Cosmic ray acceleration and Balmer emission from RCW 86 (G315.4 – 2.3)
Résumé
Context. Observation of Balmer lines from the region around the forward
shock of supernova remnants (SNRs) may provide valuable information on the shock dynamics
and the efficiency of particle acceleration at the shock.Aims. We calculated the Balmer line emission and the shape of the broad
Balmer line for parameter values suitable for SNR RCW 86 (G315.4 – 2.3) as a function of
the cosmic-ray (CR) acceleration efficiency and of the level of thermal equilibration
between electrons and protons behind the shock. This calculation aims at using the width
of the broad Balmer-line emission to infer the CR acceleration efficiency in this
remnant.Methods. We used the recently developed non linear theory of diffusive
shock-acceleration in the presence of neutrals. The semianalytical approach we developed
includes a description of magnetic field amplification as due to resonant streaming
instability, the dynamical reaction of accelerated particles and the turbulent magnetic
field on the shock, and all channels of interaction between neutral hydrogen atoms and
background ions that are relevant for the shock dynamics.Results. We derived the CR acceleration efficiency in the SNR RCW 86
from the Balmer emission. Since our calculation used recent measurements of the shock
proper motion, the results depend on the assumed distance to Earth. For a distance of 2
kpc the measured width of the broad Balmer line is compatible with the absence of CR
acceleration. For a distance of 2.5 kpc, which is a widely used value in current
literature, a CR acceleration efficiency of 5–30% is obtained, depending upon the
electron-ion equilibration and the ionization fraction upstream of the shock. By combining
information on Balmer emission with the measured value of the downstream electron
temperature, we constrain the CR acceleration efficiency to be ~20%.
Domaines
Astrophysique [astro-ph]
Origine : Publication financée par une institution
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