The explosion of a supernova releases almost instantaneously about 10
 $$^{51}$$
 ergs of mechanic energy, changing irreversibly the physical and chemical properties of large regions in the galaxies. The stellar ejecta, the nebula resulting from the powerful shock waves, and sometimes a compact stellar remnant, constitute a supernova remnant (SNR). They can radiate their energy across the whole electromagnetic spectrum, but the great majority are radio sources. Almost 70 years after the first detection of radio emission coming from an SNR, great progress has been achieved in the comprehension of their physical characteristics and evolution. We review the present knowledge of different aspects of radio remnants, focusing on sources of the Milky Way and the Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief overview of theoretical background, analyze morphology and polarization properties, and review and critically discuss different methods applied to determine the radio spectrum and distances. The consequences of the interaction between the SNR shocks and the surrounding medium are examined, including the question of whether SNRs can trigger the formation of new stars. Cases of multispectral comparison are presented. A section is devoted to reviewing recent results of radio SNRs in the Magellanic Clouds, with particular emphasis on the radio properties of SN 1987A, an ideal laboratory to investigate dynamical evolution of an SNR in near real time. The review concludes with a summary of issues on radio SNRs that deserve further study, and analysis of the prospects for future research with the latest-generation radio telescopes.

/pdf/radio-emission-from-supernova-remnants-4mk773qf8z.pdf

Radio emission from supernova remnants

Radio and X-ray observations of the growing class of Crab-line supernova remnants (''plerions'') suggest that a central pulsar dominates their evolution. We model a plerion as a spherical, homogeneous bubble of relativistic particles and magnetic field inflated by a pulsar amid uniformly expanding supernova ejecta. We calculate the dynamical evolution of the bubble, and the concurrent evolution of the synchrotron luminosity, for a variety of assumptions about field-particle coupling ad the nature of the ejecta. Free-free absorption keeps a model invisible at radio frequencies until approx.200 years after the supernova; after reaching a peak shortly thereafter, the radio spectral luminosity L/sub r/ drops steeply, L/sub r/ proportional t/sup -4/ to t/sup -5/. The X-ray spectral luminosity L/sub x/ generally rises slowly and can peak later than the radio, and the radio of X-ray to radio luminosities (L/sub x//L/sub r/) behaves in a complicated, non-monotomic fashion. This ratio is therefore a poor age indicator. After about 13,000 years, deceleration of the ejecta sends a reverse shock back through the remnant, compressing and rebrightening the bubble. It then fades slowly, the radio luminosity decreasing approximately as t/sup -1/.

Evolution of Pulsar-Driven Supernova Remnants

In this work, we present a leptonic, time-dependent model of pulsar wind nebulae (PWNe). The model seeks a solution for the lepton distribution function considering the full time-energy-dependent diffusion-loss equation. The time-dependent lepton population is balanced by injection, energy losses and escape. We include synchrotron, inverse-Compton (IC; with the cosmic-microwave background as well as with IR/optical photon fields), self-synchrotron Compton, and bremsstrahlung processes, all devoid of any radiative approximations. With this model in place we focus on the Crab nebula as an example and present its time-dependent evolution. Afterwards, we analyse the impact of different approximations made at the level of the diffusion-loss equation, as can be found in the literature. Whereas previous models ignored the escape term, e.g. with the diffusion-loss equation becoming advective, others approximated the losses as catastrophic, so that the equation has only time derivatives. Additional approximations are also described and computed. We study what the impact of these approaches is on the determination of the PWN evolution. In particular, we find the time-dependent deviation of the multi-wavelength spectrum and the best-fitting parameters obtained with the complete and the approximate models.

/pdf/time-dependent-modelling-of-pulsar-wind-nebulae-study-on-the-1p69n5asil.pdf

Time‐dependent modelling of pulsar wind nebulae: study on the impact of the diffusion‐loss approximations

Recent observations by the ATIC-2, CREAM and PAMELA experiments suggest that the spectrum of cosmic- ray (CR) helium is harder than that of CR protons below the knee energy and all CR spectra become hard at 100 GeV/nucleon. According to DSA theory, the spectrum of accelerated particles at a shock does not depend on CR el- ements, but depends only on the velocity profile of the shock. Thus, naively, recent CR observations seem to show that the acceleration site of CR helium is different from that of CR protons. We propose a new idea, that higher energy CRs are generated in a more helium-rich region, to explain the hardening without introducing different sources for CR helium. We argue that CRs are produced in a chemically enriched region, such as a superbubble, and the outward-decreasing abundance naturally leads to the hard spectrum of CR helium if CRs escape from the supernova remnant shock in an energy-dependent way. Our idea uses the fact that CRs escaping from SNRs generally have a different spectrum than those of the acceleration site. The runaway CR spectrum depends not only on the acceleration spectrum at shocks but also on the evolution of the maximum energy and the number of accelerated CRs. We also suggest that the spectral hardening of CRs is caused by the decreasing Mach number in the high-temperature medium. Both the inhomogeneous abundance and the high temperature can be realized in the hot superbubbles with multiple supernovae.

