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The spectral energy distribution (SED) of F-9

Multi-wavelength continuum: corrections andmeasurements

We have used the observations of F-9, taken between 1978 and 1991, at all wavelengths from the radio to X-rays to derive the nuclear continuum spectral energy distribution (SED), which will later be used in the photoionization models. The X-ray, UV, and optical lightcurves are shown in Fig. 1 (click here). All data were corrected for E(B-V)=0.035 which is derived from the combined hydrogen column density from our Galaxy and the Magellanic Stream of tex2html_wrap_inline4531 (Morini et al. 1986; Clavel & Santos-Lleó 1990). Using this color excess we applied Seaton's law (1979) to correct the ultraviolet and the optical. In the NIR we used the extinction law for this wavelength region from Rieke & Lebofsky (1985). To evaluate the contribution in the FIR due to cold interstellar dust from the galaxy disk of F-9, we used the results for normal early-type spiral galaxies from the IRAS observations derived by De Jong et al. (1984). The resulting disk flux of 120 and 370 mJy at respectively 60 and 100 tex2html_wrap_inline4533, has been subtracted from the IRAS fluxes for F-9 in these bands. The subtraction of the stellar contribution in the optical and the near IR will be discussed in Sect. 3.1.1. Unless otherwise specified, we will use observed wavelength in the following discussion.

  figure443
Figure: Light curves of the UV, optical and X rays continua. a) FES light curve (tex2html_wrap_inline4535. b) tex2html_wrap_inline4537 light curve. The full drawn line is the cubic spline interpolation used to derive the UV fluxes at times when no simultaneous observations were available. c) tex2html_wrap_inline4539 light curve (Table 2 (click here)). d) tex2html_wrap_inline4541) light curve (Table 3 (click here)). N.B. At minimum (JD tex2html_wrap_inline4543 3000) the object was too faint to be detected with the FES

3.1.1. The ultraviolet and optical

The ultraviolet continuum fluxes in Table 1 have been measured in three `` line-free" windows centered at 1171 (1158-1184), 1400 (1390-1410) and 1910 (1895-1925) Å, and are corrected for E(B-V)=0.035 extinction. When more than one spectrum was available on the same day, both fluxes and errors have been averaged. Since tex2html_wrap_inline4549 is rather noisy -it is measured at the short wavelength side of the geocoronal emission- and tex2html_wrap_inline4551 already has a small contribution of the Balmer continuum and the FeII emission (Small Blue Bump), both present above tex2html_wrap_inline4553 (rest wavelength), we will use the tex2html_wrap_inline4555) as reference UV continuum. For those cases where the data in the UV and at other wavelengths are not exactly simultaneous, we use an interpolation of the UV light curve fitted with a cubic spline using a smoothing factor of tex2html_wrap_inline4557(reduced) = 3.75 as shown in Fig. 1 (click here)b to the epoch of the non-UV observation. Table 1 also gives the FES counts corrected for E(B-V)=0.035. Ground-based optical photometry between 1979-1990 of F-9 has also been reported by Lub & De Ruiter (1992). We have corrected their B band fluxes at 4298 Å measured with an aperture radius of tex2html_wrap_inline4563 for E(B-V)=0.035 (versus tex2html_wrap_inline4567 as used by them; De Ruiter, private communication). These values are given in Table 2 (click here) together with the corresponding UV fluxes, derived from the interpolated ultraviolet lightcurve. Both the FES and the B lightcurve are shown together with tex2html_wrap_inline4571) and X-rays in Fig. 1 (click here).

