Properties of dusty warm absorbers and the case of IRAS 17020+4544

Abstract

We present a study of the properties of dusty warm absorbers and point to some important consequences for the resulting X-ray absorption spectra of AGN. Pronounced effects of the presence of dust in warm material are (i) an apparent flattening of the observed X-ray spectrum in the ROSAT band due to a sequence of absorption edges and a shift to lower gas ionization, (ii) the presence of a strong carbon edge at 0.28 keV, and (iii) the expectation of increased time variability of the warm absorber parameters. The first two effects can be drastic and may completely hamper an X-ray spectral fit with a dusty warm absorber even if a dust-free one was successfully applied to the data. In order to demonstrate facets of the dusty warm absorber model and test the recently reported important, albeit indirect, evidence for dusty warm material in the Narrow-Line Seyfert 1 galaxy IRAS 17020+4544 we have analyzed ROSAT PSPC and HRI observations of this galaxy. The X-ray spectrum can be successfully described by a single powerlaw with index Gamma_x = -2.4 plus small excess cold absorption, or alternatively by a steeper intrinsic powerlaw (G_x = -2.8) absorbed by a dusty warm absorber. The findings are discussed in light of the NLSy1 character of IRAS 17020 and consequences for NLSy1s in general are pointed out. In particular, the presence of dusty warm gas results in a steeper intrinsic powerlaw than observed, thus exaggerating the `FeII problem'. It also implies weaker potential warm-absorber contribution to high-ionization Fe coronal lines.