![]() | ![]() |
Astron. Astrophys. 331, 925-933 (1998) 4. DiscussionRX J0947.0+4721 is denoted as a QSO (rather than a Seyfert) by its
optical luminosity of Table 4. Parameters of the narrow line QSOs. A comparison of RX J0947.0+4721 with the narrow line quasars shows
that their X-ray parameters are quite similar: the luminosities in the
ROSAT band are of the same order of magnitude, and the photon indices
are practically identical. This latter observation may be taken as an
indication that the steepness extends towards fairly high rest frame
energies. However, the indication must not be overstressed. The value
of A better description of the observed spectrum can be achieved by thermal models, however, both the blackbody and thermal bremsstrahlung models underestimate the flux above 1 keV. The analysis in Sect. 2.1has shown that the hard component of the spectrum cannot be totally explained with the neighbouring harder source which is too weak. It must therefore be intrinsic to RX J0947.0+4721. A good fit is obtained with a blackbody modeling the soft excess plus a power law as hard component, modified by Galactic absorption. For an accretion disk model this means that the X-rays originate from a small region with little variations in temperature. This interpretation assumes a thermal origin for the X-ray emission
in the inner parts of the accretion disk. There is, however, an
indication against this assumption. No change of the hardness ratio
with increasing count rate is observed, which means that the
temperature of the emitting region remains constant over nearly one
order of magnitude in luminosity. Over this luminosity range
temperature variations should be easily detectable from the
correlation Model calculations for a spectrum with fixed power law component,
and all flux changes attributed to a temperature change in the
blackbody component, show that only small deviations from the average
flux This is illustrated in Fig. 9, which shows the ratio of high
to low count rate spectra for the data (crosses with error bars) and
six simulations (lines) with ratios
A luminosity change in dependency of the radius (pulsations with
constant temperature, In Guilbert & Rees (1988), the central engine produces a
non-thermal spectrum of hard X-rays and Models including warm absorbers may be another possible explanation of NLS1 phenomena. A detailed description of such a model applied to Mrk 766 can be found in Leighly et al. (1996). Unfortunately, Mrk 766 is more a Sy 1.5 than a NLS1 (Osterbrock & Pogge 1985), and it may be inappropriate to generalize the results for this special object to the whole NLS1 class. The application of such a model to RX J0947.0+4721 would conflict with the lack of spectral variability, anyway. Changes of ionisation parameter and/or column density of the warm absorber would cause changes in spectral shape. Spectral changes are expected even if the variations arise via changes in the central source itself since photoionisation contributes to the ionisation structure of the absorber. The lack of spectral changes for RX J0947.0+4721 is a strong argument against warm absorber models. Further model constraints can in principle be derived from timing
analysis. The shortest variation significantly detected for
RX J0947.0+4721 corresponds to a decrease Blazar-like activity, known to produce rapid X-ray variations, is
usually not applied to NLS1 galaxies because NLS1 objects do not show
properties which are typical for jet activities, as there are flat
spectrum radio emission, strong polarisation, and nearly featureless
multifrequency spectra. RX J0947.0+4721 may be different in that the
possibility of a flat radio spectrum (i.e. If the short time variations are confirmed, a Schwarzschild black
hole will no longer be a proper model. The Schwarzschild limit is
passed if large amplitude variations ( The large amplitude variation of a factor ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: March 3, 1998 ![]() |