E. Landi (1), F. Reale (2,3), and P. Testa (4) ((1) Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, MI, USA; (2) Dipartimento di Scienze Fisiche ed Astronomiche, Universita’ di Palermo, Palermo, Italy; (3) INAF-Osservatorio Astronomico di Palermo, Palermo, Italy; (4) Smithsonian Astrophysical Observatory, Cambridge, MA, USA)
In this paper, we carry out tests on the Monte Carlo Markov Chain (MCMC) technique with the aim of determining: 1) its ability to retrieve isothermal plasmas from a set of spectral line intensities, with and without random noise; 2) to what extent can it discriminate between an isothermal solution and a narrow multithermal distribution; and 3) how well it can detect multiple isothermal components along the line of sight. We also test the effects of 4) atomic data uncertainties on the results, and 5) the number of ions whose lines are available for the DEM reconstruction. We find that the MCMC technique is unable to retrieve isothermal plasmas to better than Delta log T = 0.05. Also, the DEM curves obtained using lines calculated with an isothermal plasma and with a Gaussian distribution with FWHM of log T = 0.05 are very similar. Two near-isothermal components can be resolved if their temperature separation is Delta log T = 0.2 or larger. Thus, DEM diagnostics has an intrinsic resolving power of log T = 0.05. Atomic data uncertainties may significantly affect both temperature and peak DEM values, but do not alter our conclusions. The availability of small sets of lines also does not worsen the performance of the MCMC technique, provided these lines are formed in a wide temperature range. Our analysis shows the present limitations in our ability to identify the presence of strictly isothermal plasmas in stellar and solar coronal spectra.
Complete preprint ==> http://arxiv.org/abs/1112.2857