Astron. Astrophys. 339, 327-336 (1998)

6. Discussion

1H0419-577 displays a combination of X-ray continuum and line properties which is somewhat at odds with other Seyfert 1s. A simple power-law is a good description of the data in the whole 1.8-40 keV band, with a rather flat index (). Both thermal and non-thermal Comptonization models can produce such flat spectra (Svensson 1994, Haardt et al. 1997), although in rather extreme conditions. No iron emission line was detected: the upper limit on the EW of a narrow (broad) iron line from neutral iron is 90 (250) eV. We extrapolated from the ASCA Nandra et al. (1997a) sample the 5 objects whose 3-10 keV photon index is consistent with 1H0419-577 one within the statistical uncertainties (cfr. Table 2 in their paper: NGC3227, NGC3783, Mrk841, NGC6814 and MCG-2-58-22). The average observed EW in these objects () is significantly higher then the 1H0419-577 upper limit. The constraints on the line properties are however not very tight. It is worthwhile to notice that the line upper limits are consistent, within the statistical uncertainties, both with the EW vs. luminosity anti-correlation discovered by Nandra et al. (1997b) and with the line detected in the ASCA spectrum by Turner et al. (1998).

The 1H0419-577 2-10 keV luminosity is erg s^-1, which points to a borderline case between Seyferts and quasars. The lack of Compton reflection and/or fluorescent iron line in the X-ray spectra of quasars is still puzzling, given the fact that these features seem almost universal in low luminosity objects. A possible explanation assumes that the disk surrounding the central black hole becomes substantially ionized when the primary luminosity increases (Matt et al. 1993). That would decrease the contrast between reflected and direct continua and shift the line emission centroid towards energies corresponding to ionized iron stages; eventually, the intensity of the iron line would fade away when iron turns more and more into a completely ionized stage. No ionized line or Compton reflection are required by the 1H0419-577 data, but the constraints on the spectral parameters are again too loose.

The comparison between MECS results and 1992 ROSAT and 1996 ASCA observations demonstrates that at least the 0.5-2.5 keV has changed by a factor of in flux while turning from a soft () to a hard () state. ASCA data suggested the presence of a soft excess above the extrapolation of a rather hard high-energy power-law below 0.7 keV (Turner et al. 1998), which would have had a much lower flux and/or effective temperature than observed by ROSAT. Some authors (Walter & Fink 1993, Puchnarewicz et al. 1996) suggested that the optical to soft X-ray continuum could be seen as part of the same "Big Bump" (BgB), to which a hard power-law with typical is underlying. Variability by a factor of few seems to be a common property of soft X-ray radio-quiet AGN (Mannheim et al. 1996) It was recently suggested that such a variability is connected to the transient nature of disk emission around - black holes with near-Eddington accretion (Grupe et al. 1998). In order to qualitatively check these scenarios, we show the optical to X-ray SED in Fig. 8. Simultaneous MECS and optical data seem to suggest that the peak of the energy density lies around  Å, as typically observed in Radio Quiet Quasars (RQQ, Laor et al. 1997). If the PSPC data represents a soft excess which is still present also at the epoch of the Beppo-SAX observation, a simple extrapolation of the data suggests that such a soft excess cannot be reconnected in a single component with the optical to hard X-ray spectrum. The soft X-rays are clearly variable, and a corresponding dynamics of the optical/UV emission are required to occur as well if the optical emission is tightly related to it. However, strong variations in the optical band have not been observed yet.

Fig. 8. Optical to X-ray SED. Both Beppo-SAX MECS (filled circles ) and ROSAT/PSPC (empty squares ) data are shown together with the Beppo-SAX simultaneous optical spectrum

A way to characterize the soft X-ray and optical properties of QSO is through the multiwavelength "point-to-point" spectral indices and . We follow hereinafter the definition by Laor et al. (1997), where and (f is the flux density). We extrapolated the density flux at 3000Å from the optical spectrum presented in Sect. 2. For 1H0419-577 and . These values are in good agreement with the outcomes of the RIXOS sample analysis (Puchnarewicz et al. 1996), but flatter than in the complete PG sample of Wilkes et al. (1994) and in the optically selected sample of Laor et al. (1997) (for both ). A possible explanation for this discrepancy is that optically selected samples tend to be biased in favor of objects with intense optical/X-ray ratios. 1H0419-577 is also consistent, within the statistical dispersion, with Walter & Fink sample (1993, ), which is selected upon the X-ray brightness (the difference in induced by the different choice of the reference optical energy between Laor et al. (1997) and Walter & Fink (1993) - 3000Å vs. 2650Å - is well within the statistical dispersion).

