DEM Analyses with the Utrecht Codes
International Astronomical Union Colloquium
We address the inversion problem of deriving the differential emission measure (DEM) distribution D(T) =nenHdV/d log T from the spectrum of an optically thin plasma. In the past we have applied the iterative Withbroe-Sylwester technique and the Polynomial technique to the analysis ofEXOSATspectra of cool stars, but recently we have applied the inversion technique discussed by Craig & Brown (1986) and Press et al. (1992) in the analysis ofEUVEspectra of cool stars. The inversion problem-a
... ersion problem-a Fredholm equation of the first kind-is ill-posed and solutions tend to show large, unphysical oscillations. We therefore apply a second-order regularization, i.e., we select the specific DEM for which the second derivative is as smooth as is statistically allowed by the data. We demonstrate the importance of fitting lines and continuumsimultaneously,discuss the effect on the DEM of continuum emission at temperatures where no line diagnostics are available, and address possible ways to check various model assumptions such as abundances and photon destruction induced by resonant scattering.