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Register a new userBroad Balmer line emission and cosmic ray acceleration efficiency in supernova remnant shocks
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Giovanni Morlino Dr.
Publication date
2013
fields
Physics
and research's language is
English
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Balmer emission may be a powerful diagnostic tool to test the paradigm of cosmic ray (CR) acceleration in young supernova remnant (SNR) shocks. The width of the broad Balmer line is a direct indicator of the downstream plasma temperature. In case of efficient particle acceleration an appreciable fraction of the total kinetic energy of the plasma is channeled into CRs, therefore the downstream temperature decreases and so does the broad Balmer line width. This width also depends on the level of thermal equilibration between ions and neutral hydrogen atoms in the downstream. Since in general in young SNR shocks only a few charge exchange (CE) reactions occur before ionization, equilibration between ions and neutrals is not reached, and a kinetic description of the neutrals is required in order to properly compute Balmer emission. We provide a method for the calculation of Balmer emission using a self-consistent description of the shock structure in the presence of neutrals and CRs. We use a recently developed semi-analytical approach, where neutral particles, ionized plasma, accelerated particles and magnetic fields are all coupled together through the mass, momentum and energy flux conservation equations. The distribution of neutrals is obtained from the full Boltzmann equation in velocity space, coupled to Maxwellian ions through ionization and CE processes. The computation is also improved with respect to previous work thanks to a better approximation for the atomic interaction rates. We find that for shock speeds >2500km/s the distribution of broad neutrals never approaches a Maxwellian and its moments differ from those of the ionized component. These differences reflect into a smaller FWHM than predicted in previous calculations, where thermalization was assumed. The method presented here provides a realistic estimate of particle acceleration efficiency in Balmer dominated shocks.
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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 present results from an optical study of two young Balmer-dominated remnants of SNIa in the Large Magellanic Cloud, 0509$-$67.5 and 0519$-$69.0, in an attempt to search for signatures of efficient cosmic-ray (CR) acceleration. We combine proper motion measurements from HST with corresponding optical spectroscopic measurements of the H$alpha$ line at multiple rim positions from VLT/FORS2 and SALT/RSS and compare our results to published Balmer shock models. Analysis of the optical spectra result in broad H$alpha$ widths in the range of 1800-4000 km s$^{-1}$ for twelve separate Balmer-dominated filaments that show no evidence for forbidden line emission, the corresponding shock speeds from proper motion measurements span a range of 1700-8500 km s$^{-1}$. Our measured values of shock speeds and broad H$alpha$ widths in 0509$-$67.5 and 0519$-$69.0 are fit well with a Balmer shock model that does not include effects of efficient CR acceleration. We determine an upper limit of 7%/$chi$ (95% confidence) on the CR acceleration efficiency for our ensemble of data points, where $chi$ is the ionization fraction of the pre-shock gas. The upper limits on the individual remnants are 6%/$chi$ (0509$-$67.5) and 11%/$chi$ (0519$-$69.0). These upper limits are below the integrated CR acceleration efficiency in the Tycho supernova remnant, where the shocks predominantly show little H$alpha$ emission, indicating that Balmer-dominated shocks are not efficient CR accelerators.
In the last few years several experiments have shown that the cosmic ray spectrum below the knee is not a perfect power-law. In particular, the proton and helium spectra show a spectral hardening by ~ 0.1-0.2 in spectral index at particle energies of ~ 200-300 GeV/nucleon. Moreover, the helium spectrum is found to be harder than that of protons by ~ 0.1 and some evidence for a similar hardening was also found in the spectra of heavier elements. Here we consider the possibility that the hardening may be the result of a dispersion in the slope of the spectrum of cosmic rays accelerated at supernova remnant shocks. Such a dispersion is indeed expected within the framework of non-linear theories of diffusive shock acceleration, which predict steeper (harder) particle spectra for larger (smaller) cosmic ray acceleration efficiencies.
In the past few years, gamma-ray astronomy has entered a golden age. At TeV energies, only a handful of sources were known a decade ago, but the current generation of ground-based imaging atmospheric Cherenkov telescopes has increased this number to more than one hundred. At GeV energies, the Fermi Gamma-ray Space Telescope has increased the number of known sources by nearly an order of magnitude in its first 2 years of operation. The recent detection and unprecedented morphological studies of gamma-ray emission from shell-type supernova remnants is of great interest, as these analyses are directly linked to the long standing issue of the origin of the cosmic-rays. However, these detections still do not constitute a conclusive proof that supernova remnants accelerate the bulk of Galactic cosmic-rays, mainly due to the difficulty of disentangling the hadronic and leptonic contributions to the observed gamma-ray emission. In this talk, I will review the most relevant cosmic ray related results of gamma ray astronomy concerning supernova remnants.
Supernova remnants (SNRs) are believed to accelerate particles up to high energies through the mechanism of diffusive shock acceleration (DSA). Except for direct plasma simulations, all modeling efforts must rely on a given form of the diffusion coefficient, a key parameter that embodies the interactions of energetic charged particles with the magnetic turbulence. The so-called Bohm limit is commonly employed. In this paper we revisit the question of acceleration at perpendicular shocks, by employing a realistic model of perpendicular diffusion. Our coefficient reduces to a power-law in momentum for low momenta (of index $alpha$), but becomes independent of the particle momentum at high momenta (reaching a constant value $kappa_{infty}$ above some characteristic momentum $p_{rm c}$). We first provide simple analytical expressions of the maximum momentum that can be reached at a given time with this coefficient. Then we perform time-dependent numerical simulations to investigate the shape of the particle distribution that can be obtained when the particle pressure back-reacts on the flow. We observe that, for a given index $alpha$ and injection level, the shock modifications are similar for different possible values of $p_{rm c}$, whereas the particle spectra differ markedly. Of particular interest, low values of $p_{rm c}$ tend to remove the concavity once thought to be typical of non-linear DSA, and result in steep spectra, as required by recent high-energy observations of Galactic SNRs.
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