The four year variation timescale implies that the soft excess emission region is smaller than 1 pc, and therefore associated with the nuclear region, where the primary continuum is expected to undergo strong reprocessing. If reprocessing is the ultimate cause of the soft excess, the energy balance requires that any cut-off is confined at energies 100 keV; that does not contrast the average behavior of Seyfert galaxies (Gondek et al. 1996). Any change of the flux/temperature of the soft excess has to be causally related to the variation of the high-energy spectrum. The lack of response of the soft X-rays to a change of the 0.7-10 keV photon index within two weeks (Turner et al. 1998) puts any reprocesser farther than 10⁴ Schwarzschild radii from a 10 black hole. Domination of scattering in the soft X-ray emission is unlikely because of the variability in the PSPC and HRI data (Turner et al. 1998).

An appealing alternative possibility is that the difference between the measured steepness of the PSPC and MECS spectra () is due to a change of the primary continuum itself. Haardt & Maraschi (1993) have shown that a disk corona, Comptonizing the soft photons supplied by a Shakura-Sunayev disk, can naturally reproduce the medium X-ray spectra shape typically observed in Seyfert 1. Interestingly enough, and in 1H0419-577 are very close to the expected spectral indices if the coronal plasma undergoes a transition between a scattering optically thin (, ) and thick (, ) regime (Haardt, Maraschi & Ghisellini, 1997). A pair dominated corona would require an increase of the 2-10 keV flux by more than 4 orders of magnitude for a steepening to be achieved. On the contrary, if we extrapolate the PSPC best fit power-law into the 2-10 keV band, the luminosity is  erg s^-1, which is comparable with the MECS measurement. A pair-saturated plasma can be therefore ruled out. A measurable side-effect in such a scenario would be an increase by an order of magnitude of the ratio between the flux in the 2-10 keV band above keV. The hypothesis of a change of the Comptonized spectral shape could provide also a good explanation for the relative faintness of the iron line emission, if the Compton component responds not simultaneously to the - basically unknown - pattern of variability of the primary emission.

Another possibility to explain a change in the primary continuum spectral shape comes from the analogy between Seyfert galaxies and Black Hole Candidates (BHC). BHC are well know to display two different intensity and spectral states: a high and soft one, characterized above 10 keV by a power-law spectrum with photon index , and a low and hard one, with typical . Such an analogy had been first suggested to explain the very soft () ASCA spectrum of the NLSy1 galaxy RE1034+39 (Pounds et al. 1995).

Ebisawa et al. (1996) have recently suggested that the difference between the two states is due to different Comptonization mechanisms in a viscuosless shock two-phase accretion disc. Thermal Comptonization in a disk with a very low accretion rate (and therefore low thermal soft emission) would yield a hard spectrum with , regardless of the absolute black hole mass. If, however, the accretion rate increases, more soft photons are supplied, and the post-shock region cooling becomes more efficient. The cooler inflow towards the nuclear black hole is responsible for a steep non-thermal emission. The resulting spectrum in a quasi-Eddington regime is the combination of the thermal emission from the optically thick disk and of a power-law with . In this framework, the observed variability pattern in 1H0419-577 would therefore suggest a transition phase from a bulk motion to a thermal motion regime due to a change of the accretion rate. Typical in BHC in high state are  keV; scaling , a blackbody temperature corresponds to a black body mass . The bolometric luminosity of the blackbody component as measured by PSPC on 1H0419-577 is and self-consistently in this case . However, it must be kept in mind that the determination of a blackbody temperature sensitively depends on the energy band where the spectral fits are performed and that the spectral deconvolution in the PSPC spectra of 1H0419-577 is not unique. We are then far from considering this coincidence as a confirmation of the proposed scenario.

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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