3.1.2. The stellar contribution to the optical continuum

The FES light curve (Fig. 1 (click here)) is very similar to the UV, but its amplitude, as is common in Seyfert I galaxies, is considerably smaller with tex2html_wrap_inline4575. The Cross Correlation Function (CCF; Gaskell & Peterson 1987) of the FES counts with tex2html_wrap_inline4579) peaks at tex2html_wrap_inline4581 (Fig. 2 (click here)c), indicating that there is no delay between the variations at these wavelengths. As a consequence of this both fluxes are highly correlated (Fig. 3 (click here)c):
equation465
(tex2html_wrap_inline4583 is in units of tex2html_wrap_inline4585) with r=0.938. Using the calibration of Imhoff & Wasatonic (1986) one can in an approximate way convert the FES counts to Johnson V-band fluxes. Using a color index of tex2html_wrap_inline4591 for the total central flux of F-9 (Griersmith & Visvanathan 1979) we obtain FEStex2html_wrap_inline4593(tex2html_wrap_inline4595) = 0.024 tex2html_wrap_inline4597 FES(cts), changing relation (1) into:
equation473
Here we have added the estimated error of 6%, associated with the calibration used (Imhoff & Wasatonic 1986), to the correlation errors. Both relations (1 and 2) show a non-negligible constant term most likely due to the stellar contribution from the central bulge of the underlying galaxy. This contribution dilutes the intrinsic nuclear variation of the AGN itself and causes the smaller amplitude of the optical variations with respect to the UV. Similar results are obtained from the optical data of Lub & De Ruiter (1992) and their light curve is also very similar to the UV one (Fig. 1 (click here)c) and its amplitude, tex2html_wrap_inline4601, is similar to that of the FES. The CCF of tex2html_wrap_inline4605) with tex2html_wrap_inline4607) peaks at tex2html_wrap_inline4609 (Fig. 2 (click here)c), confirming the strict simultaneity at the time resolution of 96 days of the variations at these wavelengths. Linear correlation gives (Fig. 3 (click here)d):
equation484
r=0.969, using the interpolated tex2html_wrap_inline4615 light curve. Also here the non-zero constant term is most likely due to the stellar contribution and is the main cause for the smaller amplitude of the tex2html_wrap_inline4617 variations compared to the UV. The constant terms in these relations causes the apparent hardening of the UV-optical spectral index tex2html_wrap_inline4619 (tex2html_wrap_inline4621; e.g. Table 2 (click here) for tex2html_wrap_inline4623) with increasing UV brightness. Since we want to use the observed AGN SED later as input spectrum for the photoionization models at different brightness levels, we need to correct the SED for these contributions. The constant term in Eqs. (2) and (3) is dominated by the stellar contribution, however many optical FeII multiplets and the forbidden lines of tex2html_wrap_inline4625, 4959 as well as tex2html_wrap_inline4627 contribute both to the FES counts and tex2html_wrap_inline4629. To evaluate the Fe II contribution, we use the results of Wamsteker et al. (1985), deriving a value of tex2html_wrap_inline4631, from high resolution spectra and FeII model fitting. Subtracting this from the constant in Eq. (2) we obtain (tex2html_wrap_inline4633 tex2html_wrap_inline4635 erg stex2html_wrap_inline4637 cmtex2html_wrap_inline4639 Åtex2html_wrap_inline4641 (for the V band). Similarly we estimated the FeII contributing to the tex2html_wrap_inline4645, using the same FeII model and the bandpass of this band (Lub & Pel 1977). This resulted in tex2html_wrap_inline4647. Subtracting this from the constant in relation (3) gives (tex2html_wrap_inline4649 (for the tex2html_wrap_inline4651). In the absence of optical spectra or images, taken specifically for this purpose, we can compare the results of this procedure with other independent determinations of the stellar contribution to the nuclear flux in those wavelengths were it is important. Griersmith & Visvanathan (1979) used a De Vaucouleurs law (tex2html_wrap_inline4653 distribution) for the brightness distribution, which they combined with their measured brightness distribution to calculate the stellar galaxy flux in V for any aperture. For the FES (taken in V) with an aperture radius of tex2html_wrap_inline4659, we obtain tex2html_wrap_inline4661, consistent with the above. For the tex2html_wrap_inline4663 aperture radius of tex2html_wrap_inline4665, and taking the color index for the stellar population in the galaxy tex2html_wrap_inline4667 (Griersmith & Visvanathan 1979), we obtain (taking tex2html_wrap_inline4669) tex2html_wrap_inline4671, also in good agreement with the above. Since the stellar emission of the galaxy estimated by Griersmith & Visvanathan (1979) is in good agreement with those derived by us we will use it in what follows. This value is approximately half of that derived for tex2html_wrap_inline4673 by Lub & De Ruiter (1992), but they used a disk-like exponential brightness distribution and may have underestimated the contributions due to FeII considerably. A disk distribution might not be appropriate for the apertures used by Lub & de Ruiter (tex2html_wrap_inline4675), because Griersmith & Visvanathan (1979) suggest that the nuclear bulge extends out to tex2html_wrap_inline4677. Véron-Cetty et al.\ (1991) find also that a tex2html_wrap_inline4679 distribution gives a good representation of their observations of the galaxy surface brightness distribution for F-9. With their value of tex2html_wrap_inline4681 and tex2html_wrap_inline4683 for the galaxy, we derive the expected contribution to the FES counts to be tex2html_wrap_inline4685 and to the tex2html_wrap_inline4687. Since these independent estimates of the stellar contribution to the optical measurements of the nuclear brightness are all consistent within the measurement uncertainties, we can be confident that all constant components in the observations of the nuclear region of F-9 are properly allowed for in the derivation of the SED of the active galactic nucleus. Then, using the slopes of the relations (2) and (3), we obtain tex2html_wrap_inline4689 and tex2html_wrap_inline4691, equal within the errors. As the second value has a smaller error, we will use from now on a UV-optical spectral index tex2html_wrap_inline4693. After correction for E(B-V)=0.035, instead of E(B-V)=0.06 used by Wamsteker et al. (1985), this value is similar to the index obtained from their Table 1, tex2html_wrap_inline4699.

