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Register a new userEffects of Neutral Hydrogen on Cosmic Ray Precursors in Supernova Remnant Shock Waves
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Many fast supernova remnant shocks show spectra dominated by Balmer lines. The H$alpha$ profiles have a narrow component explained by direct excitations and a thermally Doppler broadened component due to atoms that undergo charge exchange in the post-shock region. However, the standard model does not take into account the cosmic-ray shock precursor, which compresses and accelerates plasma ahead of the shock. In strong precursors with sufficiently high densities, the processes of charge exchange, excitation and ionization will affect the widths of both narrow and broad line components. Moreover, the difference in velocity between the neutrals and the precursor plasma gives rise to frictional heating due to charge exchange and ionization in the precursor. In extreme cases, all neutrals can be ionized by the precursor. In this paper we compute the ion and electron heating for a wide range of shock parameters, along with the velocity distribution of the neutrals that reach the shock. Our calculations predict very large narrow component widths for some shocks with efficient acceleration, along with changes in the broad- to-narrow intensity ratio used as a diagnostic for the electron-ion temperature ratio. Balmer lines may therefore provide a unique diagnostic of precursor properties. We show that heating by neutrals in the precursor can account for the observed H$alpha$ narrow component widths, and that the acceleration efficiency is modest in most Balmer line shocks observed thus far.
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Radiative transfer in hydrogen lines in supernova remnant (SNR) shock waves is studied taking into account the population of the hydrogen atom 2s-state. Measurements of Balmer line emission, especially of H~$alpha$, are often relied upon to derive physical conditions in the SNR shock. On the other hand, Lyman series photons, especially Ly~$beta$, are mostly absorbed by upstream hydrogen atoms. As a result, atoms are excited to the 3p state, and then emit H~$alpha$ by the spontaneous transition from 3p to 2s. Thus, the nature of H~$alpha$ depends on how many Ly~$beta$ photons are converted to H~$alpha$ photons. Moreover, the Balmer lines can be scattered by the 2s-state hydrogen atoms, which are excited not only by collisional excitation but also by the Lyman-Balmer conversion. It is shown for example that the H~$alpha$ photons are scattered if the shock propagates into an H~$_{rm I}$ cloud with a density of $sim30~{rm cm^{-3}}$ and a size of $sim 1$~pc. We find that the line profile of H~$alpha$ becomes asymmetric resulting from the difference between line centre frequencies among the transitions from 3s to 2p, from 3p to 2s and from 3d to 2p. We also find that the broad-to-narrow ratio of H~$alpha$, which is often used to estimate the ion-electron temperature equilibrium, varies at most $simeq 10$ per cent depending on the ionization degree of the upstream medium because of incomplete conversion of Lyman lines to Balmer lines.
Linearly polarized Balmer line emissions from supernova remnant shocks are studied taking into account the energy loss of the shock owing to the production of nonthermal particles. The polarization degree depends on the downstream temperature and the velocity difference between upstream and downstream regions. The former is derived once the line width of the broad component of the H$alpha$ emission is observed. Then, the observation of the polarization degree tells us the latter. At the same time, the estimated value of the velocity difference independently predicts adiabatic downstream temperature that is derived from Rankine-Hugoniot relations for adiabatic shocks. If the actually observed downstream temperature is lower than the adiabatic temperature, there is a missing thermal energy which is consumed for particle acceleration. It is shown that a larger energy loss rate leads to more highly polarized H$alpha$ emission. Furthermore, we find that polarized intensity ratio of H$beta$ to H$alpha$ also depends on the energy loss rate and that it is independent of uncertain quantities such as electron temperature, the effect of Lyman line trapping and our line of sight.
We report on the results from H{alpha} imaging observations of the eastern limb of Tychos supernova remnant (SN1572) using the Wide Field Planetary Camera 2 on the Hubble Space Telescope. We resolve the detailed structure of the fast, collisionless shock wave into a delicate structure of nearly edge-on filaments. We find a gradual increase of H{alpha} intensity just ahead of the shock front, which we interpret as emission from the thin (~1) shock precursor. We find that a significant amount of the H{alpha} emission comes from the precursor and that this could affect the amount of temperature equilibration derived from the observed flux ratio of the broad and narrow H{alpha} components. The observed H{alpha} emission profiles are fit using simple precursor models, and we discuss the relevant parameters. We suggest that the precursor is likely due to cosmic rays and discuss the efficiency of cosmic-ray acceleration at this position.
In shock precursors populated by accelerated cosmic rays (CR), the CR return current instability is believed to significantly enhance the pre-shock perturbations of magnetic field. We have obtained fully-nonlinear exact ideal MHD solutions supported by the CR return current. The solutions occur as localized spikes of circularly polarized Alfven envelopes (solitons, or breathers). As the conventional (undriven) solitons, the obtained magnetic spikes propagate at a speed $C$ proportional to their amplitude, $C=C_{A}B_{{rm max}}/sqrt{2}B_{0}$. The sufficiently strong solitons run thus ahead of the main shock and stand in the precursor, being supported by the return current. This property of the nonlinear solutions is strikingly different from the linear theory that predicts non-propagating (that is, convected downstream) circularly polarized waves. The nonlinear solutions may come either in isolated pulses (solitons) or in soliton-trains (cnoidal waves). The morphological similarity of such quasi-periodic soliton chains with recently observed X-ray stripes in Tycho supernova remnant (SNR) is briefly discussed. The magnetic field amplification determined by the suggested saturation process is obtained as a function of decreasing SNR blast wave velocity during its evolution from the ejecta-dominated to the Sedov-Taylor stage.
The Cosmic Ray (CR) physics has entered a new era driven by high precision measurements coming from direct detection (especially AMS-02 and PAMELA) and also from gamma-ray observations (Fermi-LAT). In this review we focus our attention on how such data impact the understanding of the supernova remnant paradigm for the origin of CRs. In particular we discuss advancement in the field concerning the three main stages of the CR life: the acceleration process, the escape from the sources and the propagation throughout the Galaxy. We show how the new data reveal a phenomenology richest than previously thought that could even challenge the current understanding of CR origin.
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