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Author(s):
K. Masai
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Title:
Nonequilibria in Thermal Emission from Supernova Remnants
Date of publication:
Dec. 1993
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Key words:
collisionless shock wave, supernove remnant, nonequipartition, nonequilibrium ionization, line emission
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Abstract:
Nonequipartition between the electron and the ion temperatures, and nonequilibrium ionization are discussed in view of thermal X-ray emission from young supernova remnants (SNRs). For electron heating due to Coulomb collisions with ions in the postshock region, an analytical solution is derived for the electron temperature under the adiabatic condition. By Taylor expansion of the solution, we obtain the electron temperature as a function of the electron density, time, and the shock velocity or the shock temperature T_s^. With equipartition time t _Eeq, the solution shows that the electron temperature increases to be approx 0.1T_s and 0.3T_s in 10^-3t_Eeq and 10^-2t_Eeq, respectively. The result is applied to the self-similar solutions for SNRs in the adiabatic phase, i.e., the free expansion phase and the Sedov phase. For SNRs expanding into a uniform density medium, our results predict much slower variation of the electron temperature than the case of full equipartition within the shock. We show the electron temperature plateau on the shocked matter of an SNR, and give analytical expressions of the electron temperature and the resultant free-free luminosity. In the free expansion phase, the electron temperatures of the reverse-and the blast-shocked matter are close to each other in comparison with the case of full equipartition, and their ratio is nearly constant for the ejecta envelope steepness. This results in a larger difference in the free-free luminosities of the two shocks than the case of full equipartition. Ionization processes are discussed in the framework of eigen-value problem to find the key matrix element which is a pivot of the transformation and gives the characteristic time constant. The ionization time approx 10^12(n_e)^-1 s for the electron density n_e^, and has no systematic dependence either on the electron temperature or the atomic number. The emission from the shocked matter can scale as n_et , but its superposition on a t-old SNR expanding into a uniform density medium may be represented by a lower value than n_et expected from the ambient matter density and the age. Also the effect of the nonequilibria on the level populations is discussed in view of line emission processes. While innershell ionization followed by fluorescence lines is important in compression waves, radiative recombination followed by cascade lines is important in rarefaction waves. The rarefaction occurs and adiabatically cools the shocked matter rapidly when the shock wave breaks out of the dense circumstellar matter into the rarefied ambient medium.
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