  figure511
Figure: Variability features of the UV-optical continuum. a) The power spectrum (Press & Teukolsky 1988 algoritm) of the variations for tex2html_wrap_inline4701. The peak at 4903 days represents the total temporal range of the data and is not a real variability time scale. b) ACF (Autocorrelation Function; Gaskell & Peterson 1987) for tex2html_wrap_inline4705. c) CCF for tex2html_wrap_inline4709 (solid line), for tex2html_wrap_inline4711 (dashed line), FES counts (dot-dashed line) and tex2html_wrap_inline4713 (dotted line) with respect to tex2html_wrap_inline4715. The formal delay between these is respectively, tex2html_wrap_inline4717, tex2html_wrap_inline4719, tex2html_wrap_inline4721, and tex2html_wrap_inline4723

  figure519
Figure 3: Correlations between the continuum fluxes at different wavelengths versus the UV flux tex2html_wrap_inline4725. Linear regression lines are shown as dotted lines. a) tex2html_wrap_inline4727. b) tex2html_wrap_inline4729. c) FES counts (tex2html_wrap_inline4731. d) tex2html_wrap_inline4733. e) tex2html_wrap_inline4735(1.15 tex2html_wrap_inline4737)



1

  table532
Table 2: Ultraviolet and optical continuum

3.1.3. The infrared

Near infrared (NIR) observations of F-9 in J (1.15 tex2html_wrap_inline4839), H (1.58 tex2html_wrap_inline4843), K (2.10 tex2html_wrap_inline4847) and L (3.35 tex2html_wrap_inline4851) have been reported in Glass (1986) and in Clavel et al.\ (1989). From these data we only use those coinciding in temporal coverage with the UV observations and only the data with an aperture radius of 6'' to avoid variations in stellar light contributions in the photometry. All fluxes have been corrected for E(B-V)=0.035.

