Volume 558, October 2013
|Number of page(s)||6|
|Published online||27 September 2013|
Broad Balmer line emission and cosmic ray acceleration efficiency in supernova remnant shocks
1 INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
2 INFN/Gran Sasso Science Institute, viale F. Crispi 7, 67100 L’Aquila, Italy
Received: 31 May 2013
Accepted: 27 July 2013
Context. Balmer emission may be a powerful diagnostic tool for testing 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 the 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 generally only a few charge exchange (CE) reactions occur before ionization in young SNR shocks, equilibration between ions and neutrals is not reached, and a kinetic description of the neutrals is required to properly compute Balmer emission.
Aims. We provide a method for calculating Balmer emission using a self-consistent description of the shock structure in the presence of neutrals and CRs, which also accounts for the non-Maxwellian distribution of neutrals.
Methods. 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 an improvement over previous work thanks to a better approximation of the atomic interaction rates.
Results. We find that for shock speeds ≳2500 km s-1, the distribution of broad neutrals never approaches a Maxwellian and its moments differ from those of the ionized component. These differences lead to a smaller FWHM than predicted in previous calculations, where thermalization was assumed.
Conclusions. The method presented here provides a realistic estimate of particle acceleration efficiency in Balmer-dominated shocks.
Key words: shock waves / ISM: supernova remnants / cosmic rays / acceleration of particles
© ESO, 2013
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