find that a given shock should not be characterized by a single Mach number. We find that the bulk of the radio emission comes from gas with T>5x10^7, \rho~10^(-28)-10^(-27) g/cm^3, with magnetic field strengths of 0.1-1.0 \mu G and shock Mach numbers of M~3-6. We present an analysis of the radio spectral index which suggests that the spatial variation of the spectral index can mimic synchrotron aging. Finally, we examine the polarization fraction and position angle of the simulated radio features, and compare to observations.

A DEEPCHANDRAOBSERVATION OF THE OXYGEN-RICH SUPERNOVA REMNANT 0540-69.3 IN THE LARGE MAGELLANIC CLOUD

Using our deep ~120 ks Chandra observation, we report on the results from our spatially-resolved X-ray spectral analysis of the "oxygen-rich" supernova remnant (SNR) 0540-69.3 in the Large Magellanic Cloud. We conclusively establish the nonthermal nature of the "arcs" in the east and west boundaries of the SNR, which confirms the cosmic-ray electron acceleration in the supernova shock (B ~ 20-140 microG). We report tentative evidence for Fe overabundance in the southern region close to the outer boundary of the SNR. While such a detection would be intriguing, the existence of Fe ejecta is not conclusive with the current data because of poor photon statistics and limited plasma models. If it is verified using deeper X-ray observations and improved plasma models, the presence of Fe ejecta, which was produced in the core of the supernova, near the SNR's outer boundary would provide an intriguing opportunity to study the explosive nucleosynthesis and the ejecta mixing in this young core-collapse SNR. There is no evidence of X-ray counterparts for the optical O-rich ejecta in the central regions of the SNR.

A Silicon Array for Cosmic-Ray Composition Measurements in CALET

The CALorimetric Electron Telescope (CALET) mission is proposed for a long exposure observation of high energy cosmic rays and gamma radiation, taking advantage of the JEM-EF facility on the International Space Station. The instrument is optimized for the search of nearby sources of acceleration of cosmic ray electrons in the TeV energy range. Its large collection power also allows for precision studies of the elemental composition of VHE nuclei and of their spectral features. The charge identification of the incoming particle is performed by a double-layered array of pixelated silicon sensors, covering a seamless sensitive area of the order of 1 m 2 . The conceptual design of the array and its front-end electronics are presented.

Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569

Using the Effelsberg radio telescope at 4.85GHz and 8.35 GHz we discovered large symmetric lobes of polarized radio emission around the strongly HI deficient Virgo cluster spiral galaxy NGC4569. These lobes extend up to 24 kpc from the galactic disk. Our observations were complemented by 1.4 GHz continuum emission from existing HI observations. This is the first time that such huge radio continuum lobes are observed in a cluster spiral galaxy. The eastern lobe seems detached and has a flat spectrum typical for in-situ cosmic ray electron acceleration. The western lobe is diffuse and possesses vertical magnetic fields over its whole volume. The lobes are not powered by an AGN, but probably by a nuclear starburst producing >10^5 supernovae which occurred ~30 Myr ago. Since the radio lobes are symmetric, they resist ram pressure due to the galaxy's rapid motion within the intracluster medium.

Magnetic-Reconnection and Current-Sheet Model for the Radio Arc and Threads in the Galactic Center

We propose a new mechanism to explain the radio arc and threads in the Galactic center by current sheets produced by local magnetic shears due to the interaction of a moving cloud and a vertical field based on three-dimensional magnetohydrodynamical simulations. Magnetic reconnection and acceleration of cosmic-ray electrons in the current sheet would result in a high contrast of the radio emissivity inside and outside the arc and threads.

High Energy Radiation Generated at Boundary Shear Layers of Relativistic Jets

AbstractA simple model of cosmic ray electron acceleration at the jet boundary yields a power law particle energy distribution of ultra-relativistic electrons with an energy cut-off growing with time, and, finally, a growing particle bump at the energy where energy gains equal radiation losses. For such electron distribution, in tens-of-kpc scale jets, we derived the observed time-varying spectra of synchrotron and inverse Compton radiation, including Comptonisation of synchrotron and cosmic microwave background photons. Slowly varying spectral index along the jet in the ‘low frequency’ spectral range is a natural consequence of boundary layer acceleration. Variations of the high energy bump of the electron distribution can give rise to anomalous behaviour in the X-ray band in comparison to the lower frequencies.

Surfatron and stochastic acceleration of electrons at supernova remnant shocks.

The surfatron offers the possibility of particle acceleration to arbitrarily high energies, given a sufficiently large system. Surfatron acceleration of electrons by waves excited by ions reflected from supernova remnant (SNR) shocks is investigated using particle simulations. It is shown that surfatron and stochastic acceleration could provide a seed population for the acceleration of cosmic ray electrons at SNR shocks.