Edelson & Malkan (1987) report on four IRAS observations at 12, 25, 60 and 100 tex2html_wrap_inline4857 made over a period of 14 days, and find no significant variability in any of these bands. We use here for each band the mean flux and the rms of the individual measurements. The values for the mean epoch tex2html_wrap_inline4859, are tex2html_wrap_inline4861, tex2html_wrap_inline4863, tex2html_wrap_inline4865 and tex2html_wrap_inline4867 for 12, 25, 60 and 100 tex2html_wrap_inline4869, respectively. Subtracting the disk dust contribution at 60 and 100 tex2html_wrap_inline4871 (120 and tex2html_wrap_inline4873, respectively. See above), the fluxes for these two bands become tex2html_wrap_inline4875 and tex2html_wrap_inline4877. Note the agreement between the 12 tex2html_wrap_inline4879 IRAS point and the tex2html_wrap_inline4881 measurement (also at 12 tex2html_wrap_inline4883) taken tex2html_wrap_inline4885 earlier (see Fig. 8 (click here) by Clavel et al. 1989), confirming that the 12 tex2html_wrap_inline4887\ radiation is not variable. The shape of the light curves in J (1.15 tex2html_wrap_inline4891), H (1.58 tex2html_wrap_inline4895), K (2.10 tex2html_wrap_inline4899) and L (3.35 tex2html_wrap_inline4903) are very similar to the UV (see Clavel et al. 1989), with a considerably smaller amplitude at tex2html_wrap_inline4905, tex2html_wrap_inline4907, tex2html_wrap_inline4909 and tex2html_wrap_inline4911, respectively. Clavel et al.\ (1989) derived delays of respectively tex2html_wrap_inline4913, tex2html_wrap_inline4915, tex2html_wrap_inline4917 and tex2html_wrap_inline4919 for J, H, K and L, while we have obtained that the J band and the UV fluxes are related by (Fig. 3 (click here)e):
equation576
with r=0.950, or, converting all units to tex2html_wrap_inline4935:
equation583

The constant term in relations (4) and (5) is again dominated by the stellar contribution, while also the H, K, and L fluxes are affected, but to a progressively lesser extent.

The stellar emission in the near IR for F-9 with an aperture radius of 6'', using the surface brightness distribution in V (with an error of 3%) of Griersmith & Visvanathan (1979) and color indexes of V-K=2.99, tex2html_wrap_inline4951, tex2html_wrap_inline4953 and tex2html_wrap_inline4955 (Clavel et al.\ 1989) is tex2html_wrap_inline4957, tex2html_wrap_inline4959, tex2html_wrap_inline4961 and tex2html_wrap_inline4963 for J, H, K and L, respectively. Glass (1986) comments that these color indexes are consistent with the population found in normal galaxies (V-K=2.99 is characteristic of giant and supergiant stars of K3-K4 spectral-type (Johnson 1966)). This stellar contribution at J (tex2html_wrap_inline4977 tex2html_wrap_inline4979) is similar, within the errors, to the constant term in Eqs. (4) or (5). Therefore, the behavior in the J-band can be fully understood in terms of a variable source, which is a direct extension of the UV-optical power-law, and a constant part due to the stellar contribution. Thus there appears to be no evidence for the presence of a steep constant IR power law, extending into the optical, as suggested by Edelson & Malkan (1986, 1987) and Carleton et al.\ (1987). For the AGN (without the stellar contribution) we obtain an intrinsic tex2html_wrap_inline4983 spectral index of tex2html_wrap_inline4985, equal to the index obtained between 1400 and 4298 Å, confirming that the variable J flux is a direct extension of the UV-optical.

