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Astron. Astrophys. 354, 411-422 (2000)

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6. Summary and conclusions

We have presented a study of the X-ray spectral and temporal properties of four NLSy1-like galaxies and a radio-loud quasar. The results can be summarized as follows:

QSO 0117-2837. This NLSy1 galaxy shows a very steep X-ray spectrum with [FORMULA] [FORMULA] when fit with a simple powerlaw, among the steepest spectra reported for NLSy1s. Alternatively, the spectrum can be described by a flat powerlaw with soft excess of [FORMULA] keV, or a warm-absorbed flat powerlaw. The extreme X-ray spectral steepness of QSO 0117-2837 is unexpected in the light of its fairly large FWHM of H[FORMULA]. With these properties QSO 0117-2837 fills the `zone of avoidance' in the [FORMULA]-[FORMULA] diagram. The X-ray lightcurve of QSO 0117-2837 reveals constant source flux.

RX J0134-4258. This source underwent a drastic X-ray spectral transition from steep ([FORMULA] [FORMULA]) to flat ([FORMULA] [FORMULA]) between two ROSAT observations separated by 2 yr, while the mean countrate remained nearly constant. We examined several scenarios that might account for this peculiar behavior, with focus on the presence of a warm absorber. We find that a reaction of the ionized material to continuum changes requires non-equilibrium effects to be at work. Alternatively, and more likely, a cloud of warm gas may have passed our line of sight. This latter scenario shares some similarity with the one proposed by Rodriguez-Pascual et al. (1997) based on UV observations of a sample of NLSy1s. Variability of both components in the framework of a powerlaw-plus-soft-excess spectral description provides an alternative explanation.

NGC 4051. We analyzed all ROSAT PSPC observations of this well-known Seyfert galaxy including previously unpublished ones. Variability by a factor of [FORMULA]30 in countrate is detected. The mean X-ray luminosity varies less but still by a factor of [FORMULA]7, whereas the properties of the warm absorber (U, [FORMULA]; averaged over individual pointings) change by only a factor of [FORMULA]2, indicating that the bulk of the warm absorber is either of low density or out of photoionization equilibrium.

Mrk 1298. The ROSAT PSPC spectrum of this source shows clear signs of warm absorption, confirming Wang et al. (1996). Based on detailed warm-absorber modelling, we predict the contribution of the ionized absorber to opt-UV-EUV emission lines which is found to be quite weak. Repeated rapid variability by a factor [FORMULA]2 within time intervals of 800s is detected.

4C +74.26. We considered this quasar as a candidate for a dusty warm absorber. We show that such a model successfully fits the ROSAT X-ray spectrum of this source and resolves the discrepancy between the underlying powerlaw index derived from ROSAT and ASCAobservations.

X-ray spectral complexity in NLSy1s and multi-phase BLR equilibrium. Given the increasing spectral complexity of NLSy1 galaxies, with often two or even three components contributing to the X-ray spectral shape, we examined the influence of different spectral shapes on BLR cloud multi-phase equilibrium, in an attempt to determine which spectral shapes dominates on average (within the limits of this scenario). We find that both, a steep powerlaw spectrum, and a strong EUV-X-ray excess component narrow down the range where a stable multi-phase equilibrium is possible.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000