Evidence for ionized accretion discs in five narrow-line Seyfert 1 galaxies

Abstract

We present the results of fitting ASCA spectra of six narrow-line Seyfert 1 (NLS1) galaxies with the ionized reflection models of Ross & Fabian. We find that five of the galaxies (TON S 180, PKS 0558–504, Ark 564, Mrk 335 and PG 1244+026) are well fitted by the ionized disc model, and these are often better fits than the alternative models considered. The sixth galaxy, NGC 4051, has additional spectral complexity that cannot be well described by a simple ionized disc model or any of the other alternative models. The highest luminosity NLS1 galaxy considered, PKS 0558–504, does not have a well-constrained ionization parameter or reflection fraction. This is because it is difficult to distinguish between highly ionized, highly reflective discs and moderately ionized discs with low reflection fractions. The four galaxies with well-constrained fit parameters are consistent with having inclination angles lying between 15° and 30°. Furthermore, these four sources are consistent with having a disc emissivity law that varies as r−2.5. These last two properties are also typical of broad-line Seyfert 1 galaxies. We find little or no indication of a correlation between the reflection fraction and the photon index of the underlying continuum. All six of the NLS1s we considered show evidence for a broad Fe Kα line, but none of the line centroids is consistent with emission from highly ionized Fe. This is most likely the result of the line being redshifted because of relativistic effects. We note that sources with larger ionization parameters tend to have larger Fe Kα EWs. We interpret this as evidence that ionized Fe features are making their presence felt in the spectra. Since most of our sources have steep spectra, highly ionized features are predicted even by the new variable density reflection models. The one source we analyse with a photon index less than two has the lowest ionization parameter in our sample. We conclude that our result is the strongest evidence yet that NLS1 might have ionized accretion discs. This result gives further weight to the hypothesis that these objects contain rapidly accreting black holes.