The X-rays

Morini et al. (1986) have summarized the F-9 hard X-ray observations from 1970 to 1984 obtained with the Uhuru, Ariel 5, HEAO (1 and 2) and EXOSAT satellites. We use here again only data after 1978, for comparison with the UV continuum. Two GINGA observations on 1989 November 20-21 and in 1990 November 22-25 are included (Makino, private communication). The tex2html_wrap_inline4989 fluxes are listed in Table 3 (click here) together with the interpolated UV flux and the light curves are shown in Fig. 1 (click here)d with the UV and the optical ones. The GINGA data give hard X-ray spectral indexes of tex2html_wrap_inline4991 (tex2html_wrap_inline4993) and tex2html_wrap_inline4995 (tex2html_wrap_inline4997), both with a fixed column density of tex2html_wrap_inline4999 and also show a strong Fe emission line centered at tex2html_wrap_inline5001, with an equivalent width of tex2html_wrap_inline5003 on tex2html_wrap_inline5005 and tex2html_wrap_inline5007 and tex2html_wrap_inline5009 on tex2html_wrap_inline5011. The only Soft X-ray observation has been taken with the ROSAT satellite at the same date as the second GINGA observation (tex2html_wrap_inline5013), simultaneous with one of the UV spectra (in the context of the RIASS program: De Martino et al. 1991). Walter et al. (1995) find that for a column density of tex2html_wrap_inline5015 two power laws are required. So, he obtains for the GINGA spectrum a spectral index of tex2html_wrap_inline5017 and for the tex2html_wrap_inline5019 flux of 3.04 tex2html_wrap_inline5021 very similar to Makino's values, and for the ROSAT spectrum, tex2html_wrap_inline5023 and tex2html_wrap_inline5025 tex2html_wrap_inline5027, indicative of a strong soft X-ray excess.

  table606
Table 3: Ultraviolet and X-rays continuum

3.1.5. Radio

The only radio observation reported for F-9 has been made at 4.7 GHz (Véron-Cetty et al. 1991) with the Australian Telescope on 24 December, 1990 (tex2html_wrap_inline5059) and they give a tex2html_wrap_inline5061 upper limit to the flux of 5 mJy, classifying F-9 as a weak radiosource.

3.2. Variability characteristics

3.2.1. The ultraviolet

The tex2html_wrap_inline5063 light curve (Fig. 1 (click here)b) shows a wide range of variability over the duration of the IUE observations. It shows a striking large and relatively slow flux decrease since the first observations (1978-August-02, tex2html_wrap_inline5065) through 1984 October 29 (tex2html_wrap_inline5067). After this decline, the source brightened again until 1987 June (tex2html_wrap_inline5069), where it reached tex2html_wrap_inline5071 of its maximum brightness. The maximum amplitude of the whole light curve, expressed as the ratio of the maximum to the minimum flux (R), is tex2html_wrap_inline5075 (in order to reduce the formal error on tex2html_wrap_inline5077 the minimum flux and its error were taken to be the average of the three lowest values and their rms).

Pure Fourier techniques can not be used with unequally sampled data. In this case other algorithms can be applied to allow for the non-uniform sampling. We applied the algorithm by Press & Teukolsky (1988) to the UV light curve to obtain the frequency power spectrum. This is shown in Fig. 2 (click here)a, where different significance levels (0.05, 0.001) for a random noise hypothesis are also indicated. Small values of this level indicate a highly significant periodic signal. In the figure three main maxima can be observed, corresponding to typical variability time scales of 4903, 182 and 56 days, respectively. The strongest peak at 4903 days is associated with the total duration of the sampling, and does not represent a real variability time scale. Since the mean interval between two consecutive observations is 96 days, we can only expect to resolve variations on longer time scales than this. On the other hand the irregular nature of the sampling could still allow the detection of faster variations. This could indicate the existence of the 56 days peak as a real variability time scale, but since it can not be confirmed from the present data, we conclude that 182 days represents the dominant typical variability time scale of F-9 at 1400 Å.
Both tex2html_wrap_inline5079 and tex2html_wrap_inline5081 are strongly correlated with tex2html_wrap_inline5083 and the linear correlation analysis (Figs. 3 (click here)a, b) gives:
equation627
with a correlation coefficient r=0.967; and
equation631
with r=0.986. In both cases, the constant term is small compared to the flux and not significantly different from zero. Most likely the slight deviation from zero of the constant term in relation (6) is caused by the non-linearity and poorly determined calibrations in this region of the IUE camera in the window below 1214 Å, while in relation (7) it is due to the small FeII and Balmer continuum contribution to the flux in the 1910 Å window. The strict proportionality of the fluxes in these windows indicates that, in spite of the large amplitude, the brightness variation takes place with constant spectral index between 1171 - 1910 Å. The relations (6) and (7) give spectral indexes of tex2html_wrap_inline5095, tex2html_wrap_inline5097 and tex2html_wrap_inline5099. These two last spectral indexes (tex2html_wrap_inline5101 and tex2html_wrap_inline5103) are somewhat steeper than the intrinsic UV-optical one (tex2html_wrap_inline5105, Sect. 3.1.2). This is most likely caused by the contribution of the Balmer continuum which is correlated with the UV flux (Sect. 4.1) and some FeII contribution (see Figs. 6 (click here)a-c), resulting in an apparent steepening of the spectral index. Because the Balmer continuum is a nebular component, it is more meaningful to use a large wavelength range to calculate the spectral index linking the UV to the optical, where we have allowed for the removal of non-AGN associated flux in the spectrum, as done in Sect. 3.1.2.