/pdf/cosmic-ray-helium-hardening-3akha8em8x.pdf

Cosmic-ray helium hardening

We study four young pulsar wind nebulae (PWNe) detected in TeV γ-rays, G21.5–0.9, G54.1+0.3, Kes 75, and G0.9+0.1, using the spectral evolution model developed and applied to the Crab Nebula in our previous work. We model the evolution of the magnetic field and the particle distribution function inside a uniformly expanding PWN considering a time-dependent injection from the pulsar and radiative and adiabatic losses. Considering uncertainties in the interstellar radiation field (ISRF) and their distance, we study two cases for each PWN. Because TeV PWNe have a large TeV γ-ray to X-ray flux ratio, the magnetic energy of the PWNe accounts for only a small fraction of the total energy injected (typically a few × 10–3). The γ-ray emission is dominated by inverse Compton scattering off the infrared photons of the ISRF. A broken power-law distribution function for the injected particles reproduces the observed spectrum well, except for G0.9+0.1. For G0.9+0.1, we do not need a low-energy counterpart because adiabatic losses alone are enough to reproduce the radio observations. High-energy power-law indices at injection are similar (2.5-2.6), while low-energy power-law indices range from 1.0 to 1.6. The lower limit of the particle injection rate indicates that the pair multiplicity is larger than 104. The corresponding upper limit of the bulk Lorentz factor of the pulsar winds is close to the break energy of the broken power-law injection, except for Kes 75. The initial rotational energy and the magnetic energy of the pulsars seem anticorrelated, although the statistics are poor.

Study of Four Young TeV Pulsar Wind Nebulae with a Spectral Evolution Model

W28 is one of the archetype supernova remnants (SNRs) interacting with molecular clouds. H.E.S.S. observation found four TeV sources which are coincident with the molecular clouds (MCs) around W28, but Fermi LAT detected no prominent GeV counterparts for two of them. An accumulative diffusion model is established in this Letter and the energetic protons colliding the nearby MCs are considered to be an accumulation of the diffusive protons escaping from the shock front throughout the history of the SNR expansion. We have fitted the gamma ray spectra of the four sources and naturally explained the GeV spectral break of the northeastern source (source N) and the nonsignificant GeV emission of the southern sources A and C. The distances of sources A and C from the SNR centre are found to be much larger than those of sources N and B, which may be the basic reason for the faint GeV gamma rays of the two former sources.

Gamma rays from molecular clouds illuminated by accumulated diffusive protons from supernova remnant W28

Recent observations reveal that spectral breaks at ~GeV are commonly present in Galactic gamma-ray supernova remnants (SNRs) interacting with molecular clouds and that most of them have a spectral ($E^2dF/dE$) "platform" extended from the break to lower energies. In paper I (Li & Chen 2010), we developed an accumulative diffusion model by considering an accumulation of the diffusive protons escaping from the shock front throughout the history of the SNR expansion. In this paper, we improve the model by incorporating finite-volume of MCs, demonstrate the model dependence on particle diffusion parameters and cloud size, and apply it to nine interacting SNRs (W28, W41, W44, W49B, W51C, Cygnus Loop, IC443, CTB 37A, and G349.7+0.2). This refined model naturally explains the GeV spectral breaks and, especially, the "platform"s, together with available TeV data. We find that the index of the diffusion coeffcient \delta\ is in the range of 0.5-0.7, similar to the galactic averaged value, and the diffusion coefficient for cosmic rays around the SNRs is essentially two orders of magnitude lower than the Galactic average, which is a good indication for the suppression of cosmic ray diffusion near SNRs.

Gamma-rays from molecular clouds illuminated by accumulated diffusive protons. II: interacting supernova remnants

G54.1+0.3 is a Crab-like pulsar wind nebula (PWN) with the highest $\gamma$-ray to X-ray luminosity ratio among all the nebulae driven by young rotation-powered pulsars. We model the spectral evolution of the PWN and find it difficult to match the observed multi-band data with leptons alone using reasonable model parameters. In lepton-hadron hybrid model instead, TeV photons come mainly from $\pi^0$ decay in proton-proton interaction and the observed photon spectrum can be well reproduced. The newly discovered infrared loop and molecular cloud in or closely around the PWN can work as the target for the bombardment of the PWN protons.

Lepto-Hadronic Origin of gamma-rays from the G54.1+0.3 Pulsar Wind Nebula

G54.1+0.3 is a Crab-like pulsar wind nebula (PWN) with the highest γ -ray to X-ray luminosity ratio among all the nebulae driven by young rotation-powered pulsars. We model the spectral evolution of the PWN and find it difficult to match the observed multiband data with leptons alone using reasonable model parameters. In a lepton–hadron hybrid model, however, TeV photons come mainly from π 0 decay in proton–proton interaction and the observed photon spectrum can be well reproduced. The newly discovered infrared loop and molecular cloud that lie in or closely around the PWN can work as the target for the bombardment of the PWN

/pdf/lepto-hadronic-origin-of-g-rays-from-the-g54-1-0-3-pulsar-266msy5s8i.pdf

Lepto-hadronic origin of γ-rays from the G54.1+0.3 pulsar wind nebula

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Gamma rays from molecular clouds illuminated by accumulated diffusive protons from supernova remnant W28

Gamma-rays from molecular clouds illuminated by accumulated diffusive protons. II: interacting supernova remnants

Lepto-Hadronic Origin of gamma-rays from the G54.1+0.3 Pulsar Wind Nebula

Lepto-hadronic origin of γ-rays from the G54.1+0.3 pulsar wind nebula