ISM absorption

"disnht: modeling X-ray absorption from distributed column densities"

Abstract:
Collecting and analysing X-ray photons over either spatial or temporal scales encompassing varying optical depth values requires knowledge about the optical depth distribution. In the case of sufficiently broad optical depth distribution, assuming a single column density value leads to a misleading interpretation of the source emission properties, nominally its spectral model. We present a model description for the ISM absorption in X-ray spectra at moderate energy resolution (CCDs), extracted over spatial or temporal regions encompassing a set of independent column densities. The absorption model (named disnht) approximates the distribution with a log-normal one and is presented in table format (readable by xspec). The solution table and source code are made available and can be further generalised or tailored for arbitrary optical depth distributions encompassed by the extraction region. The disnht absorption model presented and its generalised solution are expected to be relevant for present and upcoming large angular scale analyses of diffuse X-ray emission (e.g. eROSITA, Athena missions).
Description:
We provide the tabulated optical depth values as a function of energy in the range 0.03-60 keV, computed for distributions of hydrogen column densities. The table parameters describe the column desnity distribution, assumed to be log-normal, through an average value AVG and a standard deviation STD (in dex units of cm²). AVG and STD parameters are in the ranges 19.5-25.5 dex and 0.0-1.0 dex respectively. The .fits tables can be directly interpreted from the XSPEC X-ray spectral analysis software as a multiplicative model through the command etable{<TABLE_NAME>}. In this way the spectrum is multiplied by a factor exp(-TAU), where TAU is the tabulated value per energy bin. Energy bins have been fixed on a grid that oversamples the moderate energy resolution of current CCD X-ray spectrometers, by dividing the 0.03-60 keV band in 5000 log-spaced channels. TAU spectra are also provided as flat ascii tables for all AVG and STD values. Flat ascii and .fits tables are provided for different values of assumed ISM metal abundances: Anders, E. " Grevesse, N. (1989); Asplund et al. (2009) and Lodders et al. (2003). The source code used to produce the tables is also provided (make_disnht_custom.py) and described, along with its usage, in a dedicated README file (make_disnht_custom_README.txt).
For results derived with this code please cite
Locatelli N., Ponti G. & Bianchi S. A&A 2022.
For further information please contact nlocat_AT_mpe.mpg.de.