The proportionality between the three UV fluxes and the similarity of their maxima and minima in the light curves, suggests the absence of lag between them. The cross correlation function (CCF; Gaskell & Peterson 1987) of tex2html_wrap_inline5109) and tex2html_wrap_inline5111 with tex2html_wrap_inline5113 are shown in Fig. 2 (click here)c and as a reference the auto-correlation function, ACF of tex2html_wrap_inline5117, in Fig. 2 (click here)b. These peak at tex2html_wrap_inline5119 and at tex2html_wrap_inline5121, respectively. This absence of a delay in the variability between the continuum windows, suggest that a coherent emission mechanism is responsible for the continuous emission at all three wavelengths or otherwise the variations are isothermal.

The X-rays

The light curve of the integrated X-ray flux between 2 and 10 keV is clearly different from the UV and optical ones (Fig. 1 (click here)d). The X-ray light curve decreases from a maximum to a minimum value, after which the flux raises again to its initial level of 1978. The amplitude of variations in this time interval is tex2html_wrap_inline5123 for the decline and tex2html_wrap_inline5125 for the increase. This is contrary to the UV continuum, which shows a much larger amplitude of tex2html_wrap_inline5127 (as calculated from the interpolated values), in the decrease and an amplitude of tex2html_wrap_inline5129, similar to the X-rays, when brightening. This indicates that both UV and X-rays vary equally in the increase after the minimum, but not in the initial decrease from 1978. This suggests that the good correlation between X-rays and UV breaks down at high levels of the UV continuum (Fig. 4 (click here)b), as has also been seen in other AGN such as NGC 4151 (Perola et al. 1986) and NGC 5548 (Clavel et al. 1992). Figure 4 (click here)b shows a good correlation whereas the UV flux is weak (tex2html_wrap_inline5131), while at UV flux (tex2html_wrap_inline5133) the X-rays appear to remain constant. For the intermediate levels, unfortunately, no data are available. This behavior is obviously reproduced in the individual UV-X-rays spectral indexes shown in Table 3 (click here). For the low levels the mean spectral index is tex2html_wrap_inline5135, whereas for the high levels it is tex2html_wrap_inline5137, resulting in a significantly steeper spectrum at the high UV brightness, as shown in Fig. 4 (click here)c.

  figure653
Figure: The Balmer Continuum (BaC), X-rays (tex2html_wrap_inline5139) Flux, and the UV to X-rays spectral index tex2html_wrap_inline5141 as a function of the UV continuum brightness tex2html_wrap_inline5143. a) BaC (3837 Å) derived from the data of Lub & De Ruiter (1992); see Table 2 (click here). b) tex2html_wrap_inline5145); see Table 3 (click here). c) UV-X-rays spectral index tex2html_wrap_inline5147

