Signatures of the non-Maxwellian κ-distributions in optically thin line spectra^⋆

I. Theory and synthetic Fe IX–XIII spectra

¹ Department of Applied Mathematics and Theoretical Physics, CMSUniversity of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
e-mail: j.dudik@damtp.cam.ac.uk
² Department of Astronomy, Physics of the Earth and Meteorology, Faculty of Mathematics, Physics and Informatics, Comenius University, 842 48 Bratislava, Slovak Republic
³ Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic
e-mail: elena@asu.cas.cz

Received: 3 May 2014
Accepted: 24 July 2014

Abstract

Aims. We investigate the possibility of diagnosing the degree of departure from the Maxwellian distribution using single-ion spectra originating in astrophysical plasmas in collisional ionization equilibrium.

Methods. New atomic data for excitation of Fe ix – Fe xiii are integrated under the assumption of a κ-distribution of electron energies. Diagnostic methods using lines of a single ion formed at any wavelength are explored. Such methods minimize uncertainties from the ionization and recombination rates, as well as the possible presence of non-equilibrium ionization. Approximations to the collision strengths are also investigated.

Results. The calculated intensities of most of the Fe ix – Fe xiii EUV lines show consistent behaviour with κ at constant temperature. Intensities of these lines decrease with κ, with the vast majority of ratios of strong lines showing little or no sensitivity to κ. Several of the line ratios, especially involving temperature-sensitive lines, show a sensitivity to κ that is of the order of several tens of per cent, or, in the case of Fe ix, up to a factor of two. Forbidden lines in the near-ultraviolet, visible, or infrared parts of the spectrum are an exception, with smaller intensity changes or even a reverse behaviour with κ. The most conspicuous example is the Fe x 6378.26 Å red line, whose intensity incerases with κ. This line is a potentially strong indicator of departures from the Maxwellian distribution. We find that it is possible to perform density diagnostics independently of κ, with many Fe xi, Fe xii, and Fe xiii line ratios showing strong density-sensitivity and negligible sensitivity to κ and temperature. We also tested different averaging of the collision strengths. It is found that averaging over 0.01 interval in log(E [ Ryd ]) is sufficient to produce accurate distribution-averaged collision strengths Υ(T,κ) at temperatures of the ion formation in ionization equilibrium.