Radio Emission from a Young Supernova Remnant Interacting with an Interstellar Cloud: Magnetohydrodynamic Simulation with Relativistic Electrons

We present two-dimensional MHD simulations of the evolution of a young Type Ia supernova remnant (SNR) during its interaction with an interstellar cloud of comparable size at impact. We include for the first time in such simulations explicit relativistic electron transport. This was done using a simplified treatment of the diffusion-advection equation, thus allowing us to model injection and acceleration of cosmic-ray electrons at shocks and their subsequent transport. From this information we also model radio synchrotron emission, including spectral information. The simulations were carried out in spherical coordinates with azimuthal symmetry and compare three different situations, each incorporating an initially uniform interstellar magnetic field oriented in the polar direction on the grid. In particular, we modeled the SNR-cloud interactions for a spherical cloud on the polar axis, a toroidal cloud whose axis is aligned with the polar axis, and, for comparison, a uniform medium with no cloud. We find that the evolution of the overrun cloud qualitatively resembles that seen in simulations of simpler but analogous situations: that is, the cloud is crushed and begins to be disrupted by Rayleigh-Taylor and Kelvin-Helmholtz instabilities. However, we demonstrate here that, in addition, the internal structure of the SNR is severely distorted as such clouds are engulfed. This has important dynamical and observational implications. The principal new conclusions we draw from these experiments are the following. (1) Independent of the cloud interaction, the SNR reverse shock can be an efficient site for particle acceleration in a young SNR. (2) The internal flows of the SNR become highly turbulent once it encounters a large cloud. (3) An initially uniform magnetic field is preferentially amplified along the magnetic equator of the SNR, primarily because of biased amplification in that region by Rayleigh-Taylor instabilities. A similar bias produces much greater enhancement to the magnetic energy in the SNR during an encounter with a cloud when the interstellar magnetic field is partially transverse to the expansion of the SNR. The enhanced magnetic fields have a significant radial component, independent of the field orientation external to the SNR. This leads to a strong equatorial bias in synchrotron brightness that could easily mask any enhancements to electron-acceleration efficiency near the magnetic equator of the SNR. Thus, to establish the latter effect, it will be essential to establish that the magnetic field in the brightest regions are actually tangential to the blast wave. (4) The filamentary radio structures correlate well with turbulence-enhanced magnetic structures, while the diffuse radio emission more closely follows the gas-density distribution within the SNR. (5) At these early times, the synchrotron spectral index due to electrons accelerated at the primary shocks should be close to 0.5 unless those shocks are modified by cosmic-ray proton pressures. While that result is predictable, we find that this simple result can be significantly complicated in practice by SNR interactions with clouds. Those events can produce regions with significantly steeper spectra. Especially if there are multiple cloud encounters, this interaction can lead to nonuniform spatial spectral distributions or, through turbulent mixing, produce a spectrum that is difficult to relate to the actual strength of the blast wave. (6) Interaction with the cloud enhances the nonthermal electron population in the SNR in our simulations because of additional electron injection taking place in the shocks associated with the cloud. Together with point 3, this means that SNR-cloud encounters can significantly increase the radio emission from the SNR.

Acceleration of cosmic ray electrons by ion-excited waves at quasiperpendicular shocks

The standard model of cosmic ray electron acceleration requires electrons to be preaccelerated to mildly relativistic energies. It has been suggested that energy transfer from waves, generated by gyrotropic ions reflected from quasi-perpendicular shocks, could provide the necessary pre-acceleration. The distribution of shock-reflected ions at any upstream point is more likely to consist of two beams rather than a gyrotropic ring. Wave excitation in the presence of both types of ion distribution is studied. It is shown that gyrotropic or beam ions, reflected from shocks associated with supernova remnants, can excite waves capable of accelerating electrons to beyond the required injection energies. The wave group velocity along the shock normal can be approximately equal to the shock velocity: such waves are not rapidly convected away from the shock, and can thus grow to a high level. Moreover, waves satisfying this condition which also have phase velocities parallel to the magnetic field ranging from the electron thermal speed to relativistic speeds are excited in high Mach number shocks with a low ratio of electron plasma frequency to cyclotron frequency. Bulk electrons can then be accelerated to the required energies withinthe region in which shock-reflected ions are present. This is consistent with a suggestion, based on a comparison between Wolf-Rayet stars and radio Supernovae, that there exists a threshold perpendicular shock speed (between 1 and 3 per cent of the speed of light) above which the efficiency of electron injection increases by several orders of magnitude.

The strength of galactic magnetic fields

THE magnitudes of galactic magnetic fields are usually estimated from measurements of the radio synchroton emission arising from acceleration of cosmic-ray electrons in the magnetic field. To interpret the emission spectrum, it is usually assumed that the energy density in the magnetic field is equal to that in the cosmic-ray protons (which are assumed to outnumber the electrons by 100:1, as they do in our Galaxy). There is, however, no compelling justification for this assumption of energy equipartition. Here we use measurements by the Compton Gamma Ray Observatory1,2 of the γ-ray flux from the Magellanic clouds together with radio continuum data to estimate the strength of the magnetic fields in these galaxies without having to invoke energy equipartition. We find that the assumption of energy equipartition is not valid in these irregular galaxies, and that the validity cannot be restored by changing the electron/proton ratio. Supporting evidence for our conclusion comes from radio and X-ray observations (M. G. Watson, personal communication) of the starburst galaxy M82. Our results imply that these galactic fields are too large to have been generated by dynamo action alone, and we suggest that recent star formation might instead provide the generating mechanism.

Acceleration of cosmic-ray electrons by large-scale galactic spiral shocks ? An observational report

The hypothesis that large-scale shocks accompanying spiral density waves are the main sources of cosmic-ray electrons for whole galactic disks has been tested utilizing results of 10.7 GHz observations of spiral galaxies. It is suggested that the 10.7 GHz emission of galactic disks is largely controlled by star-forming processes, and that large-scale shocks apparently play a minor role.