The spectral energy distribution (SED) from radio to X-rays

In Fig. 5 (click here) we show the intrinsic F-9 continuum SED (observed), from radio to X-rays, as described above for four different levels of continuum brightness. To allow direct comparison with the standard conventions to display SED in the context of different wavelength domains we show both the frequency spectrum tex2html_wrap_inline5151 as well as the energy spectrum tex2html_wrap_inline5153 versus tex2html_wrap_inline5155. The first one allows easy evaluation of the spectral index (slope of the curve) and the second one is the usual way to represent the SED, where a flat SED corresponds to equal energy per decade. In absence of observational support for significant variability in the FIR and radio domain we have used at these wavelengths the single value determined above for all UV and X-ray brightness levels of F-9. For the near IR-hard X-rays range, we use the results of the variability study in each band with respect to the UV one. We will in what follows refer to the four levels of UV brightness as it `` high" (tex2html_wrap_inline5157), `` intermediate-high" (tex2html_wrap_inline5159), `` intermediate-low" (tex2html_wrap_inline5161) and `` low" (tex2html_wrap_inline5163), corresponding to the interpolated tex2html_wrap_inline5165) for each date. We have chosen these dates because for three of them there exist values for the tex2html_wrap_inline5167) (see Table 3 (click here)) and for the tex2html_wrap_inline5169, when there are no X-rays data, we interpolated linearly between the two values nearest to this date, obtaining tex2html_wrap_inline5171 tex2html_wrap_inline5173. For each date, the J, H, K and L fluxes have been taken by interpolating on the light curves (corrected for the stellar contribution). These observed complete SED's will be applied later, to define the input continuum spectra for the photoionization models in the detailed evaluation of the line intensities and the variability characteristics to disentangle the conditions in the BLR. The flux values used from radio to X-rays, in units of tex2html_wrap_inline5183 (and between round brackets in units of mJy), are given in Table 4 (click here).

  table674
Table 4: Flux values used in the SED

From the combined optical, NIR (J) and ultraviolet variability study we found that the spectrum in this domain can be represented, after allowing for the galaxy contribution, by a power law spectrum with a constant spectral index of tex2html_wrap_inline5253 (independently of the brightness). It is then justified to parametrize the central continuum spectrum with a UV-optical-J power law of the type tex2html_wrap_inline5255, where tex2html_wrap_inline5257 is determined from the observed tex2html_wrap_inline5259 for the different brightness levels. In the hard X-rays we use a power law with tex2html_wrap_inline5261 and the constant value is determined from the observed tex2html_wrap_inline5263). This spectral index is taken from Morini et al. (1986) and is within the errors the same as that independently determined from the GINGA observations (see Sect. 3.1.4). There is only one observation with ROSAT, simultaneous with GINGA and IUE. Since the wavelength ranges around the soft X-rays are variable, the soft X-rays are likely to vary also. We can then not use this unique datum for the four brightness levels defined before (Fig. 5 (click here)). However, the simultaneity with the hard X-rays and the UV on tex2html_wrap_inline5265, allows us to characterize at least part of the big blue bump shape (Fig. 5 (click here)c). In this figure we have used the ROSAT-GINGA spectral model by Walter (see Sect. 3.1.4), and the J-UV power law with tex2html_wrap_inline5267 fitted to the tex2html_wrap_inline5269 in this date. A strong soft X-ray excess is clearly seen with respect to the hard X-ray power law, suggesting that the soft X-rays merge smoothly into the decrease of the big blue bump, which must than have its maximum at energies less than 0.15 keV (tex2html_wrap_inline5271; tex2html_wrap_inline5273).

  figure702
Figure 5: Different representations of the SED of the F-9 continuum. a) tex2html_wrap_inline5275 versus tex2html_wrap_inline5277 from radio to X-rays for four levels of UV brightness (see Sect. 3.3). b) tex2html_wrap_inline5279 versus tex2html_wrap_inline5281 from radio to X-rays for four levels of UV brightness. c) SED (tex2html_wrap_inline5283 versus tex2html_wrap_inline5285) for the unique simultaneous observation IUE-ROSAT-GINGA (tex2html_wrap_inline5287). The extrapolation of the hard X-rays (tex2html_wrap_inline5289) into the soft X-rays clearly indicates a soft X-rays excess. This suggest that the turnover of the big blue bump, which starts at tex2html_wrap_inline5291, occurs in this range


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